The unsustainable use of ecosystems by human societies has made 75–95% of the terrestrial biosphere to be altered by anthropogenic stressors, with considerable impacts on its fauna and flora (Kennedy et al., 2019; Ellis et al., 2021; Wagner et al., 2021). Among the myriad of threatened organisms are bees (Hymenoptera: Anthophila), a highly diversified group of pollinators represented by more than 20.000 described species globally (Michener, 2007; Zattara and Aizen, 2021). Their collapse in both abundance and diversity is a critical issue for ecosystems and human societies given the invaluable ecosystem services they provide in the perpetuation of numerous wild plants and crops worldwide (Matias et al., 2017; Ollerton, 2017; Porto et al., 2020). As for most threatened insects (Wagner, 2020; Wagner et al., 2021), bee decline involves climate change, landscape destruction, the misuse/overuse of pesticides, the arrival of invasive species and the transmission of pathogens as predominant drivers (Cariveau and Winfree, 2015; Carvalheiro et al., 2020; Mathiasson and Rehan, 2020; LeBuhn and Vargas Luna, 2021).

Bumblebees (Bombus spp.), known by ~270 species at the global scale, are a clade of bees mostly widespread across America, Europe and Asia, with distributions encompassing all biomes except extremely xeric deserts (Williams, 1998; Williams and Jepsen, 2021). These pollinators are increasingly used as models in a large array of scientific fields including ecology (Phelan et al., 2021), evolution (Tian et al., 2019; Wood et al., 2021b) and biogeography (Ghisbain et al., 2020; Potapov et al., 2021). Most concerningly however, bumblebees have increasingly drawn the attention of scientists worldwide due to the dramatic regressions of their populations across all continents they occupy (Bartomeus et al., 2013; Kerr et al., 2015; Aizen et al., 2019; Rasmont et al., 2021). These repeated patterns of decline have converted them into popular conservation entities, making them the second most studied bees worldwide after the honeybee Apis mellifera (Cameron and Sadd, 2020).

Against this context of global bee decline, Wood et al. (2020) discussed the relevance of using the A. mellifera as a sensor for wild bee decline. The authors showed that despite some of the responses of honeybees to anthropic stressors being similar to those of wild bees, this managed species is not a suitable surrogate for detecting declines in wild bees. In this review, I raise a similar concern for bumblebees, discussing their relevance as models to global changes for other wild bees.

Bumblebees have always been immensely popular due to their great cultural value in north temperate regions (Rasmont et al., 2021). The overall colorful appearance, abundance and diversity of bumblebees in areas frequented by natural historians of the northern hemisphere have made them especially well-represented in museum collections (Kleijn and Raemakers, 2008; Cameron et al., 2011; Wood et al., 2019, 2021b). This has led scientists to rapidly acquire paramount quantities of data in comparison to other groups of wild bees (Nieto et al., 2014). As a direct benefit of having acquired so much biogeographic data on these culturally “cherished” animals is the speed at which their decline has been reported in Europe (Free and Butler, 1959; Williams, 1986; Rasmont and Mersch, 1988; Williams and Osborne, 2009). Among the array of insects that have shown dramatic declines since then, bumblebees showed an additional characteristic that would allow them to get even more attention: their ability to be domesticated (Velthuis and van Doorn, 2006). The immense economic importance of this management for global agriculture, and later the subsequent use of managed colonies for research purposes rapidly ranked bumblebees higher than ever in our conservation priorities (Cameron and Sadd, 2020; Rasmont et al., 2021). For all these reasons, bumblebees were intensively investigated compared to the rest of wild bees and are therefore usually used as main examples to illustrate the decline of other bee species. Research about their decline is now extensively cited in the scientific literature to illustrate the regression of other native/wild bees (e.g., literature cited in Carvalheiro et al., 2020; Reilly et al., 2020; Kammerer et al., 2021; Zattara and Aizen, 2021) but also of pollinators in general (e.g., in Potts et al., 2010; Sevenello et al., 2020; Dicks et al., 2021), and of even higher taxonomical levels such as insects and arthropods (e.g., in Harvey et al., 2020; Høye, 2020; Wagner, 2020; Halsch et al., 2021). It is not uncommon that works exclusively focusing on bumblebees are cited as references to support statements about threats to wild, native bees (e.g., in Winfree, 2010; Phelps et al., 2018; Vega-Hidalgo et al., 2020) or to pollinators in general (e.g., in Potts et al., 2016; Dicks et al., 2021). However, are bumblebees objectively that convincing as models for wild bee decline?

The end of the 1980s was marked by an important event in the study of bumblebees: the commercial domestication of the buff-tailed bumblebee Bombus terrestris (Velthuis and van Doorn, 2006). Beyond the purely economic interest of this feat, the availability of colonies of this species on demand has allowed scientists in many countries to thoroughly study its ethology, metabolism and their resilience to different drivers of decline such as heat waves or pesticides. All in all, the domestication of B. terrestris has made it possible to test ex situ the impacts of the Anthropocene on a bee that is different from the honeybee, and with an unprecedented possibility of experimental replication. Recent studies have however highlighted the over-performance of B. terrestris against global stressors (e.g., heat stress) compared to other bumblebees (Zambra et al., 2020; Martinet et al., 2021a,b), making it an exception among bumblebees in the Anthropocene (Ghisbain et al., 2021).

The metabolic and physiological performances of B. terrestris have been mostly discussed in the current context of geographic expansion and invasions in the Anthropocene (Geslin et al., 2017; Aizen et al., 2020; Ghisbain et al., 2021). Very quickly after its domestication began, the buff-tailed bumblebee became invasive in very many areas of the globe where it was originally absent (Rendoll-Carcamo et al., 2017; Aizen et al., 2019; Chandler et al., 2019). This expansion syndrome is relatively uncommon among bumblebees and other bees in the context of global change, being again the exception rather than the rule (Rasmont et al., 2015, 2021; Ghisbain et al., 2021). This pattern of geographic expansion in the buff-tailed bumblebee may be explained by a series of traits whose combination in a single species is relatively exceptional. Bombus terrestris has a large and highly flexible diet (Rasmont et al., 2008), produces larger pupae than other bumblebees even on resources of lower quality (Romain Moerman et al., 2016), copes well with a variable climate and extreme weather events (Zambra et al., 2020), is an early-emerging bumblebee with the ability to be active in winter (Stelzer et al., 2010) and seems to have an inherently high dispersal ability (Rasmont et al., 2015). Instead, numerous other bumblebee species display a more restricted diet (Wood et al., 2019, 2021b), cannot stand pronounced heat waves (Martinet et al., 2021a), are univoltine and tend to decline or remain stable rather than expand (Cameron et al., 2011; Cameron and Sadd, 2020). Overall, this drastic difference between B. terrestris and all other bumblebees is an additional point questioning the relevance of using it as model for other wild, solitary species.

The well-established and well-studied patterns of decline experienced by several bumblebees worldwide are very often used as example to illustrate the parallel non-Bombus bee decline, both at local (Drossart et al., 2019) and global scales (Potts et al., 2010, 2016). As a case study, I will focus here on the comparatively well-known European bee fauna, for which assessments of both bumblebee and wild bee population changes have been thoroughly reported (Nieto et al., 2014; Michez et al., 2019; Wood et al., 2020). In the latest European Red List of Bees, ~9% were assessed “threatened” against the IUCN criteria (Nieto et al., 2014). Importantly, more than a half of all European bees could not be assessed due to an insufficient amount of available data, making them fall in the category “Data Deficient.”

Figure 1 shows that regardless of the inclusion or not of the Data Deficient category, bumblebees appear to be especially threatened when separated from all other bees at the continental scale. Although results from Red Lists at national levels and following the same IUCN criteria also highlight the particular sensitivity of bumblebees against global changes (e.g., Drossart et al., 2019; Gogala, 2019; Quaranta et al., 2019), comparisons of that type remain nebulous. Indeed, the “Data Deficient” non-Bombus bees could include a substantial proportion of threatened species, making threatened non-Bombus bee ranging from ~4 to ~60%, respectively, if none or all data deficient were considered as threatened (Nieto et al., 2014). In addition, given that the Red Listing of Nieto et al. (2014) includes all historic data of European bees (including from areas where species have now vanished) and that bumblebee records have always been more represented in the last two centuries, both the EOO (Extent of Occurrence) and AOO (Area of Occupancy) calculations of the latter have benefitted from higher spatial resolution at the continental level. Overall, these fundamental sampling differences emphasize the difficulty to extrapolate trends from bumblebee to wild bee conservation, highlighting the need for an updated and more extensive sampling of the European bee fauna.

The fact that bumblebees are imperfect models for understanding wild bee conservation does not make them completely irrelevant for the understanding of the basic ecological requirements of wild bees. Research on bumblebees has provided key insight into the importance of maintaining connected habitats with an adequate supply of high-quality resources for all bees (Potts et al., 2016; Drossart and Gérard, 2020). The lessons learned from bumblebees concerning the impacts of biological pollution caused by domesticated organisms must be a warning signal to avoid such repercussions on other bees (Aizen et al., 2019; Bartomeus et al., 2020). The pathogen spillover by domesticated species is, for instance, a significant cause of decline in native bumblebee fauna and another growing concern for other native wild bees (Graystock et al., 2016). Similarly, the impacts of chemical pollution caused by plant protection products, repeatedly tested in bumblebees, should also be a cause for alarm for other bees (Sgolastra et al., 2020). Last but not least, the overall impact of climate change on bumblebees as a driving force leading to species extirpation (Kerr et al., 2015; Rasmont et al., 2015) can be used as a preliminary understanding of how climatic variations could affect the population dynamics of bees.

Although species-specific understanding of population trajectories would be ideal for selecting the bee species that are at greater risk, the paramount diversity of ecological traits (e.g., activity period, specialization in nesting substrate, social level, diet breadth) of wild bees at the global scale forces conservation biologists to use model species as surrogates. However, as for honeybees (Wood et al., 2020), the responses of bumblebees and wild bees to similar threats are generally not comparable. Although many aspects resulting from bumblebee sociality such as colony homeostasis, optimized food intake and broader diet breadth could make them particularly resilient under specific stressors, their overall tendency to be cold-adapted makes them especially disadvantaged in a warming environment. Overall, these results reinforce the need for conservation actions that consider a broader array of ecological diversity in wild bees, which in turn requires additional research into their biological requirements.

The author confirms being the sole contributor of this work and has approved it for publication.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.