All model results, regardless of climate change scenarios or assumptions about dispersal capacity, suggest that significant declines of bumblebee species across much of North America are likely. Models reveal large range losses even in scenarios where dispersal abilities are estimated using the highest recorded dispersal rates for an invasive bumblebee species (10 km/year)30. Across a range of realistic dispersal rates, few bumblebee species are likely to maintain stable geographical range sizes, let alone track warming rapidly into historically-unoccupied areas beyond species’ current ranges. Even pollinators known to disperse at especially high rates under climate change, such as some butterfly species, have accumulated substantial climate debt29,63 – the lag between species actual colonization of new areas and the rate required given the pace of climate change –, a prospect our results suggest is likely to be especially severe among bumblebee species.

Even assuming unlimited dispersal, about half of modeled bumblebee species’ ranges are projected to decline in all future climate scenarios. Large interspecies variation arose because of differences in spatial shifts of species’ climatically suitable range into new areas and the extent to which areas in their current ranges were forecasted to become unsuitable. Range losses were focused in regions that are also strongly disturbed by various anthropogenic activities (e.g. agricultural and developed land). Areas where multiple species expanded (i.e. hotspots) were concentrated in regions poleward of current distributions, where developed or agricultural lands are limited62. The extent of hotspots was largest in areas east of the Rockies where the geographical trajectories of anticipated climate changes are mostly northward64. The relatively uniform topography of eastern regions generates weak climate gradients, so large latitudinal shifts are necessary for species to track changing climatic conditions17,65. Comparatively smaller distances must be traveled to track similar climatic changes within mountainous areas13. Under high or unlimited dispersal, range changes are more accentuated in 2050 than 2070 for some species, but only under strong climate mitigation (RCP2.6). The relatively low losses at most species’ trailing edge of this moderate RCP scenario were compensated more easily at the leading edge in 2070 while losses were not significantly worsened. Species-specific traits, however, ultimately determine species’ capacities to reach newly suitable areas in the future, regardless of environmental or anthropogenic barriers13,17, and may explain variation associated with bumblebees’ realised responses to climate change66. Bumblebees’ ability to reach and persist in new areas, necessary to maintain their range over time, is particularly important within agricultural areas where ecosystem services hold important economic and ecological value for human populations4.

Widespread species richness declines were predicted for agricultural areas of North America, where substantial changes in pollination networks are already expected due to known threats like land-use change, various chemicals used by the agro-industry, parasites and pathogens67. Bumblebee richness decline is expected to impair pollination services4. Subsequent negative impacts on food yields68 and on human welfare are also likely69. Changes in landscape connectivity and host plant abundance could drastically alter expected impacts4. Our results show that several species could be vulnerable to climate change in agricultural areas across the North American continent. Several species seem likely to require management intervention with a broad-scale perspective, to prevent potentially synergistic effects of agricultural practices and climate change on bumblebee decline.

Our approach is conservative in its assessment of potential range losses among bumblebees by excluding measures of potential land-use impacts from models. Including land-use data at appropriate spatial scale in Maxent modeling processes would probably assess species niches more broadly than purely on climatic grounds70. However, historical land-use data for North America is coarse thematically, temporally, and spatially. Detailed land-use models that address these limitations of resolution for the future do not exist but, regardless, are necessarily highly uncertain relative to climate models71.

Interactions between climate change and land-use change are expected to exacerbate species range losses in the future72, suggesting that future range declines may be even steeper than those reported here. Land-use change may hinder species dispersal under climate change while local climate changes may similarly interact with, and impair, species’ responses to land-use changes73. Land-use changes are likely to contribute especially to widespread biodiversity losses under strong climate mitigation scenarios (RCP2.6) due to rapid expansions of infrastructure like biofuel plantations72. Conversely, climate changes are likely to exert more severe negative impacts than land use changes under RCP8.572, which most closely reflects the trajectory of current emissions64,74. Consequently, range declines we project in this study likely provide an optimistic view of species realised future ranges, emphasizing the urgent need for effective bumblebee conservation strategies.

Less than 1% of areas we identify as hotspots for bumblebee range expansion are currently protected, according to the World Database on Protected Areas (WDPA). Recent efforts to rapidly expand protected area networks in Canada to improve prospects for conservation are likely to benefit many taxa, including bumblebees. Accounting for ecosystem services and species’ range dynamics consequent to climate changes are central to these efforts75.

Findings support the need for effective mitigation strategies that could benefit multiple species simultaneously, and aim to increase the likelihood of successful colonizations in areas beyond those occupied historically. Our data revealed relatively large and concentrated hotspots overlapping across climate scenarios, supporting broad-scale management plans for multiple bumblebee species. These hotspots include candidate sites where assisted colonisation efforts could be concentrated and would benefit several species76. Such efforts might help species maintain broader geographical ranges than would otherwise be possible, reducing the prospects of species extinctions and erosion of pollination services associated with rapid climate change26,28,77,78. Assisted colonisation precludes introductions of species across biogeographical boundaries28,79, avoiding the creation of non-analog ecological communities and should account for risks of introducing novel genotypes of pathogens to new areas. Bumblebees’ high risk of decline under climate change, the relative practicality of translocation, and manageable costs of relocating small numbers of fertilized queens in the spring, together, suggest that assisted colonisation should be evaluated alongside conventional conservation strategies focusing on habitat characteristics28. Assisted colonisation remains controversial on ethical grounds and due to potential risks of relocated species becoming locally invasive26,80,81,82.

Mitigation strategies can be directed toward areas where climate changes poses the greatest risks to bumblebee populations8,83. Landscape management to protect habitat across species’ dispersal pathways or corridors could facilitate range shifts84,85. Species rely on habitat availability beyond their historical range to disperse and track shifting climate conditions86. Further, habitat management in areas where climate-driven species losses are concentrated could slow range losses. Among species included in this study, such areas are subject to intensive land uses62. Habitat management can decrease pressures on bumblebees87, reducing the impacts of other threats and providing micro-refugia enabling bees to escape thermal extremes50. Microclimatic heterogeneity improves probabilities of survival at the edge of species’ distributions88 where they are most vulnerable to other threats89.

Even though species distribution models mirror mechanisms that modulate species distribution and hold ecological significance to predict past and future distribution37, they make a number of simplifying assumptions90,91. Models assume that the selected environmental variables are the main contributors to the position of species’ ranges, but other biotic and abiotic interactions have an effect on their distributions92. SDMs can also underestimate species’ niches because they assume that all suitable habitat is colonised, even though biotic or anthropogenic barriers can prevent species’ ranges from reaching equilibrium with environmental conditions90. Impacts of these assumptions are alleviated at the continental scale because climate is a main contributing factor to species distributions at this scale21. Further, factors like pesticides and land use change have not been observed to interfere with bumblebee range shifts in Europe and North America to date at those broad spatial extents7. SDMs can produce useful insights into how environmental factors shape species’ geographical ranges independently of complex species interactions or contingent factors50, but testing models over time to test their validity after periods of observed climate change is important93,94,95.

Bumblebee species’ dispersal capacities and generalized losses of climatically suitable areas make it likely that most bumblebee species included here will see substantial range losses in the coming decades. Widespread range losses for North American bumblebees seem likely even when assuming improbably rapid dispersal that has only been observed in the most extreme instances of Bombus terrestris invasion of new environments. Bumblebee species are particularly effective pollinators4 and their projected declines are especially pronounced in agricultural areas, so global change-induced erosion of those services could have both ecological and pronounced economic significance96,97. Discussions around whether, where, and for which species assisted colonisation should be considered are warranted, as are expanded efforts to manage habitats to retain species in areas where climatic conditions are likely to become less suitable in the near future.