DEPARTMENT OF BIOLOGICAL AND ENVIRONMENTAL SCIENCES OLIGOLECTIC BEE SPECIES: An understudied group in Global Change impacts? Monika Böttcher Degree project for Bachelor of Science with a major in biology BIO 603, Bachelor´s degree project, 30 higher education credits First cycle Semester/year: 2023 Supervisor: Julia Osterman, Deptartment of Biology and. Environmental Science Examiner: Åslög Dahl, Deptartment of Biology and. Environmental Science Andrena vaga ♀ Photo: Julia Osterman Content Abstract .................................................................................................................. 3 Introduction ............................................................................................................ 4 Global Change...................................................................................................................................... 4 Apiformes - Bees ................................................................................................................................. 4 Objectives of this study ........................................................................................................................ 6 Methods ................................................................................................................. 6 Collection of data ................................................................................................................................. 6 Oligolectic species ............................................................................................................................... 7 Database searches ................................................................................................................................ 7 Results .................................................................................................................... 9 Oligolecty ............................................................................................................................................. 9 Disproportionalities in Red lists ......................................................................................................... 11 Oligolectic bees and Global Change impacts ..................................................................................... 12 Topics ................................................................................................................................................. 12 Discussion ............................................................................................................ 21 Method discussion.............................................................................................................................. 21 Terminology ....................................................................................................................................... 22 Global change impacts on oligolectic bees ........................................................................................ 23 Are oligolectic bees particularly vulnerable? ..................................................................................... 27 Why so few studies? .......................................................................................................................... 29 Conclusions .......................................................................................................... 30 Oligolecty ........................................................................................................................................... 30 Oligolectic bees .................................................................................................................................. 30 Further research.................................................................................................................................. 31 Words of thanks ................................................................................................... 31 References ............................................................................................................ 31 Appendixes .......................................................................................................... 34 Appendix 1: Hit result list (references) .............................................................................................. 34 Appendix 2: A revised list of oligolectic bees in Sweden 2023 ......................................................... 49 Appendix 3: Current prevailing red list status of oligolectic bees ..................................................... 51 Appendix 4: Combination of search words ........................................................................................ 54 Abstract Global change is considered the primary cause of the decline in bees worldwide, posing a significant threat to crucial pollination services they provide, carrying negative economic and ecological implications. Despite the extensive research conducted on the responses of bee communities to anthropogenic impacts, the focus has predominantly been on commercially interesting bees. In contrast, studies on solitary wild bees are notably scarce, especially on oligolectic bees (i.e. pollen specialists), despite their significant representation, accounting for up to 30% of species in some regions. This study seeks to address important knowledge gaps surrounding oligolecty and the responses of oligolectic bee species to global change. Objectives include providing a comprehensive explanation of "oligolecty"; provide a revised list of Swedish oligolectic species; reviewing current knowledge on global change impacts, indications of the potential vulnerability of oligolectic bees, and quantitatively presenting the distribution of research studies on global changes and bees. Existing knowledge has been drawn from scientific articles via global databases, reports, and experts. The used method is partly qualitative and partly quantitative. This study also reveals obscurities and misleading generalizations. Possible reasons for the sparse number of studies, what consequences this may have and what can be done to change this are discussed to some extent. Key words: Solitary bee, global change, oligolecty, red list, taxonomy 3 Introduction Global Change Global changes are a large variety of anthropogenic drivers (figure 1A) and different authors’ points out the main stressors with some slight differences. Five major global change stressors: landscape alteration, agricultural intensification, climate change, invasive species, and spread of pathogens have been identified as the main drivers of wild bee declines and extinctions (Gonza´ lez-Varo et al. 2013). Although LeBuhn & Luna (2021) mention that the drivers of pollinator declines vary, they also specifies enhancing recognition of important drivers such as; impacts of pollution, notably lead and other heavy metals, pesticide use and diseases, leading to reduced species richness and abundances. Rasmussen and colleagues (2022) highlights habitat destruction, changed (intensified) land use in agriculture, the use of plant protection products, climate change and invasive species as the broad- scale threats to the diversity of pollinators. Figure 1A: Global change stressors Figure 1B: Global change interactions Global change consists of multiple factors, and while it involves various factors, it is essential to understand the impact of individual drivers (figure 1A). Therefore, clarity of the interaction effects (figure 1B) of the decline in wild bee populations with multiple natural and anthropogenic stressors is crucial (Meeus et al. 2018). Anthropogenic alterations in modern landscapes encompass a mix of stressors that synergistically affect various species. Many of these species play pivotal roles in ecosystem functionality. The combined impact of these stressors can diminish reproduction and survival rates in beneficial insects such as bees, potentially resulting in population decline. Additionally, these stressors may influence behaviours related to resource acquisition and nesting (Stuligross et al. 2023). Apiformes - Bees Bees (Apiformes) are insects belonging to the order Hymenoptera and there are seven bee families in the world, of which six are found on all continents except Antarctica, the seventh family is endemic to Australia (Hanson, 2018). At species level there are around 20 000 bee species worldwide (Raine & Rundlöf 2023). In Sweden there are 280 bee species spread over the six families mentioned above and 68 (24 %) of them are specialized in their pollen foraging, they are so called oligolectic bees (pers. comm. with Björn Cederberg). 4 The six families, where and how they live (Falk & Lewington 2015) is shortly presented here: • Family Megachilidae; various nesting, but cavity nesting dominates, • Family Andrenaidae; typically ground nesting, solitary • Family Colletidae; solitary, Colletes - mostly ground nesting, Hylaeus – cavity nesting • Family Melittidae; typically ground nesting, solitary • Family Apidae; various nesting, contains both solitary and eusocial species • Family Halictidae; usually ground nesting, contains both solitary and eusocial species Oligolectic bees Among solitary bees, there exist "thousands" of species classified as oligolectic (pollen specialists), as elucidated by Cane in 2011. The term "thousands" denotes the extensive diversity within this category. Michener's classification identifies 69 out of 443 genera across six bee families as exhibiting oligolecty. Extrapolating from these figures, the global average of oligolectic bee species stands at approximately 9% (1491 out of 17187 bee species) as per Michener's data from 2007. Geographically, the prevalence of oligolectic species is highest in the southernmost regions of Europe, gradually diminishing as one move northward (Pekkarinen 1998) [166]. Many oligolectic bees also exhibit dependency on specific habitat types, with their limitations primarily dictated by the availability of suitable habitats and nest sites rather than host plants. Additionally, these bees may manifest preferences within their chosen habitat, necessitating heterogeneity. This preference for diverse habitat features accommodates the distinct needs of these species, which utilize different parts of the habitat for pollen collection and nest construction, as indicated by Bogusch et al. in 2020 [40]. Oligolectic bees and their host plants Oligolectic bees and their host plants are linked elements in biological communities. One important factor is the host plant´s role in bee reproduction. It is common for female bees of the genus Andrena (sand bees) to become so closely associated with flowers of a specific species that it is the only place males can, with relative certainty, find his female counterpart (Hanson 2018). For oligolectic wild bees to be able to maintain viable local populations, the plants from which they collect their pollen must be abundant (Linkowski et al. 2004). These bees disappear from their habitats if their forage plants disappear or if the populations become so scarce that they no longer constitute a secure food resource (Rasmussen et al. 2022). Biesmeijer and colleagues (2006) studied bee (and hoverfly) assemblages in Britain and the Netherland and their results showed clearly that pollen specialists and their obligate outcrossed hostplants were declining in parallel. If a pollen specialist disappears from an area where its host plant exists, it does not necessarily mean that its host plant also disappears, instead the pollination network can change (i.e. another species takes over, usually a generalist). An example is the areas with arable heath in Uppsala County where the diversity and frequency of flower visitors is dominated by Apis mellifera (honey bee) and flies (order Diptera). In that case, the honey bee and the fly indicate a disturbed ecology where specialists are missing (Larsson & Sjödin 2010). Burkle and colleagues (2013) looked at the changes in pollination networks, and found that about 50% of the species of bees that existed 120 years ago no longer existed. Moreover, more specialists than generalists disappeared, despite their host plant still remaining (Burkle et al. 2013). 5 Objectives of this study This study aims to; provide a comprehensive explanation of the terminology “oligolecty”, quantitatively presenting and reviewing current knowledge, identify deficiencies and knowledge gaps of global change stressors on oligolectic bees, present indications of the potential vulnerability of oligolectic bees, and suggest explanations to the limited knowledge in the field. Additional to that, a revised list of the oligolectic bee species in Sweden is provided, as appendix 2. Methods Descriptions of how this study is performed are here presented stepwise, additional aspects can be found in Method discussion (page 20) and specifications are attached as appendixes. Collection of data A large number of published studies and reports related to wild bees, oligolecty and global change have been read. Scientific articles have been searched via global databases; Web of Science (WoS), ScienceDirect, Google Scholar, etc. Some facts originate from established institutions or authorities such as Sweden Observation Species Centre (Artportalen) & Artfakta) and the Swedish University of Agricultural Sciences (SLU). Personal communication with the Swedish entomologist Björn Cederberg (part of the Swedish expert committee of Hymenoptera) has also formed the basis for certain parts of this study. Studies, other than those that were the result of the quantitative search, have been selected in slightly different ways, mainly because the relevance to this study, but certain prioritization has taken place for articles written by authors whose studies within the subject in question I have read and judged to be reliable (Potts, Biesmeijer, Cane, Westrich, Müller, Kuhlmann, Westerfelt and Bogusch among others). References studies included in these articles have also been used. Some studies have been recommendations from Björn Cederberg or my supervisor Julia Osterman. Other studies have been selected for other reasons, for example their choice of terms, methods or results descriptions made me question them. It should be emphasized that the quantitative results, regarding the extent to which studies on the effects of global change include oligolectic bees, should be viewed only as an indication rather than actual fact. This then; 1) the overall interpretation of the studies is largely based only on the title and abstract of the study and, i.e. for many of these studies no qualitative assessment has been made in this study; 2) it cannot be excluded that if the database search is performed using a different method, it could generate more results; 3) more studies have been discovered that were not included in the results list from the database searches, even though the choice of keywords should have included them; 4) during the course of the study, several new studies in the field have been published. Some of those who were not included (3 & 4) in the search, as well as additional studies extracted from reference lists mentioned earlier, are however, included and discussed in the study. In the result section they are referred to as additional studies. 6 Oligolectic species This study also provides a updated list of the Swedish oligolectic species, their hostplants and a refined degree of their oligolecty. The revised compilation (appendix 2) of oligolectic species occurring in Sweden is based on the lists Pettersson and colleagues (2004) and Linkowski and colleagues (2004) presented in their reports. Sources used to update these lists are; Swedish Observation Species Center (Artportalen, Artfakta), Bees Wasps & Ants Recording Society (BWARS), Steven Falk's book; Field Guide to the Bees of Great Britain and Ireland (2015), the Norwegian Biodiversity Information Center (Artsdatabanken), Finnish Biodiversity Info Facility (Artdatacenter), Global Biodiversity Information Facility (GBIF) and Denmark's national Artportal. The list has then been fact-checked by the Swedish entomologist Björn Cederberg. The species included in the updated list of oligolectic bees in Sweden, were searched in the other countries' red lists, some could not be found, which could mean that; 1) they do not appear in the country 2) they can go by a different name. Sources of oligolectic current prevailing red list status in Scandinavian countries are the Biodiversity Information Centers of; Sweden (Artportalen); Norway (Artsdatabanken); Finland (Finnish Biodiversity Info Facility) and Madsen`s “Den danske Rødliste 2019” and the complete list is attached as appendix 3. It should be emphasized that some species might be considered as polylectic by other researchers. The revised list of oligolectic bees is used as a reference in this study for which bees are oligolectic, it should then be noted that it is based on the oligolectic bees found in Sweden. In some sections of this study, the classification of polylecty/oligolecty that has been made is questioned and in those cases bees, in Sweden considered broadly oligolectic are not included, as they could also be considered polylectic. Database searches All database searches described here were performed in; Web of Science Core Collection, all editions, and in all of the searchable fields using one query. Web of Science will henceforth be referred to as WoS. All searches were performed between 2023-10-10 and 2023-10-24; specific dates are included in Appendix 4. The database searches to see if the number of hits differs depending on the choice of term and the search combinations were as follows: • "bee*" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") • "bee*" AND ("oligol*" OR "pollen speciali*") • "bee*" AND ("food speciali*" OR "diet speciali*") NOT ("oligol*" OR "pollen speciali*") • "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") • "bee" AND ("oligol*" OR "pollen speciali*") • "bee" AND ("food speciali*" OR "diet speciali*") NOT ("oligol*" OR "pollen speciali*") 7 Quantitative search – global change effects For the quantitative distribution of global change effects, different keywords have been pooled together in into different groups. The reality is different as global change effects, to varying degrees, are interactive and directly or indirectly affect other areas. The Land alteration group includes search terms related to land use or changes in the landscape or the layout of the land. Chemicals (mainly linked to agriculture) itself have been placed in the group; (Agro-) chemicals, Invasive species and Pathogens are included in the same group and climate-related keywords are placed in the group; Climate Change. Competition, Mismatch and Nutritional deficiency are a separate group as they, more or less, are indirect effects of other impacts. The searches were performed with the search combination: ("bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND ("global change search term"), where the “global change search term” was exchanged for every new search, some examples are; habitat loss; urbanization; pesticide; invasive species etc. (all search combinations, can be found in Appendix 4). There are also two additional search groups where words related to Synergism and Threats, respectively, is included; These two groups were added due to the fact that there are interactive effects within global impacts and pollen specialist bees are threatened. All these search results (268 studies) were pooled together as a marked list in Web of Science. Selection of data In the earlier mentioned marked list in Web of Science, all doublets were automatically excluded. The marked list (169 studies) was then exported as a full record (available in Appendix 1). Not to potentially be affected by keywords, author or other records, two columns relevant to the examination; the title and the abstract, were copied to another work book. Then all irrelevant studies were excluded and the rest (69 studies) were read through (the abstracts) and classified after what kind of global change the study focused. Some of the studies were read in full as the abstract raised questions and others because they appeared to be of special interest. In the result section, where the found studies are reviewed, these classifications are slightly different as the content of many studies did not really fit the groups used in the search. Several studies employing expressions such as "oligolectic bees/species" in their conclusions, presented in the abstracts, were analyzed by scrutinizing the supplementary material to identify the specific species encompassed within each study. Appendix 1 is, additional to present all the studies that resulted from the search, serving as a reference list of the selected studies reviewed herein, when reviewed or referred to in the study, a reference number is marked with square brackets [number] that correlates with the underlined number in the appendix. In the appendix all numbers of selected studies are underlined. Studies considered irrelevant are written with grey text. Assessed classification (what study field) can be read in a separate column, as can also the result of the analyses, where red textboxes indicates misleading abstracts, found bias, orange 8 Results Oligolecty Oligolecty and polylecty are terms used in bee species facts to describe the degree of specialization for pollen collection. Bees collecting pollen from species within a single plant family are oligolectic (=“few-gathering”) and those collecting from two or more plant family are polylectic (=”poly- gathering”) (Nilsson 2013). In the article “Oligolectic bee species in northern Europe” (1989), Pekkarinen discusses the concepts of poly-, oligo- and monolecty. The terms oligolecty and polylecty were introduced by Robertson in 1925 to describe the degree of specialization for pollen collection in bees. Oligo- is a prefix indicating few/a small number (of something)/a few/small and comes from the Greek combining form of olígos. The suffix -lectic comes from the Greek lektos, which means chosen/selected (noun) or légō, “to choose; to arrange; to gather”), from Proto-Indo-European *leǵ- (“to collect, gather”) (Wikipedia 2023). Although the term oligolectic has existed for almost 100 years, the word specialist (mostly in a combination) is often used instead. It is then important to reflect on what the author actually means by specialist. If you look up the word specialized in a biology dictionary, the definition is; “having special adaptation to a particular ecological niche which often results in wide deviation from the presumed ancestral form. Such specializations evolve and may result in niche limitations” (Thain & Hickman 2004). A bee can be a pollen specialist as well as a habitat specialist; therefore would the word specialist not be accurate. It is common with combinations such as; diet specialist and food specialist but these terms might give the impression that the bee is selective in its diet for nutritional reasons or tastes of the pollen. Oligolecty is a term that refers to the collection of pollen, not to which food a bee eats or to which diet it goes (diet = "a specific allowance or selection of food, to control weight or for health reasons" (Collins 2003)). The term "oligolecty" specifically refers to the behaviour of being specialized in collecting pollen from a limited range of plant species. Oligolectic bees often have specific requirements for larval development, and collect specific types of pollen to provision their larvae and the choice of pollen is linked to meeting those requirements (and to some extent the specific plant species' availability). Oligolecty is an example of how bees have evolved specialized behaviours to maximize their reproductive success in their respective environments. By focusing their foraging efforts on specific plant species, these bees ensure that their offspring receive optimal nutrition while also contributing to the pollination and reproduction of their preferred plants. It's more accurate to use "pollen specialist" or "host plant specialist" to convey the idea that the bee specializes in collecting pollen from specific plant species for the purpose of provisioning its larvae. This terminology reflects the biological and ecological aspects of oligolecty more accurately. Different forms of the word oligolecty • As a noun for the phenomenon itself; oligolecty (Robertson 1925) points out that oligolecty is the correct word and not oligolectism, (monolecty, oligolecty, polylecty). • As a noun for a bee with this degree of pollen specialization: oligolege (plural oligoleges), it is the word that is most common in English (monolege, oligolege, polyleges) • As an adjective for the quality of a bee; oligolectic (monolectic, oligolectic, polylectic). 9 Monolecty, oligolecty or polylecty The terms monolecty, oligolecty, and polylecty have conventionally served as a classification framework for categorizing bee species based on the number of plant taxa from which they collect pollen. Unfortunately, these terms have not been consistently applied, primarily due to challenges in compiling the taxonomic spectrum of pollen use, insufficient data regarding the host plants utilized by different bee species, and limitations in certain analytical methods that hinder valid comparisons. It is crucial to emphasize that these classifications are ultimately about the fidelity of bees to specific plants and should not be conflated with floral constancy—a dynamic attribute exhibited by individual bees. Even in the case of highly polylectic bees, each bee, during individual rounds of pollen collection, consistently gathers pollen from a single plant species without switching to another (Cane & Sipes 2006). This behavior, as elucidated by Michener in 2007, does not constitute oligolecty but rather represents an efficient strategy for pollen collection. The variable specialization of bees in collecting pollen within a taxonomic range of host plants, distinct from floral constancy, is an intrinsic and species-specific trait (Cane & Sipes 2006). Another way to put it: while the specialization of bees in relation to flora is likely influenced by inherent neural or morphological constraints, floral constancy is a learned behavior unique to each individual bee. This constancy has the potential to shift with new opportunities or vary among individuals of the same species at the same time and location (Michener 2007). Monolecty is employed when a bee exclusively gathers pollen from a single flower species or a few closely related flower species, as outlined by Westrich in 1990. However, a perspective articulated by Linkowski and colleagues (2004) introduces the term "narrow oligolecty" as a preferable alternative to monolecty, asserting that "mono" specifically conveys the notion of collecting from only one species, a behavior observed in very few bee species. This rarity of pollen specialization to a single plant species is corroborated by additional studies, such as Rasmussen et al. in 2021. Crone et al. (2023) employs the term "strict foraging specialists" for bee’s exclusively foraging on one plant species. Pekkarinen (1998) [166] further highlights the fluidity in distinguishing between oligolectic and polylectic species, noting the presence of intermediate species and spatial variations in pollen specialization within the same species. The terminology for these intermediate species varies among authors; Linkowski and colleagues (2004) use "narrow oligolecty" along with "moderate" and "broad oligolecty” while Praz and colleagues (2008) uses “strictly oligolectic” and “broadly oligolectic”. It should be mentioned that there is a fourth term; mesolecty, although the use of it is not as widespread as the others. Rasmussen and colleagues (2020) utilize the term "mesolecty" for bees that forage pollen across a narrow range of plant families, as seen in their work and in the study by Praz et al. (2008). However, Cane and Sipes (2006) consider the term mesolectic to be a substitute for “broadly oligolectic”. Apart from the term monolectic (defined as pollen specialized on one plant species), it is difficult to specify exactly where the boundary is between oligolecty and polylecty. The division is more or less arbitrary and has been the basis of endless debates (Cane & Sipes 2006). Most oligoleges are univoltine (having one generation per year) thus adult emergence and bloom of their hostplant are and have to be synchronized. In case of desynchronization, different oligolectic species appears to acts either as; obligate oligoleges, refusal of provisioning or nesting as long as the hostplant is unavailable or as facultative oligoleges, turning to some substitutional pollen source. There is some advantages by await hostplant bloom such as reduced risk of predation, nest invasion and general wearness (Cane & Sipes 2006). 10 Even though some oligolectic bees display a remarkable specialization in their pollen preferences to the extent that their larvae can only thrive on pollen derived from a limited pool of plant families or genera (Cane 2011) the classification of pollen specialization is essentially of an ethological character and visible morphological qualifications are not necessarily required when compared to other related bee species (Pekkarinen 1998) [166]. Behavioural specializations can serve as a driving force in the evolutionary process and often encompass various aspects such as the daily timing of floral visits, the preparation of pollen grains through moistening and packing for return flights to the nest, or the vibration of flowers (buzzing) to effectively release pollen grains. Morphological specialization encompasses features such as the density and type of hairs tailored for the collection of different-sized pollen grains, the presence of flattened spatulate hairs adapted for gathering oil, and the development of extremely elongate mouthparts designed to access hidden nectar sources (Rasmussen et al. 2020). After accounting the classification criteria for monolecty, oligolecty, and polylecty, the subsequent sections of this study will use "oligolecty", encompassing monolectic species, unless other is stated. Disproportionalities in Red lists Already in 1998 Pekkarinen pointed out that 32 of the oligolectic bee species in Finland were listed as threatened in England, southwestern Germany or Poland (Pekkarinen 1998), and in 2004 Pettersson and his colleagues presents the fact that the red-listed oligolectic species (and their parasitic bees) are greatly over-represented in the Swedish national Red list of threatened wild bee species (Pettersson et al. 2004). A more recent study shows that in the Red List of bees of Czechia, a larger proportion is comprised of oligoleges (97 of 166; 58%) than that of polyleges (139 of 306; 45%) (Bogusch et al. 2020) [40]. Swedish oligolectic species and their listings in the National Red List of Sweden; Norway; Denmark; Finland and the listing in IUCN Red List are presented in Table 1. Table 1: Listing of oligolectic bees in National Red Lists and in the IUCN Red List. Red listed (%) is the part of red listed oligolectic bees out of the total number of oligolectic bees. The species lists are attached as appendix 3. Country Sweden Denmark Norway Finland IUCN Year 2020 2019 2021 2019 2012-14 LC = Least concern LC 39 30 29 32 31 NT = Near Threatened NT 9 5 3 2 10 VU = Vulnerable VU 6 6 6 3 1 EN = Endangered EN 5 5 3 6 3 CR 3 1 2 2 0 CR = Critical endangered DD - - - - 23 DD = Data deficiency RE 3 4 3 2 - RE = Regionally extinct Total* 67 58 46 47 68 Red listed 23 17 14 13 14 Red listed (%) 34 % 29 % 30 % 28 % 21 % * = number of all found species in respectively country including species not assessed (NE) and species listed as not appliable (NA) Sources: https://www.artportalen.se/Occurrence/TaxonOccurrence/16/2002991 [Visited 2023-10-10] http s://artsdatabanken.no/lister/rodlisteforarter/2021 [Visited 2023-10-18] https://ecos.au.dk/forskningraadgivning/temasider/redlistframe/soeg-en-art [Visited 2023-10-27] http s://punainenkirja.laji.fi/sv/results?type=speci es&year=2019&redListGroup= [Visited 2023-10-28] https://www.iucnredlist.org/ [Visited 2023-11-05] 11 Oligolectic bees and Global Change impacts A first search in WoS with a combination of words related to; Threat, Nutrient deficiency and Climate change was used (see more in Methods) generated 928,030 result hits. Therefor the search terms had to be pooled into different hypothetical groups, which in WoS generated all together 289 results (Figure 2). After removal of doublets and screening of the abstracts of which studies that classified as relevant, a total of 69 studies remained. The specified search terms within each group, as well as search combination groups are available in Appendix 4. These remaining studies were then reclassified after topics, to better serve the purpose of this study. The new classifications are quantitatively presented in Figure 3 and will in various extensions be reviewed in the subsequent sections, following the same topic order as in the figure. As there are very few studies or not cover the subject enough in some of the sections, other studies are included for an enhanced understanding of the different global change impacts. These studies are placed after the reviewed hit results as, “additional studies” in the headline. Figure 2: Number of results generated from data Figure 3: The quantitative distribution after base searches in the different groups of pooled search re-classification based on studied topic. terms. Topics Land alteration; Biodiversity Seven studies had assessed bee diversity in their study. One was performed in a scrub oak barrens and concluded in the abstract that: increased visibility of nectar resources and sandy patches post-treatment may have promoted sand specialist and oligolectic bee species (Bried & Dillon 2012) [124]. In the study that investigated the bee community in wet meadows near Krakow, in Poland, showed that the least abundant species were disproportionately represented by oligolectic bees. Their over representation clearly indicates that species having a close association to wet meadow plant, are particularly at risk (Moron et al. 2008) [143]. Species richness in a sand steppe habitat in Eastern Austria was found to have decreased with over 50% (Dominique et al. 2023) [3]. When the composition of bee communities was compared between restored and remnant prairies, the results showed pronounced differences, and that oligolectic bees occurred more in remnant prairies (Lane et al. 2022) [27]. Not reviewed: [17], [21], [36] 12 Land alteration; Urbanization Urbanization studies dominate, comprising seventeen, with a notable exclusion [14]. A Finnish study highlights oligolectic bee preference for less urbanized areas, emphasizing the importance of focusing on oligolectic and terrestrial bee species for biodiversity preservation (Venn et al. 2023) [7]. In Brazil, a 40-year evaluation of grassland bee fauna reveals a 22% decline in species richness and abundance, attributed to intense land occupation and lack of natural area preservation (Martins et al. 2013) [119]. Urban intensity's impact on European cities' bees is explored, revealing broader pollen generalization as less sensitive to severe urbanization (Casanelles-Abella et al. 2022) [26]. Springfield's sub-urban yards house around half solitary species, noting lower abundance for oligolectic species (~10%) (Lerman & Milam 2016) [96]. Vegetation appeal for bees in wasteland areas, early-season polylectic and kleptoparasitic bees favor sub-urban, while summer emerging bees prefer urban sites (Twerd et al. 2021) [34].Bengaluru's 20 ha urban green area study estimates native bee fauna diversity and abundance, including a probable misspelling of "oligolectic" (Bhatta & Kumar 2020) [37]. Paris reveals positive associations between pollinator diversity and green space size, flowering plant richness, while impervious surfaces correlate negatively (Zaninotto et al. 2023) [8]. Berlin's urban garden study links wild bee diversity to garden and landscape traits (Felderhoff et al. 2023) [10]. Cities with fragmented green spaces exhibit reduced oligolectic species, increased social and large-bodied bees. Greater impervious surfaces relate to fewer below-ground-nesting bees. Warmer cities show lower richness, with optimal functional diversity at intermediate precipitation levels (Ferrari & Polidori 2022) [15]. A Czech study uncovers a bee and wasp biodiversity hotspot on bare loess exposed by anthropogenic activities (Heneberg & Bogusch 2020) [55]. Lastly, in Pennsylvania, ornamental plants attract polylectic bee species despite the coexistence of oligolectic species (Ericksson et al. 2020) [57]. Not reviewed: [51]; [80]; [81]; [83]; [150]. [80]; [81] are included in the discussion Land alteration: Fragmentation Five studies explored fragmentation's effects on bee populations. One focused on functional traits in the Hungarian Great Plain's natural forest steppe, revealing a close connection between fragment size and larval feeding preferences, positively impacting oligolectic bees (Török et al. 2022) [22]. Franzén et al. (2007) investigated Andrena hattorfiana behavior in small populations, finding a 2% patch emigration rate with a maximum distance of 900 m. Notably, 10% crossed areas lacking pollen plants, such as unpaved roads and stone walls, suggesting sedentary behavior and increased vulnerability to local extinction (Franzén et al. 2007) [142]. Gonçalves et al. (2014) proposed Orchid bees as ecological indicators, noting abundance increases in Apinae and oligolectic bees with larger fragment sizes, while richness of Augochlorini bees decreased (Gonçalves et al. 2014) [111]. Slagle and Hendrix (2009) found that fragmentation did not affect Andrena quintilis, an oligolectic bee species (Slagle & Hendrix 2009) [139]. In a 2008 study comparing mesic and xeric regions in North America, Minckley found higher species richness in the xeric region, with xeric habitats richer in oligolectic species. They suggested a comprehensive approach integrating phylogeny, historical biogeography, and bee-plant ecology to understand bee fauna differences (Minckley 2008) [145]. Cane et al. (2006) investigated a desert bee guild in Arizona's response to fragmentation, targeting 120 bees, including 21 pollen-specialized on the creosote bush Larrea tridentata (Cane et al. 2006) [151]. 13 Land alteration: Others Sixteen studies fitted within the field Land alteration, where seven is placed in the sub-field Biodiversity and five in the sub-field Flower strips. The four remaining should be reviewed here, however, due to lack of time two of them have not been reviewed. A positive illustration of human- made modifications in the environment that is of benefit for both civilization and the conservation of biodiversity, are railway embankments, when managed appropriately was presented in a study by Moron and colleagues (2017) [89]. Another study presented a similar positive illustration, but in Gatewick Airport where Eucera longicornis thrives (Hennessy et al. 2020) [42]. Not reviewed: [69], [132], although [132] is included in the discussion. Land alteration; Flower stripes Five studies from the comprehensive search specifically delve into flower strips. One study highlights that the composition of plant species in flower strips, commonly used to enhance pollinator-friendly agricultural landscapes, is often dictated by logistics rather than direct knowledge of bee-plant interactions. They identify 34 herbaceous key plant species crucial for attracting wild bees, contributing significantly to sustaining diverse bee populations, including 2% to 32% oligolectic or red-listed bees (Kuppler et al., 2023) [13]. Another study compares habitat patches with sown flower strips, finding that while flower strips offer abundant flowers, their species composition and flowering timing exhibit uniformity, potentially favoring only a subset of pollinator species. In contrast, existing semi-natural habitat patches along slopes, fences, or ditches have the potential to support additional species for pollinator conservation, albeit with limited political promotion. Notably, these patches attract different pollen-specialized bees than sown flower strips (von Konigslow et al., 2021) [31]. A third study in Belgium assesses bee and hoverfly abundance and diversity within flower strips, suggesting that intercropping systems with flower strips contribute to sustainable agro-ecosystems. The study documents 43 bee species, emphasizing the generalist character of the pollinator community, with the exception of the oligolectic bee Andrena nitidiuscula (Amy et al., 2018) [76]. The fourth study near Vienna focuses on flower-visiting insects, particularly wild bees, in semi-natural grassland patches and flowering strips within vineyards. It highlights the correlation between insect numbers and flower cover, underscoring the role of flowering plants in supporting pollinators. Grassland patches consistently supply nectar-producing plants, while flowering strips, dominated by short-lived sowed plant species, benefit oligolectic bees specializing in Brassicaceae or Fabaceae (Rasran, 2018) [75]. In the fifth long-term study, networks of perennial flower strips covering 10% of an agricultural landscape led to increased pollinator abundance, notably oligolectic bee species after the third year. This suggests the crucial role of diverse habitats, foraging resources, and nesting sites in supporting overall pollinator well-being (Buhk et al. 2020) [72]. Land alteration: Foraging distances Three studies with focus on forage distances were found, where one had investigated forage distances for two polylectic Osmia spp. and four oligolectic species, specifically Chelostoma florisomne, C. rapunculi, Heriades truncorum, and Hoplitis adunca, all belonging to the family Megachilidae. This study, conducted at the Munich Botanic Garden, aimed to determine forage distances, a crucial factor for assessing the critical size of fragmented habitats and implementing conservation measures such as flower strips. The study's results suggest that flower strips and nesting sites should not be located more than 150 meters apart. Notably, it should be acknowledged that in this study, data collection was aided by public visitors who reported the sightings of numbered species (Hofmann et al., 2020) [47]. 14 In another study they investigated impacts due to prolonged foraging distances, in two solitary oligolectic bee species; Chelostoma rapunculi and Hoplitis adunca. Forage distances prolonged with 500 and 600 m showed to reduce the number of brood cells produced by C. rapunculi per time unit, with 46% and 36% respectively. Forage distances prolonged with 150 m; 200 m; 300 m, showed to reduce the number of brood cells produced by H. adunca per time unit with 23%, 31% and 26% respectively. The findings underscore the critical importance of having suitable nesting and foraging habitats in close proximity for the persistence of populations and, consequently, the conservation of endangered solitary bee species (Zurbuchen et al. 2010) [137]. The last study investigated whether structures in the landscape function as impassable obstacles to pollen collecting bees. Hoplitis adunca showed no signs of such, as the bee passed both an intensely trafficked highway and a broad river. More than 130 m altitude differences did not hindered Chelostoma florisomne, neither did a dense, forest covering a distances above 450 m (Zurbuchen et al. 2010) [136]. (Agro-) Chemicals Of the 14 result hits the majority of the studies only mentioned pesticides in a general concept and the only three dealt with pesticide impacts on oligolectic bees were about species within the family Megachilidae; Osmia brevicornis, Osmia ribifloris and Heriades truncurum. These three bee species are all cavity nesting and solitary. Hellström and his colleagues underscore the importance of aligning foraging preferences and crops in pesticide risk assessments. They contend that the existing model species may not always be appropriately matched to the crops investigated, potentially leading to erroneous conclusions regarding pesticide risks in pollen and nectar. To address this, they propose Osmia brevicornis, an oligolectic European wild bee species specialized in Brassicaceae pollen, as a new model organism suitable for assessing how pesticides can impact specialist pollinators, particularly in oilseed rape, a mass flowering Brassicaceae crop. The study outlines a method for housing and administering controlled oral solutions in the laboratory, facilitating future investigations into pesticide exposure. The researchers conclude that O. brevicornis is a viable model for assessing pesticide risks both in laboratory settings and in the field. Additionally, they advocate for diversifying the species used in agricultural ecology, emphasizing the inclusion of pollen specialists. They emphasize the importance of considering the foraging preferences and dietary needs of selected model species when evaluating pesticide exposure risks and effects (Hellström et al. 2023) [9]. An additional study, proposing an oligolectic model species, explored the repercussions of pesticides on sexual communication. The aboveground oligolectic bee, Heriades truncorum, serves as an excellent model for investigating the impact of pesticides on sexual communication, given that certain aspects of its mating behavior have been previously documented. In this study, males exhibited a quicker approach towards unexposed females compared to those exposed to insecticides. Females exposed to insecticides produced reduced amounts of sex pheromone candidates and displayed less selectivity than their unexposed counterparts. Their findings suggest that insecticide exposure has a discernible impact on sexual communication, influencing both male preference and the female's assessment of male quality (Boff & Ayasse 2023) [2]. The third study introduces a method for rearing the oligolectic mason bees Osmia ribifloris sensu lato "in vitro." This approach is proposed as a valuable tool for assessing the risks associated with fungicides. Specifically, in the context of Osmia species demonstrating oligolecty, wherein they exclusively consume pollen from a specific group of plants, their inability to utilize pollen from non-host plants may heighten their vulnerability to toxicity induced by fungicides (Dharampal et al., 2018) [78]. 15 (Agro-) Chemicals – additional studies Current knowledge of pesticides is limited to very few species and the majority of the research is upon neonicotinoid insecticides (under unrealistic conditions). Bees can be exposed to pesticides all way through life; in their larvae stage; during hibernation, as they forage, when they constructs their nest and during brood care. The exposure could be oral through nectar, pollen, oil, water or by contact with air, plants, soil and other material bees are in contact with of in the environment (Raine & Rundlöf 2023). Pesticides can remain in the environment for years resulting in double exposure; if pesticide residues remain in soil and they build their nests in the ground (Sponsler et al. 2019), which approx.75 % of the 20 000 species in the world does (Raine & Rundlöf 2023). Bees can also be exposed by contact or by drinking from the guttation emitted by a plant, whose seeds have been treated with e.g. Imidacloprid. This as systematic agent transports the substance within a plant via the xylem and can even reach the leaves of the plant (Tome´ et al. 2012). The problem with pesticides can also impact bees by indirect effects, herbicides as an example, reduces the amount of flowers that produce nectar and pollen as well as host plants for the larvae of certain pollinators (Sponsler et al. 2019). There are plenty of studies on the effects of pesticides (unfortunately, these studies have mostly focused on the honey bee), and the most common sublethal effects are learning disabilities, poor memory, and aberrant foraging behavior. Learning and memory are controlled by special areas of the brain and one of them is the corpus callosum, which has the task of storing information. As the bee grows, this structure also expands and in adulthood it exhibits a high neural plasticity. Bees that consume small amounts of insecticide via either contaminated nectar or pollen can lose the ability to remember and to orient themselves in time and space (Tome´ et al. 2012). In the study Tome´ and his colleagues performed on stingless bees, the effects of imidacloprid did not appear immediately when the fully formed adult emerged, but after four days a changed walking behavior was noted. One of the study's conclusions was that if walking behavior is affected, it is likely that the bee's flying ability and foraging behavior will be even more affected. They also emphasized that the changes that may occur during larval development, induced by pesticides, may result in additional consequences (beyond the loss of adult bees) to the colony and should not be neglected. Moreover, when combinations of several different agents are used simultaneously or over time, in one and the same field, pollinators are exposed to these combinations of plant protection agents. In addition, pollinators visit many different areas (especially if the plant they prefer is not widely available) and might then be exposed to several different types of chemical preparations (Sponsler et al. 2019). Climate change Six of the results fitted best within the topic climate change. The newest published study provides long term baseline data on areas in the warm deserts of North America with minimal human impacts to be used for studies of areas where human impacts are graver and as climate change advances (Minckley & Radke 2021) [33]. In a study performed a couple of years earlier, the effects on German bees, of various factors such as habitat breadth, pollen specialization, body size, nesting sites, sociality, duration of flight activity, and time of emergence during the season, were statistically modelled and analyzed. The study exposed that a narrow habitat breadth and late-summer emergence increased vulnerability to extinction in Central European bees. Spring emergence and occurrence in urban areas, on the other hand, were found to reduce vulnerability, indicating that intensive land use particularly affects summer-active bees. The combination of these factors is currently leading to a shift in Germany's bee diversity towards warm-adapted, spring-flying, city-dwelling species (Hofmann et al. 2019) [66]. 16 Dellicour with collegues (2015) is suggesting that food resource abundance has a potential role when current patterns of genetic variation in specialists are to be determined. They had studied the impacts of past climate changes on three oligolectic Melitta species. The study illustrates that current phylogeographic patterns may have been shaped by contributions of both demographic history and ecological factors, and even though it is not a study of present climate change, the result could be of use in modelling and predictions (Dellicour et al. 2015) [109]. The fourth study delved into the temperature-dependent aspects of nesting activity and lifetime reproductive output, revealing that the positive effects of higher temperatures on bee productivity were counterbalanced by indirect costs associated with heightened parasite activity (Forrest & Chisholm 2017) [93]. In the fifth study, analyses of the bee fauna in the Munich Botanical Garden were performed in 1997/1999 and again in 2015/2017. During this period, 12 polylectic species disappeared out of 62 and 23 were added, of the oligolectic ones two disappeared out of the total 17 and 10 were added (Hofmann et al. 2018) [79]. In the sixth and last study, the relationships between environmental abiotic conditions, length of adult life, and magnitude of foraging activity in two bee species, were studied. Studied bee species were the oligolectic Andrena vaga and the polylectic Anthophora plumipes. The study suggests that life span is influenced both directly by climate and indirectly through activity patterns that are dependent on climate (Straka et al. 2014) [112]. Plant-pollinator phenological mismatch The search resulted in two studies, where one study was performed by Cerceau and colleagues (2019) [62]. They investigated the role of the oligolectic bee Arhysosage cactorum for the reproduction of Parodia neohorstii (Cactaceae) and were carried out in Brazil 2016 and 2017. Both bee and host plant are threatened, red listed species and a mismatch could substantially impact the reproductive success of both partners. Mating behaviour of Arhysosage cactorum – is associated with the pollen host plant, which is common for oligolectic bees, being a mating place where male bees wait and searches for females. But here they observed a special mating behaviour, only known for a few other species of Andrenaidae; during copulation they were flying together among cactus flowers, strongly enhance crosspollination (Cerceau et al. 2019) [62]. The other study was carried out by Schenk and colleagues (2016) where they tested the effect of temporal (0, 3 and 6 days) mismatches on fitness of three solitary bees emerging at spring; the early-spring species Osmia cornuta, the mid-spring species Osmia bicornis and the late-spring oligolectic species Osmia brevicornis. All of them exhibited severe reduced fitness after a mismatch of 6 days, as not many bees can survive without flowers that long. After a mismatch of 3 days, the two polyleges produced the same number of brood cells as under synchronized conditions, whereas the oligolectic Osmia brevicornis produced fewer brood cells. It should be mentioned that O. cornuta decreased the number of female offsprings and O. bicornis used fewer nests to spread the brood cells over, which could result in higher offspring mortality. Their conclusion was that short temporal mismatches can cause clearly reduced fitness in solitary bees. In temperate climates, the seasonal activity of most bee species is primarily regulated by temperature cues. Solitary bees that emerge early in spring have spent the winter as fully mature adults within their spacious cells. Consequently, a shortened period of warmth in the spring can trigger rapid responses in these bees, potentially resulting in temporal mismatches with their host plants. While the consequences of such mismatches on plants have been extensively studied, there is a notable paucity of research focusing on the fitness implications for the bees themselves. Temperate oligolectic bee species that exhibit early spring emergence or late autumn activity are postulated to face more pronounced negative repercussions stemming from temporal desynchronization. This elevated vulnerability is attributed to the heightened risk of emerging in the absence of their preferred interaction partners. 17 Moreover, during the early and late periods of the season, when plant biodiversity is comparatively lower, bees may encounter challenges in shifting to alternative interaction partners. They also pointed out that since metabolic functions are faster and that the total energy consumption is higher, in warm than in cold conditions, temporal mismatches in periods of warm weather could aggravate potential starvation compared to mismatches during cold periods. Whether this also applies to solitary bees remains to be seen, as the study referred to a study made by Vesterlund & Sorvari, 2014, that dealt with bumble bees (Schenk et al., 2016) [88]. Mismatch and oligolectic bees – additional studies Consequences of phenological mismatches for five wood-nesting solitary bees, representing a broad gradient of oligolecty/polylecty, were assessed during 9 years. Their published results shows that; if climate change increases phenological mismatches, negative consequences of climate change for specialist bees can be expected; a negative population growth rate for the two most specialized bee was indicated in their demographic analysis as well as a greater, nonnegative growth rate for the other three species; oligolectic bees might have lower viability and could therefore experience a greater decline than polylectic bees, from phenological mismatches. It should be noted that the results should be interpreted with caution due to uncertainties of both the data and the analysis, but this type of analysis is still a useful tool for comparisons among populations and species and its results helps to elucidate the role of phenological mismatches for the demography of wild pollinators (Vázquez et al. 2023). Plant-pollinator interaction Of four studies that dealt with plant-pollinator interactions one was only pointing out need of comparable studies about solitary, pollen specialized bee species, where the interplays among the timing of floral resource availability, the foraging behavior of bees, and characteristics such as diet breadth, sociality, and body size is examined (Olgilvie & Forrest 2017) [91]. The insight that recommended plant selections mostly benefits polylectic bees and may not support rare specialist pollinators in the Northeast America, inspired Fowler (2016) to provide a catalogue of native specialist bees and their associated host plants. This as such populations are susceptible to harm from anthropogenic threats. Further he identifies and discusses vulnerable bee-plant association, suggests pronounced emphasis on research and restoration efforts and that conservation efforts practice specifically target specialist bees (Fowler 2016) [100]. The findings in the third study signified a strong relationship between bee population size and plant population size. Findings like this are useful tools in conservation efforts, as the critical resource levels can be estimated from a pollen budget calculation (Larsson & Franzén 2007) [148]. The fourth study reviews, summarize and compile the existing knowledge in plant-pollinator interaction and, in contrast to almost every other study that focusing on (honey-) bees exhibiting pollen generalisation, the highlights are upon two often negligated groups; oligolectic and nocturnal foraging bees. It is concluded that research needs to figure out how to restore lost interactions in degraded habitats may be restored, as a stable plant-pollinator network will be a pivotal goal for conservation biology (Scott-Brown & Koch 2020) [46]. Competition & Invasive species Among the 69 search results, three studies focused on the concept of competition, with two addressing competition and invasive species, and the third examining competition within a native bee community. The first study concentrated on the endangered Perdita meconis, a specialized poppy pollinator. It revealed the invasive African honey bee's successful competition against native P. meconis, leading to the alarming absence of P. meconis and a potential local extinction in Utah. The study also noted reduced populations of another native bee species and a decline in European honey bee abundance, causing decreased fruit set in sparsely distributed poppy populations (Portman et al., 2018) [85]. 18 In the French West Indies, the second study explored various bee species and their floral hosts, highlighting the dominance of the introduced European honey bee due to its overwhelming abundance. The competitive and aggressive behavior of the honey bee displaced native bees from flowers, although ecological data on its impact in the region were lacking (Meurgey, 2016) [98]. The third study investigated competition within a species-rich, native bee community visiting creosote bush flowers in North American warm desert regions. Findings indicated that competition for pollen resources was temporary and rarely limited the native bee population. The researchers emphasized the need for comprehensive, long-term assessments of population dynamics, considering both native and non-native bee species across areas with multiple measurable flowering plant species. They underscored the importance of fundamental ecological data, noting that without such information, competing hypotheses and questions regarding competition in bee ecology cannot be adequately evaluated or resolved (Minckley et al., 2003) [169]. Competition and oligolectic bees – additional studies When it comes to competition between different species, opinions vary; many authors believe that the most sensitive to competition are the species that are oligolectic. Some other authors are of the opinion that since the oligolectic bees is so good at harvesting pollen (due to their specialization), they can handle competition with generalists. In the studies done up to 2004 on the competitive impact of the honey bee on wild bees, they have varied so much that no direct conclusion can be drawn, but it is also pointed out that competition can be important in terms of habitat shortage and fragmentation (Linkowski. et al. 2004). Roughly 50% of the plant species visited by both honey bees and wild bees are shared between the two groups. Nevertheless, existing studies predominantly highlight the shared utilization of flowers by wild bees and honey bees, without fully illustrating the extent of this overlap. There are indications that the level of resource overlap fluctuates over time and is contingent on the context; in certain environments, the overlap can be notably extensive. (Rasmussen et al. 2021). There are authors that emphasizes that honey bees quickly can exhaust forage resources due to their highly sophisticated system of recruitment and large perennial colonies (Robertson 1925). An example is in a research investigation exploring food overlap, it was observed that honey bees swiftly deplete forage resources, potentially resulting in the local extirpation of wild bee populations. These findings offer valuable parameters for decision-making in the management of honey bee colonies within regions inhabited by threatened species. Notably, the study identifies six distinct oligolectic bee species facing threats, demonstrating a food overlap exceeding 70% with honey bees. The endangered species are: Andrena lathyri, Andrena marginata, Dasypoda suripes, Dufourea halictula, Dufourea inermis, and Hoplitis anthocopoides (Rasmussen et al. 2021). Oligolectic bees Four studies specifically focus on oligolecty or oligolectic bees, standing apart from other categories. Two of these studies focus on European oligolectic bees, highlighting their disproportional occurrences in Red Lists. Pekkarinen (1989) [166] is accompanied by a study proclaiming the same announcement two decades later by Bogusch and colleagues (2020). The latter emphasizes that, regardless of the viability and abundance of host plants for specialized bees, these bees still face a higher risk of endangerment compared to polylectic bees (Bogusch et al. 2020) [40]. A study on the declining specialized bee Andrena humilis explores its pollen harvesting pattern and reproductive rate. The results reveal an exceptionally low reproductive rate, with 0.9 offspring per day and < 10 produced offspring in a lifetime, despite its efficiency as a forager. This low reproduction rate appears to be a common trait in pollen-specialized bees in the family Andrenidae, providing insight into the severe decline of these bees (Franzén & Larsson 2007) [146]. 19 The third study addresses native bee diversity, emphasizing the urgent need for taxonomic research, especially for oligolectic bees, as many remain undescribed. Approximately half of Australia’s native bees are in need of revision, with land clearing, agriculture, invasive plant species, and climate change identified as main threats to native Australian bees (Batley & Hogendoorn 2010) [141].The last study, while not explicitly about global change, investigates pollinator foraging bout specialization. Its conclusions about oligolectic bees could serve as valuable basic data in global change research or conservation efforts, potentially influencing decisions but warranting consideration for bias (Smith et al. 2019) [64]. Genetics Among the four studies addressing genetic variation, a study led by Packer and colleagues (2005) revealed reduced genetic variation within smaller and more isolated populations of oligolectic bees compared to their polylectic counterparts. Examining phylogenetically independent pairs of species from various bee families, including Colletidae, Megachilidae, Andrenidae, and Apidae, the findings supported the hypothesis that oligolectic bees are more vulnerable to extinction due to a likely reduction in their effective population size. This vulnerability suggests potential threats to mutualistic relationships between oligolectic bees and their host plants from genetic and ecological factors (Packer et al., 2005) [154]. In a study by Zayed and Packer in 2007, the lack of available data on the population genetics of solitary bees, particularly focusing on oligolectic species, was highlighted. The study focused on the population genetics within the oligolectic bee Lasioglossum oenotherae, covering 455 females from 15 populations across the bee's North American range. Results indicated regional disparities in gene flow, drift, and inbreeding (Zayed & Packer, 2007) [147]. A third study, utilizing a quantitative comparative approach to predict population genetic structure, observed no discernible effect of diet specialization but identified significant impacts of sociality on population genetic structure. The study included representatives from six bee families but notably lacked species from the Megachilidae and Melittidae families. Oligolectic species in the study included solitary species like Lasioglossum oenotherae, Peponapis pruinosa, Andrena fuscipes, Andrena vaga, and Macrotera portalis, as well as social oligolectic species like Halictus scabiosae and Bombus bifarius (López- Uribe 2019) [65]. The fourth study asserted that understanding the population genetics of pollen- specialized bees is enhanced by their work. Analyzing the population genetic structure of Colletes gigas, the main pollinator of rapeseed, China's crucial oil crop, they used a population genomic approach to explore the roles of geography and climate in genetic diversity, structure, and demographic history of C. gigas (Su et al. 2022) [19]. Synergism and interactions – additional studies Global change pressures exhibit variation in their biotic or abiotic nature, spatiotemporal scales, and potential non-additive interactions, occurring synergistically or antagonistically. However, studies on pollinator and/or pollination decline often overlook the collective consideration of these pressures (González-Varo et al., 2013). Despite yielding 89 hits, there were no studies with a primary focus on the synergism or interactions of Global Changes and oligolectic bees in the result list. This observation aligns with a previous study by Straub and colleagues (2022), who systematically assessed the interactive effects of pesticides and pathogens on wild bees, revealing a limited number of relevant studies conducted in one laboratory and solely on social bees (bumblebees and stingless bees) (Straub et al., 2022). Terrestrial ecosystems face various simultaneous pressures, highlighting the crucial need to understand the interactive effects between them. This knowledge is vital for biodiversity conservation and the preservation of ecosystem services, as the impact of one pressure can be magnified or mitigated by the effects of another (Gonza´ lez-Varo et al. 2013). 20 Modern landscapes undergo anthropogenic alterations that introduce a mix of stressors affecting various species synergistically. Many of these species, especially bees, play crucial roles in ecosystem functionality. The combined impact of these stressors may reduce reproduction and survival rates in beneficial insects, potentially leading to population decline. Additionally, these stressors can influence behaviors related to resource acquisition and nesting (Stuligross et al., 2023). The decline in wild bee populations is primarily attributed to human activities, particularly land use changes that significantly alter the composition and diversity of accessible plants and food sources (Parreño et al., 2022). Pesticides and the depletion of food resources from flowering plants are two stressors that often interact, jointly affecting bee fitness. The impact of these stressors on essential behaviors such as foraging and nesting can restrict pollination services and hinder population persistence. Therefore, understanding these sublethal effects is crucial for a comprehensive grasp of the challenges faced by bees (Stuligross et al., 2023). Discussion Method discussion Trying to cover such a big field as Global change and its impacts has definitely proved to be a challenge. It is important to remember that this study has focus on a broader level, to point out the lack of research done in the field. This makes it difficult to present uncomplicated and clear results. Nevertheless, a serious attempt to split or break down the results into digestible parts has taken place within a defined time frame in many of the fields. It would of course be preferred if all 79 found studies were read in its whole, but that would call for some assistants or co-workers. By mostly using abstracts or number of search hits, trying to present the distribution of found studies between different topics and to what extent they cover global impacts on oligolectic bees, might have caused bias. Terminology and subjectivity Investigation of the term oligolecty revealed gradations of the term, all of which might not be included. Subjective values on the importance of clarity and correct definitions might have influenced; the accurate use and possible consequences of inaccurate use of the term oligolecty presented, and thus maybe not consistent with the generally accepted view. Explanations of why “inaccurate” words, e. g. food specialist, are used instead have not been investigated; in the majority of the cases, it is likely more due to other reasons than lack of comprehension. Well aware of a personal stand in that question, database searches have been conducted with diet- and food specialist included, all other variants has been excluded though (read more in “Selection of search terms”). Database searches and “know how” To be sure of having all relevant studies within a field (in this case global change) require a high level in “knowhow”. Original plans of what was to be sought and presented have had to be changed time after time as it turned out to be; far too many result hits (several thousands) or too many completely irrelevant studies have been included in the hit list. During the course of the work, new concepts and words, relevant to this study, have also appeared or a letter has been missed, resulting in searches having to be redone, time after time. Different databases also has different ways of conducting a search, e. g. some has limitations of how many search terms you can use and some can use * as a “flashcard” to get all variants of a certain word (oligol* gives you oligolectic, oligolecty, oligoleges etc.) while others uses a ‘ to do the same. This has resulted in “trial and error” repeatedly and took a lot of time that could have been spent in understanding of studies. 21 Selection of search terms Versions of oligolectic: Exclusion of “plant specialist” has probably led to search result bias due to the “accurate” word combination; “host-plant specialist”. The mistake was detected when there was not enough time to correct it. Other terms used than; oligolecty, oligolectic, oligolege, pollen specialist, food specialist and diet specialist, were excluded. Bias in search results, due to the exclusion of words such as; “specialized solitary bee” or “specialist bee” is more than likely, but that is done deliberately, to point out the importance of correct term usage. Possible bias in search result can have occurred due to the fact that it might be other “accurate” words or expressions that have been overseen or due to misspell. Overseen words that were realized before deadline of this study but too late to add: desertification, UV-increase, synchronization, etc. Selection of studies The use of abstract as a selective method, due to the attempt to cover a big field as global change impacts on oligolectic, turned out to be nearly an impossible mission as well as an eye-opener. As the insight of oligolectic bees and global change impacts expanded the more excluded studies, considered irrelevant, became to be of relevance due to the importance of basic understanding of oligolectic bees. Examples of such research are within the fields of; evolution, pollen ecology, visual and olfactory floral cues, nesting biology, bar-coding, plant-pollinator networks, reproduction, conservation and many more. However, studies within those fields are mainly excluded, and there might also be some overseen studies that ought to be included. Different point of views of what is of relevance has also played a part, and subjectivity might have influenced the selection of studies. Terminology There is also considerable uncertainty regarding the degree of specialization in bee species concerning their choice of pollen-collecting plants. A species considered oligolectic today may be reclassified as polylectic, and vice versa, due to a lack of reliable data. Many classifications are founded on older observations, some of which may be as simple as noting a bee on a particular plant. Such observations can be fallible, as female bees may interact with flowers for purposes other than pollen collection, such as feeding on nectar, mating, or resting. If the host plant from which an oligolectic species primarily collects pollen were to disappear, the species would adapt by collecting pollen from alternative plants (personal communication with B. Cederberg). The consequences of such a shift in larval food pollen sources can have adverse impacts on larval development though, potentially leading to increased mortality. In its fully developed state, the species may experience compromised overall health, rendering it more vulnerable to diseases and other stressors, or it may suffer from impaired reproductive capacity. Correct term usage When using the term oligolectic or pollen specialist, this ensures that the concept and its meaning are clear and cannot be misunderstood (Figure 4). If e.g. the word specialist is used, it can result in uncertainty about what the terms actually stand for and if there is no explanation or definition for them, this can lead to misinterpretation in the worst case. Oligolecty is also a word that occurs in many other languages, which facilitates translations and reduces the risk Figure 4: The figure shows some possible consequences of not using the correct term oligolectic alt. pollen specialist of incorrect translations. 22 Correctness and clarity in the use of terms in general also give increased credibility to the study being read, while incorrect terms could make the reader wonder to what extent the study is reliable; has the author really considered the true meaning of oligolecty and why is the correct term not used? This, in turn, can lead to the reader also opting out of other studies written by the same author. Database search results - oligolecty It may also happen that a study, where the correct term is not used, is excluded in a database search. Different databases use slightly different ways to specify certain words and include synonyms and not all of them have a comprehensive competence in that area, which means that the results can vary. The search on Web of Science with the word bee* and three different search variants showed a difference in the number of hits; oligolectic or pollen specialist together with food or diet specialist generated 704 results, when food or diet specialist where excluded the number of results was 492, while food or diet specialist and not oligolectic or pollen specialist generated 212 results. If * was removed from the word bee, 610, 287 and 21 results were generated respectively. This indicates that exclusion may be a reasonable assumption. Global change impacts on oligolectic bees There are studies that include oligolectic bees, albeit not to a large extent. This scarcity could be explained by one of the two aspects that have become most apparent during this review; The first is the lack of taxonomic expertise, which has become evident in several of the reviewed studies. Oligolecty is not a static condition but can change over time or with new knowledge, meaning that classifications need to be revised. Many researchers use older lists where many species, previously misclassified as polylectic, have been reclassified as oligolectic through new observations. Ideally, these researchers should recognize that this could be the case, as demonstrated by, for example, Hofmann and colleagues (2019), who ensured that species' new classifications were updated before using them in statistical tests and modelling (Hofmann et al. 2019) [66]. The need for accurate taxonomic data is crucial to avoid introducing bias into the research. Taxonomy goes beyond mere nomenclature, serving practical purposes in diverse fields such as biodiversity studies, conservation efforts, and agriculture. It extends beyond the assignment of names, providing a systematic framework to understand the natural world. Through taxonomic revisions, valuable information is generated, documenting variations in colour and morphology, enhancing predictability by revealing shared behaviours and ecologies among closely related species. Additionally, it offers insights into distribution patterns, phenology, and the intricacies of associated organisms like parasites and food plants. Figure 6: The main causes to oligolectic bees’ scarce inclusion in The second aspect is the oligolectic research studies within the field of global change impacts bees under study. The family that overwhelmingly dominates is Megachilidae, a commercially important group in agriculture. Unfortunately, the threat situation for the other families is significantly greater than that for Megachilidae. While the average threat in Europe is 9.2%, the family Megachilidae has a threat level of 1.1%, whereas the Melittidae and Colletidae families exhibit significantly higher levels; 18.9% and 12.8%, respectively (Nieto et al 2014). 23 These two aspects together imply that knowledge of oligolectic bees does not increase directly but only leads to increased knowledge of oligolectic bees belonging to the Megachilidae family (Figure 6).To what extent oligolectic bees are included in studies on global change impacts is obscure due to many factors that complicate an evaluation, especially when performed on a broader level. What has been identified is described in the subsequent sections. Obscurity or generalization? A conclusion read in one of the abstracts was; “the diversity of plant pollen in oligolectic bee species nesting tubes were higher in residential gardens compared to bushland habitats”, but when looking into the full study it turns out that there were only three oligolectic species included in the investigation; Megachilidae; Megachile canifrons, M. fabricator, and Rozenapis ignita (Fernandes et al. 2022) [14]. One study performed in a scrub oak barrens and concluded in the abstract that; increased visibility of nectar resources and sandy patches post-treatment may have promoted sand specialist and oligolectic bee species. But a closer look in the study revealed that only four species had been found, namely; Andrena braccata, A. hirticincta, A. placata, and A. simplex (Bried & Dillon 2012) [124]. Another study employing a quantitative comparative approach to predict population genetic structure, no discernible effect of diet specialization was observed. However, the study identified significant impacts of sociality on population genetic structure. The representatives utilized in the study encompassed six bee families: Andrenidae, Apidae, Colletidae, Halictidae, Megachilidae, and Melittidae. Notably, upon scrutiny of the supplementary materials, it became apparent that species from the Megachilidae and Melittidae families were absent. Among the sampled families, Apidae predominated with 32 species, of which 14 belonged to the genus Bombus. The remaining families included two species of Colletidae, four of Halictidae, and three of Andrenidae. The oligolectic species identified in the study were as follows: solitary species encompassed Lasioglossum oenotherae (Halictidae), Peponapis pruinosa (Apidae), Andrena fuscipes, Andrena vaga, and Macrotera portalis (Andrenidae). Social oligolectic species comprised Halictus scabiosae (Halictidae) and Bombus bifarius (Apidae) (López-Uribe 2019) [65]. Another study, that investigated bee assemblages in cattle-grazed sites versus sites with high cheat grass cover in prairies proposed in their abstract that; “sites with high grass cover tended to support oligolectic solitary bees”. Although, upon examination of the study it became clear that the oligolectic bees, covered by this proposition, (of nine) and one in the family Megachilidae: Lithurgopsis apicalis and the other eight in the family Apidae: seven Long horn bees (5 Melissodes spp., 2 Svastra spp) and Diadasia enavata (Thapa-Magar et al. 2020) [36]. Would it not be more correct to state that: “sites with high grass cover tended to support Long horn bees”? And if seventeen sites along a gradient (levels of urbanization) were studied and the total number of oligolectic species found were six, is a conclusion like: “significant preference”, even possible then? (Venn et al. 2023) [7]. If you have a small dataset, caution should be exercised when making strong statements about preferences, as statistical significance may be harder to achieve with limited observations. 24 It ought to be possible to perform a quantitative estimation only based on abstracts, although some authors includes conclusion referring to oligolectic bees as a group in their abstracts, while in the study actually only found a smaller restricted number of oligolectic species. Authors should consider being more precise in their abstracts, specifying the scope of the study to avoid potential misinterpretations. By providing accurate and clear information in the abstract ensures that readers understand the context and limitations of your research from the outset. Using inclusive language might attract more attention or interest from a broader audience. However, it's crucial to balance this with accuracy. Certain terms or concepts might be commonly understood within a specific scientific community, but researchers should be mindful of potential misinterpretations by those outside the field. Lack of informative material and misclassification There were several studies where 1) species lists were missing or the species list could not be opened, 2) species lists without information about which species they classified as oligolectic, 3) narrowly oligolectic species incorrectly classified as polylectic. 1) Lane et al. 2022 [27], Lerman & Milam 2016 [96], Grundel et al. 2010 [132] 2) Felderhoff et al. 2023 [10], Buhk et al. 2018 [72], Gonçalves et al. 2014 [111] 3) Ferrari & Polidori 2022 [15], Twerd et al. 2021 [34] (read more of this in next section) Taxonomic misclassification There are huge knowledge gaps in taxonomic data available, and current taxonomic is often not updated. The need of correct taxonomic data is crucial not to introduce bias in the research. An example of this need is found in a study (Twerd et al. 2021) [34] performed as late as 2021, where 12 oligolectic species were wrongly considered polylectic; narrowly oligolectic species: Andrena apicata, A. clarkella, A. curvungula, A. lapponica, A. nycthermera, Colletes cunicularius; moderately oligolectic species: Colletes daviesanus, C. fodiens, C. marginatus, C. similis. If ten species out of 131 of the polylectic bees are found to be, upon further examination, not truly polylectic but rather oligolectic, when assessing the contributions of phenological groups of wild bees as an indicator of food availability in urban wastelands, this revelation could have significant implications for the study's results. The misclassification of these bees may introduce bias in the assessment of food availability, as the foraging behavior and resource utilization of oligolectic bees differ from polylectic ones. The findings may need to be re-evaluated and adjusted to account for this misclassification, ensuring the accuracy and reliability of the study's conclusions regarding food resource availability in urban wastelands. In another study they explored how city traits affect both taxonomic and functional profile of urban bee communities in 55 cities around the world and when screening the list of included bee species in the study, six species that are narrowly oligolectic was classified as polylectic: Andrena clarkella, A. curvungula, A. lapponica, A. nycthermera, A. praecox, Colletes cunicularius (Ferrari & Polidori 2022) [15]. A study that caused doubtfulness is Lerman & Milam 2016 [96] where they referred to oligolectic as a “specialists on a single plant”. 25 Questionable studies One study that concluded positive results of oligolectic bee occurrence was read more thoroughly and possibly their conclusions could be questioned. The study took place in Munich, consisting of three study sites; a - the area of the Allacher Lohe, has a marshalling yard (continuous operational since 1991) but the remaining 150 ha area has been a nature reserve since 2000; b - Virginia depot (20 ha) that were off limit between 1945-2003 (therefor harbours rare plants and animals) and then transformed into a city biotope; c - Munich Botanical garden (20 ha). The results showed; a. a decrease of 60 % (80/135) in present bees and an increase of 30 % (244/189) of absent bees, b. an increase of 37,5 % (44/32) and a decrease of 3 % (280/292) respectively; c. an increase of 35, 5 % (105/78) and a decrease of 11 % (219/246) respectively. Moreover; they were referring to one of the authors own study (Hofmann & Renner 2018) [79] performed in Munich Botanical Garden where the outcome of “German Bee Diversity” showed no phylogenetic signal in the prediction of any vulnerability detected, and therefore used simple logistic regression. To apply and use results of phylogenetically informed models performed in a botanical garden that showed that phylogeny (oligolecty included) played no role, and use them without adding the information of where that study has taken place, could be questionable. In the study it can be read; “We therefore here use simple logistic regression with two models applied to the 324 species recorded for Munich since 1795” (Hofmann & Renner 2020) [51]. The two models were flight duration and seasonality. If the results used in a simple logistic regression analysis are incorrect, the outcome can lead to uncertain or erroneous conclusions. Incorrect results used as input for a statistical analysis can result in biased or misleading outcomes, which, in turn, can affect the interpretation of the results and any decisions based on them. The study could very well be in order, but due to the fact that it aroused questions, it some way failed to be clear and convincing. In one study on green roofs, the biodiversity on nine green roofs, in sub-urbans in Vienna, was investigated (Kratschmer et al. 2018) [81]. All together 2462 individuals were found, 1470 Apis mellifera and 992 individuals of wild bees where the total amount of oligolectic species found were 34. In the conclusion it could be read that occurrence of oligolectic wild bee species was low, but that they were “strongly positively affected” by floral diversity increases. When looking at the appendix of the found species found at these roofs the distribution was as shown in table 2. Table 2: Data extracted from the appendix of findings on respectively roof at the investigation performed in Vienna. Roof 1 Roof 2 Roof 3 Roof 4 Roof 5 Roof 6 Roof 7 Roof 8 Roof 9 Number of oligolectic 5 0 0 0 18 0 1 3 7 bees Number of 32 23 36 38 136 11 53 15 77 plant species Using the community weighted means (CWM) and R packages to examine characteristic traits on green roofs is a valid approach for ecological analysis. However, the key issue, in the former mentioned statement, is that of only show data where there were oligolectic bees, to show a strong positive result (Figure 5). By showing only the data where there were oligoleges, selection bias is introduced into the analysis. This means that by only considering situations where oligoleges were present, it can lead to an overestimation of the positive relationship between the examined traits and bee occurrence. 26 Moreover; excluding data where there was no oligoleges neglects important information. It's essential to consider both presence and absence data to get a comprehensive understanding of the ecological relationships. The absence of oligolectic bees might also be informative and could indicate factors that are unfavourable for oligolectic bee presence. The strong positive result observed in the data might not hold when considering a broader context. It is important to assess the relationship across a more extensive dataset to determine its generalizability. Selecting data only when oligoleges are present can lead Figure 5: Figure in the study to statistical biases and an overestimation of the significance of the by Kratschmer et al. 2018, presenting the results of the relationship. This can result in misleading or inaccurate conclusions. community weighted means Describing bee occurrence as "low" suggests a low frequency or (CWM) and R packages presented in the study. abundance of bees, which is typically associated with negative or neutral impacts. Saying it's "strongly positively affected" contradicts this by implying that the presence of more flowers has a very positive effect on oligolectic bee occurrence, especially when the presence of these bees does not consistently increase with an increased number of flowers. The few data points emphasize the need for a more thorough and comprehensive analysis, including statistical methods, to understand the complex ecological dynamics that affect the relationship between oligolectic bees and flowers. A larger and more diverse dataset is necessary to draw more reliable and meaningful conclusions about the relationship between oligolectic bee occurrence and the number of flowers on green roofs. In the review on Bee species from green roofs, conducted by Hofmann and Renner in 2017 [80], an introduced bias was notably present. Specifically, it was observed that "11% of the species found on green roofs in Vienna were oligolectic" (Kratschmer 2015). However, in the work by Kratschmer et al. in 2018, the statement is articulated as "we observed only 11% oligolectic species". In the preceding text, Kratschmer's study accurately presents the following figures: a total of 992 wild bee individuals were recorded, of which 34 individuals were oligolectic, representing 3.4% of the total. It is worth noting that there is no reference to Kratschmer (2015) in the citation list. Indeed, it should be noted that Kratschmer's initially published article (according to Web of Science, November 7, 2023) was released on December 12, 2017. Are oligolectic bees particularly vulnerable? In the majority of studies where they seriously has been investigating both oligolectic and polylectic bees the results clearly shows a greater decline of oligoleges compared to polyleges (Beismeijer et al. 2006; Bogusch et al. 2020) [40]. As a minority group within Apiformes, they are often overlooked or deemed less attractive to research funders and, consequently, researchers. The absence of taxonomic data poses challenges, as a comprehensive taxonomic description is essential for evaluating whether a species is endangered and for devising effective conservation measures. Without this information, oligolectic bees may be neglected in assessments of threat status and conservation needs. It should be emphasized that anything that poses a threat to a wild bee undoubtedly poses an even greater threat to an oligolectic bee. This is because oligolectic bees rely on a limited number of plants, and any factor affecting the host plant also impacts the oligolectic bee (see Figure 7), it is simple math. While some studies suggest that certain larvae can develop on non-host pollen, these studies do not track the larvae throughout their entire life cycle. This could potentially result in reduced fitness in adult bees, diminished reproductive success, or a shift in the male/female ratio, all of which can contribute to a decline in abundance. 27 Figure 7: Global Change multiple threats to oligolectic bee health. Fig. 1 modified from Perreno et al. (2021) Results shown in the two studies of genetic variation in oligolectic bees indicate that oligolectic bees, compared to polylectic bees, have lower levels of genetic variation. Add to that, the fact that specialist bees, in contrast to generalist bees, are forced to exist in smaller and more isolated population due to habitat loss and fragmentation. All together it point at a higher risk of endangerment or extinction for genetic and demographic reasons, due to a probably lesser capability to adapt to changing environmental conditions. Complete mismatch with floral hosts would likely cause severe fitness consequences in short-season oligolectic bees, but there are few documentations of this (Ogilvie & Forrest 2017) [91]. In temperate climates most species are triggered by the temperature, signalling the time of their seasonal activity. Those solitary bees that emerge in early spring, has spent the winter as full-fledged adults in their broad cells. Therefore could a shorter period of warmth in spring initiate quick responses in these bees, possibly result in temporal mismatches with their host plant. Consequences for plants have been well studied but research studies lack focusing on the fitness consequences for bees. Temperate (oligolectic) species occurring very early in spring or in late autumn are assumed to experience higher negative impacts of desynchronization. The higher threat is due to the risk of emerging in the absence of their preferred interaction partners, and as the plant biodiversity is lower during the season´s early and late periods, bees cannot easily switch to another potential interaction partner (Schenk et al. 2016) [88]. Then there are pesticides. There are a huge number of studies concerning sublethal and lethal effects of pesticide exposure, all done on social bees, mostly honey bees, but there some that studied bumble bees, which both have a social way of living. However, in general these studies show that the most common sublethal effects cause learning disabilities, poor memory, and aberrant foraging behavior. Such sublethal effects would impact a solitary bee much harder as they play a more important role as individual. Add to this the fact that oligolectic bees have less flexibility in their foraging, thus would face greater consequences of the sublethal effects mentioned above. 28 Why so few studies? Knowledge gaps are often pointed out in studies, but no explanations to the limited number of research studies on global change impacts and oligolectic bees are to be found. To explain it as a consequence of limited awareness about the ecological significance of oligolectic bees and the need to study them in the context of global change would be to simplify the answer. Raising awareness of possible factors explaining the limited number of studies could potentially lead to increased research in the field. The most obvious factor to this limitation is; 1) limited data availability; taxonomic bias in ecological research with more studies focused on economically significant or well-known species, lack of long- term monitoring programs or extensive datasets focusing on oligolectic bees resulting in limited comprehensive data to research upon. But there are also; 2) challenges and complexity in studying oligolectic species; understanding their responses to global changes requires a nuanced approach that considers not only the bees but also the dynamics of their interactions with particular plants, the potential impacts on both the bees and their host plants, adding complexity to study designs. This pollen specialization can make them less tractable for study. Attention are not to be forgotten, affecting 3) funding priorities; researchers may prioritize research with focus on broader topics, like overall pollinator declines or the effects of global changes on more generalist species since it may receive more attention than specific subsets such as oligolectic bees. Funding priorities often influence research focus, and could therefor make a major difference by prioritize research on global change impacts and oligolectic bees. How can this be changed? As interest in pollinator conservation and the understanding of ecosystem dynamics grows, it is possible that more studies will emerge in the intersection of global changes and oligolectic bees. Collaborative efforts among researchers, increased funding for targeted studies, and the recognition of the ecological importance of these pollen specialized bees may contribute to a more comprehensive understanding of their responses to global changes. There are researches having competence to perform studies in this field, at least in one or another aspect. It is the prerequisites that has to change; importance of oligolectic bees has to be highlighted; funders need to understand the challenges a researcher will have in such a complex field; not only commercial important bees has to be included in studies; in situ sites, suitable for studies, has to be investigated, ways to perform in vitro studies on these bees has to be investigated. Some researchers are already on their way: A study that stands out among others is Beyond generalists: "The Brassicaceae pollen specialist Osmia brevicornis as a prospective model organism when exploring pesticide risk to bees" by Hellström and colleagues (2023) [9] where their conclusion is that the oligolectic O. brevicornis is a feasible model for to assess the risk of pesticides in the laboratory and in the field. In two other studies Heriades truncurum and Osmia ribifloris are suggested as oligolectic models. Studies like that might encourage other researches to find model species, in some of the other five families of bees. And perhaps botanical gardens could be suitable places to individually study the impact that climate change has on bee biodiversity? That is suggested by Hofmann and her colleagues in their article (2018) where they analysed the bee fauna in the Munich Botanical Garden. Since the flora of the botanic garden has not changed and the protected flora of the surrounding environment has not changed for 20 years, habitat loss or loss of host plants of oligolectic species should not be reasons behind the disappearance of oligolectic species. In addition, there has been hardly any use of pesticides. The factor that remains is climate change, as the average temperature during the growing season in Munich has increased by 0.5 ºC and that winters have become almost four weeks shorter. 29 Conclusions Oligolecty That mono-/oligo-/polylecty rather is a matter of a continuum than of different categories, as Bogusch and colleagues (2020) wrote in their study, do acknowledge the potential for change and adaptation, but the key difference is in the nature of the change. A continuum represents a smooth, continuous progression without distinct categories, which might be correct in an evolutionary point of view. When it comes to spatial changes or mismatch induced changes, a continuum would not really describe the phenomena correctly. If anything it is more as a dynamic categorization; not fixed and immutable but can change over time or under different circumstances. Mono-/oligo-/polylecty are not classified within a single static category; instead, these classdifications adapts and reclassifies as needed or when new relevant information becomes available. It is important to use appropriate terms and concepts to describe the process where something can change or adapt depending on various factors, to help clarify that there is no rigid and static categorization but rather flexibility and adaptation within the system. Oligolectic bees While oligolectic bees are vital components in pollination ecology, their specific inclusion can vary depending on the research focus and objectives of different studies within broader fields. The overall conclusion of this study consists with what many other researches already have pointed out; there are huge knowledge gaps, especially for oligolectic bees. These knowledge gaps stretches from basic data, such as taxonomic, distribution and abundancy to researches done in the field of global change impacts on bees. Generalizations Using broad language in the abstract that implies a study's findings are representative of an entire group, when the study actually focused on a subset, can lead to potential misunderstandings or misinterpretations among readers who may not delve into the full study. Readers who only skim the abstract might get the impression that the study's conclusions apply universally to all oligolectic bees, which may not be the case. While the full study may provide the necessary details and context, the abstract is often the first section a reader encounter. Misleading language in the abstract can also affect the overall scientific accuracy and integrity of the research. This can lead to misunderstandings about the generalizability of the findings and if others in the scientific community or beyond might use your study's abstract as a reference without delving into the specifics. If the abstract suggests broad conclusions about all oligolectic bees, it could be cited inaccurately in other works. It is not accurate or appropriate to claim in the abstract that the study has investigated oligolectic bees in general and then draw conclusions about all oligolectic bees if the study only focused on a few species from one of the six families of bees. The abstract should accurately reflect the scope and findings of the study. Vulnerability of oligolectic bees Oligolectic bees are significantly more threatened by global change than polylectic bees; a fact that is underlined by their overrepresentation in red lists. Of the anthropogenic effects described in the text above, almost every one of them involve some form of possible extra vulnerability for oligolectic bees. The overall conclusion of the reviews is that oligolectic bees are threatened by multiple factors, as many of the wild bees are. But due to their dependence on specific hostplant, they are in general, more vulnerable and less capable to adapt to global changes. 30 Further research More basic research is needed on oligolectic bee foraging ranges, flight seasons, and floral-host associations. Additionally, studies examining bee behavioural and reproductive responses to fluctuations in resource availability are essential. Understanding how bee foraging and floral phenology has co-evolved, considering phylogenetic relatedness is crucial. Identification and protection of floral reserves near roost sites along the "nectar corridors" of threatened migratory pollinators is a crucial conservation strategy. Maintaining these corridors, which enable migratory pollinators to move between patches of plants, is essential for preserving their populations. Given the alterations in floral resource phenology due to anthropogenic environmental change, a better understanding of bee responses to global changes is necessary to anticipate their future population and community trajectories. Comparative analyses of the pollen preferences of oligolectic bee populations in different environmental contexts, along with experimental tests of behavior in settings with scarce floral hosts, are needed to predict specialist bee responses to changes in floral availability. Evaluating the relative effects of different environmental gradients on bee community composition is also crucial. There are knowledge gaps in understanding what oligolectic bees do under a lack of resources, such as whether they halt nesting or search elsewhere for their host. Closing these gaps is essential for comprehensive insights into the ecology and behavior of oligolectic bees. Words of thanks I would like to thank Björn Cederberg for all the entomological taxonomy he has contributed with during his life, and for helping me clarify concepts and classifications. To my supervisor Julia Osterman, I would like to say; thank you for your amazing patience! And thank her for the advice and tips she gave me which really improved this study. I would like to thank the Entomological Association in Stockholm for the scholarship I received, for the purchase of Stephen Falk's book on wild bees. Course leader Charlotta Kvarnemo and examiner Åslög Dahl, I thank you for being available for all of my questions. 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No. Asteraceae Pollen Provisions Protect Osmia Spear, DM; Mason Bees Am. 797- 102 Silverman, S; 2016 187 6 (Hymenoptera: Nat. 803 Forrest, JRK Megachilidae) from Brood Parasitism Milet-Pinheiro, Host choice in a bivoltine P; Herz, K; bee: how sensory BMC 103 2016 16 20 Dötterl, S; constraints shape innate Ecol. Ayasse, M foraging behaviors Wappler, T; Specialized and Labandeira, Generalized Pollen- Curr. 3092- 104 CC; Engel, 2015 25 23 Collection Strategies in an Biol. 3098 MS; Zetter, R; Ancient Bee Lineage Grímsson, F Taxonomic and behavioral components of faunal Proc. comparisons over time: Entom Goldstein, PZ; 290- 105 The bees (Hymenoptera: ol. 2015 117 3 Scott, VL 346 Antophila) of Boulder Soc. County, Colorado; Past Wash. and Present Visual and Olfactory Floral Cues of Campanula Milet-Pinheiro, (Campanulaceae) and PLoS e0128 106 P; Ayasse, M; 2015 10 6 Their Significance for Host One 577 Dötterl, S Recognition by an Oligolectic Bee Pollinator Ecology and evolution of New floral volatile-mediated 571- 107 Schiestl, FP Phytol 2015 206 2 information transfer in 577 . plants Palaearctic Chelostoma bees of the subgenus Gyrodromella Zoota 393 408- 108 Müller, A 2015 3 (Megachilidae, Osmiini): xa 6 420 biology, taxonomy and key to species Impact of past climatic Dellicour, S; changes and resource Michez, D; Mol. 1074- Climate 109 availability on the 2015 24 5 3 Melitta spp. Rasplus, JY; Ecol. 1090 Change population demography of Mardulyn, P three food-specialist bees Does multi-level Sydenham, environmental filtering MAK; Moe, determine the functional Ecogr 140- 110 SR; Totland, 2015 38 2 and phylogenetic aphy 153 O; Eldegard, composition of wild bee K species assemblages? List of found Gonçalves, Bee and wasp responses J. Land species did RB; Sydney, to a fragmented Insect 1193- alteration; not show 111 NV; Oliveira, 2014 18 6 landscape in southern Conse 1201 Fragmentati which species PS; Artmann, Brazil rv. on considered NO oligolectic Straka, J; Life span in the wild: the Cerná, K; role of activity and climate Funct. 1235- Climate 112 Machácková, 2014 28 5 Andrena vaga in natural populations of Ecol. 1244 Change L; Zemenová, bees M; Keil, P MacIvor, JS; Pollen specialization by Urban 139- 113 Cabral, JM; solitary bees in an urban Ecosy 2014 17 1 147 Packer, L landscape st. Seeking the flowers for Silva, DP; the bees: Integrating biotic Gonzalez, VH; interactions into niche Melo, GAR; Ecol. 200- 114 models to assess the 2014 273 Lucia, M; Model. 209 distribution of the exotic Alvarez, LJ; bee species Lithurgus De Marco, P huberi in South America 43 J. Publ. Issu Pg. or [ No.] Authors Article Title Vol. Study field Notes Abbrev. Year e Art. No. Vanderplanck, M; Moerman, How Does Pollen R; Rasmont, Chemistry Impact PLoS e8620 115 P; Lognay, G; Development and Feeding 2014 9 1 One 9 Wathelet, B; Behaviour of Polylectic Wattiez, R; Bees? Michez, D Dellicour, S; Inferring the mode of Mardulyn, P; colonization of the rapid Hardy, OJ; range expansion of a J. Evol. 116 Hardy, C; 2014 27 1 116-132 solitary bee from Biol. Roberts, SPM; multilocus DNA sequence Vereecken, variation NJ Milet-Pinheiro, P; Ayasse, M; The Chemical Basis of J. Dobson, HEM; Host-Plant Recognition in 12/ 1347- 117 Chem. 2013 39 Schlindwein, a Specialized Bee 11 1360 Ecol. C; Francke, Pollinator W; Dötterl, S Gosselin, M; Does Aconitum Michez, D; septentrionale chemically Ecol. Vanderplanck, 400- 118 protect floral rewards to Entom 2013 38 4 M; Roelants, 407 the advantage of ol. D; Glauser, G; specialist bumblebees? Rasmont, P Changes in wild bee fauna Land Martins, AC; of a grassland in Brazil alteration; Gonçalves, reveal negative effects Zoolo 157- 119 2013 30 2 Urbanizati RB; Melo, associated with growing gia 176 on; GAR urbanization during the grassland last 40 years Danforth, BN; The Impact of Molecular Annu. Cardinal, S; Data on Our Rev. 120 Praz, C; 2013 58 57 Understanding of Bee Entom Almeida, EAB; Phylogeny and Evolution ol. Michez, D Nesting biology and immatures of the Am. Rozen, JG; 376 121 oligolectic bee Trachusa Mus. 2012 1-24 Hall, HG 5 larreae (Apoidea: Novit. Megachilidae: Anthidiini) Territorial or wandering: Oliveira, R; how males of Carvalho, AT; Protodiscelis palpalis Apidol 674- 122 2012 43 6 Schlindwein, (Colletidae, ogie 684 C Paracolletinae) behave in searching for mates Individual lifetime pollen Natur Hagbery, J; and nectar foraging wisse 821- 123 2012 99 10 Nieh, JC preferences in bumble nschaf 832 bees ten Insect. Land Bee diversity in scrub oak Conse alteration; Bried, JT; 237- 124 patches 2 years after mow rv. 2012 5 3 Biodiversit Dillon, AM 243 and herbicide treatment Divers y, scrub . oak Milet-Pinheiro, P; Ayasse, M; Host location by visual Schlindwein, and olfactory floral cues in Behav 531- 125 2012 23 3 C; Dobson, an oligolectic bee: innate . Ecol. 538 HEM; Dötterl, and learned behavior S Collection of Pollen Grains by Centris (Hemisiella) Gonçalves, L; tarsata Smith (Apidae: Zool. 195- 126 da Silva, CI; 2012 51 2 Centridini): Is C. tarsata Stud. 203 Buschini, MLT an Oligolectic or Polylectic Species? 44 J. Publ. Issu Pg. or [ No.] Authors Article Title Vol. Study field Notes Abbrev. Year e Art. No. Filella, I; Chemical cues involved in Bioch Bosch, J; the attraction of the em. 498- 127 Llusià, J; oligolectic bee Hoplitis 2011 39 6/4 Syst. 508 Peñuelas, A; adunca to its host plant Ecol. 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J. 1498- 131 Villagra, CA; ecology of two parapatric 2010 97 9 Bot. 1510 Raguso, RA subspecies of Oenothera cespitosa (Onagraceae) Grundel, R; Floral and nesting Jean, RP; resources, habitat No list of Frohnapple, structure, and fire Ecol. 1678- Land which bees 132 2010 20 6 KJ; Glowacki, influence bee distribution Appl. 1692 alteration they consider GA; Scott, PE; across an open-forest as oligolectic Pavlovic, NB gradient Experimental Oliveira, R; demonstration of Anim. 241- 133 Schlindwein, alternative mating tactics Behav 2010 80 2 247 C of male Ptilothrix fructifera . (Hymenoptera, Apidae) The chemical ecology and Dötterl, S; Can. evolution of bee-flower 668- 134 Vereecken, J. 2010 88 7 interactions: a review and 697 NJ Zool. perspectives Mutual reproductive dependence of distylic Milet-Pinheiro, Cordia leucocephala P; (Cordiaceae) and Ann. 135 2010 106 1 17-27 Schlindwein, oligolectic Ceblurgus Bot. C longipalpis (Halictidae, Rophitinae) in the Caatinga Are landscape structures Zurbuchen, A; insurmountable barriers Land Bachofen, C; for foraging bees? A Apidol 497- alteration; Chelostoma 136 Müller, A; 2010 41 4 mark-recapture study with ogie 508 Forage florisomne Hein, S; Dorn, two solitary pollen distances S specialist species Zurbuchen, A; Cheesman, S; Long foraging distances Land Hoplitis J. Klaiber, J; impose high costs on 674- alteration; adunca, 137 Anim. 2010 79 3 Müller, A; offspring production in 681 Forage Chelostoma Ecol. Hein, S; Dorn, solitary bees distances rapunculi S A synopsis of Actenosigynes Moure, Graf & Urban, 1999 Zoota 229 138 Silveira, FA (Hymenoptera: 2009 15-24 xa 2 Colletidae)-new species, possible oligolecty and biogeographic comments Reproduction of Amorpha Land canescens (Fabaceae) Slagle, MW; Oecol 813- alteration; Andrena 139 and diversity of its bee 2009 161 4 Hendrix, SD ogia 823 Fragmentati quintilis community in a on fragmented landscape 45 J. Publ. Issu Pg. or [ No.] Authors Article Title Vol. Study field Notes Abbrev. Year e Art. No. Phylogeny of the bee family Melittidae Michez, D; Syst. (Hymenoptera: 574- 140 Patiny, S; Entom 2009 34 3 Anthophila) based on 597 Danforth, B ol. combined molecular and morphological data Batley, M; Diversity and conservation Apidol 347- Climate 141 Hogendoorn, status of native Australian 2009 40 3 ogie 354 Change K bees Small local population J. Land Franzén, M; sizes and high habitat Insect alteration; Andrena 142 Larsson, M; 2009 13 1 89-95 patch fidelity in a Conse Forage hattorfiana Nilsson, S specialised solitary bee rv. distances Moron, D; Szentgyörgyi, DIVERSITY OF WILD Land H; Wantuch, BEES IN WET alteration; M; Celary, W; Wetla 975- 143 MEADOWS: 2008 28 4 Biodiversit Westphal, C; nds 983 IMPLICATIONS FOR y; wet Settele, J; CONSERVATION meadows Woyciechows ki, M Michez, D; Patiny, S; Phylogeny and host-plant Rasmont, P; Apidologi 144 evolution in Melittidae s.l. 2008 39 1 146-162 Timmermann, e (Hymenoptera: Apoidea) K; Vereecken, NJ Faunal composition and species richness Land differences of bees Apidol 176- alteration; 145 Minckley, R 2008 39 1 (Hymenoptera: Apiformes) ogie U134 Biodiversit from two north American y regions Pollen harvesting and Ann. Franzén, M; 405- Oligolectic Andrena 146 reproductive rates in Zool. 2007 44 6 Larsson, M 414 bees humilis specialized solitary bees Fenn. The population genetics of a solitary oligolectic sweat Zayed, A; bee, Lasioglossum Heredi 397- Lasioglossum 147 2007 99 4 Genetics Packer, L (Sphecodogastra) ty 405 oenotherae oenotherae (Hymenoptera: Halictidae) Critical resource levels of pollen for the declining Biol. Plant- Larsson, M; 405- Andrena 148 bee Andrena hattorfiana Conse 2007 134 3 pollinator Franzén, M 414 hattorfiana (Hymenoptera, rv. interaction Andrenidae) Foraging patterns of the southeastern blueberry J. bee Habropoda laboriosa 149 Pascarella, JB Apic. 2007 46 1 19-27 (Apidae, Hymenoptera):: Res. Implications for understanding oligolecty Faunal makeup of wild Land bees and their flower Hisamatsu, M; Entom 137- alteration; 150 utilization in a semi- 2006 9 2 Yamane, S ol. Sci. 145 Urbanizati urbanized area in central on Japan Cane, JH; Complex responses within Land Minckley, RL; a desert bee guild Ecol. 632- alteration; 151 Kervin, LJ; (Hymenoptera: Apiformes) 2006 16 2 Appl. 644 Fragmentati Roulston, TH; to urban habitat on Williams, NM fragmentation Pollen-host specificity and evolutionary patterns of Biol. J. Sipes, SD; 487- 152 host switching in a clade Linnea 2005 86 4 Tepedino, VJ 505 of specialist bees n Soc. (Apoidea:Diadasia) 46 J. Publ. Issu Pg. or [ No.] Authors Article Title Vol. Study field Notes Abbrev. Year e Art. No. Ecological context of the evolution of self-pollination Moeller, DA; in Clarkia xantiana:: Evoluti 786- 153 2005 59 4 Geber, MA Population size, plant on 799 communities, and reproductive assurance Packer, L; Conservation genetics of Zayed, A; potentially endangered Conse Grixti, JC; mutualisms: Reduced 195- 154 rv. 2005 19 1 Genetics Ruz, L; Owen, levels of genetic variation 202 Biol. RE; Vivallo, F; in specialist versus Toro, H generalist bees Pollinator community structure and sources of spatial variation in plant- Oecol 155 Moeller, DA 2005 142 1 28-37 pollinator interactions in ogia Clarkia xantiana ssp xantiana Inverse density-dependent Antonini, Y; and density-independent Trop. 156 Martins, RP; parasitism in a solitary 2003 16 1 83-92 Zool. Rosa, CA ground-nesting bee in Southeast Brazil Biological impediments to J. Minckley, RL; measures of competition Kans. Competition Cane, JH; 306- 157 among introduced honey Entom 2003 76 2 & Invasive Kervin, L; 319 bees and desert bees ol. species Yanega, D (Hymenoptera: Apiformes) Soc. Biology and behavior of a Norden, BB; J. seasonally aquatic bee, Krombein, KV; Kans. Perdita (Alloperdita) 236- 158 Deyrup, MA; Entom 2003 76 2 floridensis Timberlake 249 Edirisinghe, ol. (Hymenoptera: JP Soc. Andrenidae: Panurginae) Phylogenetic relationships Mol. Sipes, SD; 159 within Diadasia, a group of Phylogen 2001 19 1 144-156 Wolf, PG specialist bees et. Evol. Proc. Origins and ecological Minckley, RL; R. consequences of pollen 144 265- 160 Cane, JH; Soc. 2000 267 specialization among 0 271 Kervin, L B-Biol. desert bees Sci. Plant Dobson, HEM; The ecology and evolution 161 Syst. 2000 222 4/1 63-87 Bergström, G of pollen odors Evol. Competition between the oligolectic bee Ptilothrix plumata (Anthophoridae) and the flower closing Plant Schlindwein, 183- 162 beetle Pristimerus Syst. 2000 224 4/3 C; Martins, CF 194 calcaratus (Curculionidae) Evol. for floral resources of Pavonia cancellata (Malvaceae) Spatial predictability and Minckley, RL; resource specialization of Biol. J. Cane, JH; bees (Hymenoptera: 119- 163 Linnea 1999 67 1 Kervin, L; Apoidea) at a 147 n Soc. Roulston, TH superabundant, widespread resource Affinities between Aust. southern Tasmanian 361- 164 Hingston, AB J. 1999 47 4 plants in native bee visitor 384 Zool. profiles Micro-foraging routes of Bicolletes pampeana Schlindwein, (Colletidae) and bee- Bot. 177- 165 C; Wittmann, induced pollen 1997 110 2 Acta 183 D presentation in Cajophora arechavaletae (Loasaceae) 47 [ No.] J. Publ. Issu Pg. or Authors Article Title Vol. Study field Notes Abbrev. Year e Art. No. Oligolectic bee species in Entom 205- Oligolectic 166 Pekkarinen, A Northern Europe ol. 1997 8 4 214 bees (Hymenoptera, Apoidea) Fenn. Floral resource utilization by solitary bees Annu. Wcislo, WT; (Hymenoptera: Apoidea) Rev. 257- 167 1996 41 Cane, JH and exploitation of their Entom 286 stored foods by natural ol. enemies Proc. The biology of Diadasina Entom Martins RP; distincta (Holmberg, 1903) 553- 168 ol. 1994 96 3 Antonini, Y (Hymenoptera, 560 Soc. Anthophoridae) Wash. Minckley, RL; Behavior and phenology Wcislo, WT; of a specialist bee Ecolo 1406- 169 Yanega, D; (Dieunomia) and 1994 75 5 gy 1419 Buchmann, sunflower (Helianthus) SL pollen availability 48 Appendix 2: A revised list of oligolectic bees in Sweden 2023 Species regionally extinct in Sweden is printed in grey. Species Hostplant Informal qualifier Andrena afzeliella (albofasciata) Fabaceae Moderate Andrena apicata Salicaceae: Salix Narrow Andrena batava Salicaceae: Salix Narrow Andrena clarkella Salicaceae: Salix Narrow Andrena curvungula Campanulaceae: Campanula Narrow Andrena denticulata Asteraceae Broad Andrena fulvago Asteraceae Moderate Andrena fuscipes Ericaceae: Calluna Broad Andrena gelriae Fabaceae Moderate Andrena hattorfiana Dipsacaceae: Knautia Narrow Andrena humilis Asteraceae Moderate Andrena intermedia Fabaceae Broad Andrena labialis Fabaceae Moderate Andrena lapponica Ericaceae: Vaccinium Narrow Andrena lathyri Fabaceae: Lathyrus Narrow Andrena marginata Dipsacaceae: Succisa Moderate Andrena morawitzi Salicaceae: Salix Narrow Andrena nanula Apiaceae Moderate Andrena niveata Brassicaceae Moderate Andrena nycthemera Salicaceae: Salix Narrow Andrena praecox Salicaceae: Salix Narrow Andrena ruficrus Salicaceae: Salix Narrow Andrena russula (similis) Fabaceae Moderate Andrena tarsata Rosaceae: Potentilla Narrow Andrena vaga Salicaceae: Salix Narrow Andrena wilkella Fabaceae Broad Anthophora furcata Lamiaceae Moderate Bombus consobrinus Ranunculaceae: Aconitum Narrow Chelostoma campanularum Campanulaceae: Campanula Narrow Chelostoma florisomne Ranunculaceae: Ranunculus Narrow Chelostoma rapunculi Campanulaceae: Campanula Narrow Colletes cunicularius Salicaceae: Salix Narrow Colletes daviesanus Asteraceae Moderate Colletes fodiens Asteraceae Moderate Colletes marginatus Fabaceae Moderate Colletes similis Asteraceae Moderate Colletes succinctus Ericaceae: Calluna Narrow 49 Species Hostplant Informal qualifier Dasypoda argentata Dipsacaceae Moderate Dasypoda hirtipes Asteraceae Broad Dasypoda suripes Dipsacaceae – Väddväxter Moderate Dufourea dentiventris Campanulaceae: Campanula Narrow Dufourea halictula Campanulaceae: Jasione Narrow Dufourea inermis Campanulaceae: Campanula Narrow Dufourea minuta Asteraceae Moderate Eucera longicornis Fabaceae Broad Heriades truncorum Asteraceae Broad Hoplitis adunca Boraginaceae: Echium Narrow Hoplitis anthocopoides Boraginacae: Echium vulgare Narrow Hoplitis mitis Campanulaceae: Campanula Narrow Hoplosmia spinulosa (Osmia spinulosa) Asteraceae Broad Hylaeus signatus Resedaceae: Reseda Narrow Macropis europaea Primulaceae: Lysimachia Narrow Megachile circumcincta Fabaceae Broad Megachile lagopoda Asteraceae Broad Megachile lapponica Onagraceae: Epilobium Narrow Megachile ligniseca Asteraceae Broad Megachile nigriventris Fabaceae Moderate Melitta haemorrhoidalis Campanulaceae Narrow Melitta leporina Fabaceae Broad Melitta melanura (wankowiczi) Campanulaceae: Campanula Narrow Melitta tricincta Scrophulariaceae: Odontites Narrow Osmia leaiana Asteraceae Moderate Osmia maritima Fabaceae Moderate Panurginus romani Rosaceae: Rubus idaeus Narrow Panurgus banksianus Asteraceae Moderate Panurgus calcaratus Asteraceae Moderate Rophites quinquespinosus Lamiaceae Moderate Trachusa byssina Fabaceae Broad Sources: Pettersson et al (2004), Linkowski et al. (2004), Swedish Observation Species Center (Artportalen, Artfakta), Bees Wasps & Ants Recording Society (BWARS), Steven Falk's book; Field Guide to the Bees of Great Britain and Ireland (2015), the Norwegian Biodiversity Information Center (Artsdatabanken), Finnish Biodiversity Info Facility (Artdatacenter), Global Biodiversity Information Facility (GBIF), Denmark's national Artportal. The list is fact-checked and corrected by the Swedish entomologist Björn Cederberg (part of the Swedish expert committee of Hymenoptera). The data was collected between 2023-06-01 and 2023-10-31. 50 Appendix 3: Current prevailing red list status of oligolectic bees LC = Least concern NT = Near Threatened VU = Vulnerable EN = Endangered CR = Critical endangered RE = Regionally extinct DD = Data deficiency NE = Not assessed NA = Not appliable Current prevailing red list status of bees considered to be oligolectic in Sweden. It should be emphasized that some species could be considered polylectic in other countries. Species regionally extinct in Sweden are printed in grey. Country Sweden Denmark Norway Finland IUCN IUCN Year of red list classification 2020 2019 2021 2019 2012-14 2012-2014 Andrena afzeliella (albofasciata)* NA VU RE NT RE in Finland Andrena apicata LC LC EN DD RE in Czechia Andrena batava VU DD Andrena clarkella LC LC LC LC DD RE in Switzerland Andrena curvungula NT NA DD RE in Netherlands Andrena denticulata LC LC LC LC DD Andrena fulvago LC VU VU VU DD Andrena fuscipes LC LC LC LC DD RE in Czechia Andrena gelriae EN RE EN DD Andrena hattorfiana LC LC CR LC NT Andrena humilis VU NT RE CR DD RE in Norway Andrena intermedia LC NA LC LC LC Andrena labialis NT LC RE DD Andrena lapponica LC LC LC LC LC Andrena lathyri LC VU LC EN DD Andrena marginata NT EN VU CR DD RE in Netherlands Andrena morawitzi CR EN* DD RE in Czechia Possibly RE in Andrena nanula VU NA VU DD Great Britain Andrena niveata EN RE DD Andrena nycthemera VU NA DD RE in Switzerland Andrena praecox LC LC LC LC LC Andrena ruficrus LC LC LC LC LC 51 Country Sweden Denmark Norway Finland IUCN IUCN Year of red list classification 2020 2019 2021 2019 2012-14 2012-2014 Andrena russula (similis) EN EN EN DD Andrena tarsata LC NT LC LC DD RE in Hungary Andrena vaga LC LC LC LC LC Andrena wilkella LC LC LC LC DD Anthophora furcata LC LC LC LC LC Bombus consobrinus LC LC EN LC Chelostoma campanularum LC LC LC LC LC Chelostoma florisomne LC LC LC LC LC Chelostoma rapunculi LC LC LC LC Colletes cunicularius LC LC LC LC LC Colletes daviesanus LC LC LC LC LC Colletes fodiens NT LC VU Colletes marginatus NT NT VU EN LC Colletes similis LC LC LC LC LC Colletes succinctus LC LC LC LC NT Extant in Sweden Dasypoda argentata RE NT 2013 Dasypoda hirtipes LC VU LC LC RE in Czechia; Dasypoda suripes RE CR EN Denmark; Germany; Sweden Dufourea dentiventris LC EN NT LC NT Dufourea halictula VU EN NT Dufourea inermis EN EN EN NT RE in Netherlands; Dufourea minuta CR RE RE VU NT Sweden Eucera longicornis LC LC LC LC LC Heriades truncorum LC NT LC LC LC Hoplitis adunca NA NA LC Hoplitis anthocopoides NA VU LC Hoplitis mitis NT LC 52 Country Sweden Denmark Norway Finland IUCN IUCN Year of red list classification 2020 2019 2021 2019 2012-14 2012-2014 Hoplosmia spinulosa LC VU LC LC (Osmia spinulosa) Hylaeus signatus NT DD LC Macropis europaea LC LC LC LC LC Megachile circumcincta LC LC LC LC LC Megachile lagopoda NT LC CR NT LC Megachile lapponica LC LC NT LC DD RE in Great Britain Megachile ligniseca LC RE LC DD RE in Norway Megachile nigriventris LC NA LC LC DD Melitta haemorrhoidalis LC LC LC LC LC Melitta leporina LC LC NT NT LC Melitta melanura (wankowiczi) CR EN RE in Germany Melitta tricincta NT VU NT Osmia leaiana LC LC LC VU LC Osmia maritima EN NT EN EN RE in Poland Panurginus romani LC LC LC DD Presence Uncertain Panurgus banksianus VU LC VU LC in Norway; Romania Panurgus calcaratus LC LC LC LC RE in Netherlands; Rophites quinquespinosus RE RE NT Sweden Trachusa byssina LC NA LC LC Sources of oligolectic current prevailing red list status are the Biodiversity Information Centers of; Sweden (Artportalen); Norway (Artsdatabanken); Finland (Finnish Biodiversity Info Facility), Madsen`s “Den danske Rødliste 2019” and IUCN Red list: https://www.artportalen.se/Occurrence/TaxonOccurrence/16/2002991 (Visited 2023-10-10) https://artsdatabanken.no/lister/rodlisteforarter/2021 (Visited 2023-10-18) https://ecos.au.dk/forskningraadgivning/temasider/redlistframe/soeg-en-art (Visited 2023-10-27) https://punainenkirja.laji.fi/sv/results?type=species&year=2019&redListGroup= (Visited 2023-10-28) https://www.iucnredlist.org/ (Visited 2023-11-05) 53 Appendix 4: Combination of search words Oligolecty Combination of search words Date No. hits “bee*" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 704 speciali*") "bee*" AND ("oligol*" OR "pollen speciali*") 2023-10-20 492 "bee*" AND ("food speciali*" OR " diet speciali*" OR "plant speciali*") NOT 2023-10-20 212 ("oligol*" OR "pollen speciali*") “bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 610 speciali*") "bee" AND ("oligol*" OR "pollen speciali*") 2023-10-20 287 "bee" AND ("food speciali*" OR " diet speciali*" OR "plant speciali*") NOT 2023-10-20 21 ("oligol*" OR "pollen speciali*") Global Change – “the full search” Combination of search words Date No. hits "Bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND ("Land use" OR "Urban*" OR "Fragment*" OR "Habitat loss" OR 2023-10-10 49 "Monoculture*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND ("Weed control" OR "Pesticide*" OR "Pest management*" OR "Pest control" OR "Insecticide *" OR "Herbicide*" OR "Fungicide*" OR "Fertilizer" 2023-10-10 14 OR "Biocide*" OR "Agrochemical*" OR "Pollution*" OR "Combustion*" OR "Heavy metal*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND ("Pathogen*" OR "Parasite*" OR "Disease*" OR "Virus*" OR 2023-10-10 18 "Invasive species" OR "Non-native species" OR "Exotic species") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-10 19 speciali*") AND ("competition") "bee" AND ("oligol*" OR "pollen speciali*" OR "food speciali*" OR "diet speciali*") AND ("Climate change" OR “Desiccation" OR "Dehydrat*" OR "Drought" OR "Wildfire*" OR "Heat tolerance" OR "Thermal tolerance" OR 2023-10-10 20 "Landslide*" OR "Extreme weather" OR "Heavy rain*" OR "Extreme rain*" OR "Global warming" OR "Flood*") “bee” AND (“oligol*” OR “pollen speciali*”OR “food speciali*” OR “diet speciali*”) AND (“Mismatch”) 2023-10-10 1 “bee” AND (“oligol*” OR “pollen speciali*” OR “food speciali*” OR “diet speciali*”) AND (“Nutri* defici*” OR “Malnutrition” OR “Floral resorce*” OR 2023-10-10 1 “poor flower “ OR “nectar quality*” OR “poor nutrition”) “bee” AND (“oligol*” OR “pollen speciali*”OR “food speciali*” OR “diet speciali*”) AND (“synergis*” OR “interact*” OR “multiple” OR “Additiv*” OR 2023-10-10 89 “combin*”) “bee” AND (“oligol*” OR “pollen speciali*”OR “food speciali*” OR “diet speciali*”) AND (“threat*” OR “risk” OR “decline” OR “stress*” OR “drive*” OR 2023-10-10 74 “harm*” OR “impact*” OR “impaired” OR “damage*”) Total number of hit results 268 54 Land alteration Combination of search words Date No. hits "Bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 14 speciali*") AND ("Land use") "Bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 28 speciali*") AND ("Urban*") "bee" AND ("oligol*" OR "pollen speciali*" OR "food speciali*" OR "diet 2023-10-20 19 speciali*") AND ("Fragment*") "Bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 1 speciali*") AND ("Habitat loss") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 1 speciali*") AND ("Monoculture*") "Bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND ("Land use" OR "Urban*" OR "Fragment*" OR "Habitat loss" OR 2023-10-10 49 "Monoculture*") (Agro-) Chemicals & Pollution Combination of search words Date No. hits "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 0 speciali*") AND ("Weed control") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 10 speciali*") AND ("Pesticide*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 0 speciali*") AND ("Pest management") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 1 speciali*") AND ("Pest control") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 2 speciali*") AND ("Insecticide *") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 3 speciali*") AND ("Herbicide*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 1 speciali*") AND ("Fungicide*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 0 speciali*") AND ("Fertilizer") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 0 speciali*") AND ("Biocide*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 0 speciali*") AND ("Agrochemical*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 1 speciali*") AND ("Pollution*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 0 speciali*") AND ("Combustion*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 1 speciali*") AND ("Heavy metal*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND ("Weed control" OR "Pesticide*" OR "Pest management*" OR "Pest control" OR "Insecticide *" OR "Herbicide*" OR "Fungicide*" OR "Fertilizer" 2023-10-20 14 OR "Biocide*" OR "Agrochemical*" OR "Pollution*" OR "Combustion*" OR "Heavy metal*") 55 Invasive species & Pathogens Combination of search words Date No. hits "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 4 speciali*") AND ("Pathogen*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 10 speciali*") AND ("Parasite*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 1 speciali*") AND ("Disease*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 0 speciali*") AND ("Virus*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 2 speciali*") AND ("Invasive species") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 0 speciali*") AND ("Non-native species") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 4 speciali*") AND ("Exotic species") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND ("Pathogen*" OR "Parasite*" OR "Disease*" OR "Virus*" OR 2023-10-20 18 "Invasive species" OR "Non-native species" OR "Exotic species") Competition Combination of search words Date No. hits "bee" AND ("oligol*" OR "pollen speciali*") AND ("Competition") 2023-10-10 19 "bee" AND ("oligol*" OR "pollen speciali*" OR "food speciali*" OR "diet 2023-10-10 19 speciali*") AND ("Competition") Climate changes & Mismatch Combination of search words Date No. hits "bee" AND ("oligol*" OR "pollen speciali*" OR "food speciali*" OR "diet speciali*") AND 15 ("Climate change") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND 1 ("Desiccation") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND 1 ("Dehydrat*") "bee" AND ("oligol*" OR "pollen speciali*" OR "food speciali*" OR "diet speciali*") AND 0 ("Drought") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND 2 ("Wildfire*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND 0 ("Heat tolerance") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND 0 ("Thermal tolerance") "bee" AND ("oligol*" OR "pollen speciali*" OR "food speciali*" OR "diet speciali*") AND 2023-10-20 0 ("Landslide") "bee" AND ("oligol*" OR "pollen speciali*" OR "food speciali*" OR "diet speciali*") AND 0 ("Extreme weather") "bee" AND ("oligol*" OR "pollen speciali*" OR "food speciali*" OR "diet speciali*") AND 0 ("Heavy rain*") "bee" AND ("oligol*" OR "pollen speciali*" OR "food speciali*" OR "diet speciali*") AND 0 ("Extreme rain*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND 1 ("Global warming") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND 0 ("Flood*") "bee" AND ("oligol*" OR "pollen speciali*" OR "food speciali*" OR "diet speciali*") AND ("Climate change" OR "Desiccation" OR "Dehydrat*" OR "Drought" OR "Wildfire*" OR "Heat tolerance" OR 20 "Thermal tolerance" OR "Landslide*" OR "Extreme weather" OR "Heavy rain*" OR "Extreme rain*" OR "Global warming" OR "Flood*") 56 Mismatch Combination of search words Date No. hits "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-10 2 speciali*") AND ("Mismatch") Nutrient deficiency Combination of search words Date No. hits “bee" AND ("oligol*" OR "pollen speciali*" OR "food speciali*" OR "diet speciali*") AND ("Nutri* defici*" OR "Malnutrition" OR "Floral resorce*" OR 2023-10-10 1 "poor flower " OR "nectar qualit*" OR "poor nutrition") Synergism Combination of search words Date No. hits "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 0 speciali*") AND ("Synergis*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 56 speciali*") AND ("Interact*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 0 speciali*") AND ("Multiple") 2023-10-20 "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 1 speciali*") AND ("Additiv*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 34 speciali*") AND ("Combin*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND ("synergis*" OR "interact*" OR "multiple" OR "Additiv*" OR 2023-10-10 89 "combin*") Threat Combination of search words Date No. hits "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 1 speciali*") AND ("threat") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-21 8 speciali*") AND ("risk") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2023-10-20 21 speciali*") AND ("decline") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 19 speciali*") AND ("drive*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 4 speciali*") AND ("harm*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 34 speciali*") AND ("impact*") 2023-10-21 "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 1 speciali*") AND ("impaired") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet 2 speciali*") AND ("damage*") "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND ("threat" OR "risk" OR "decline" OR "stress*" OR "drive*" OR 2023-10-10 74 "harm*" OR "impact*" OR "impaired" OR "damage*") Impossible search combination Combination of search words Date No. hits "bee" AND ("oligol*" OR "pollen speciali*"OR "food speciali*" OR "diet speciali*") AND ("threat" OR "risk" OR "decline" OR "stress*" OR "drive*" OR "harm*" OR "impact*" OR "impaired" OR "damage*") OR ("Nutrition defici*" OR "Malnutrition" OR "Floral resorce*" OR "poor flower " OR "nectar qualit*" OR "poor 2021-10-10 928,030 nutrition") OR ("Climate change" OR "Mis match" OR "Desiccation" OR "Dehydrat*" OR "Drought" OR "Wildfire*" OR "Heat tolerance" OR "Thermal tolerance" OR "Landslide*" OR "Extreme weather" OR "Heavy rain*" OR "Extreme rain*" OR "Climate change*" OR "Global warming" OR "Flood*") 57