Recent and future declines of a historically widespread pollinator linked to climate, land cover, and pesticides

Authors: JANOUSEK, WILLIAM - Us Geological Survey (USGS); DOUGLAS, MARGARET - Dickinson College; CANNINGS, SYD - Canadian Wildlife Service; CLEMENT, MARION - Us Fish And Wildlife Service; DELPHIA, CASEY - Montana State University; EVERETT, JEFFREY - Us Fish And Wildlife Service; HATFIELD, RICHARD - The Xerces Society; KEINATH, DOUGLAS - Us Fish And Wildlife Service; Koch, Jonathan; McCabe, Lindsie; MOLA, JOHN - Us Geological Survey (USGS); OGILVIE, JANE - Rocky Mountain Laboratory; RANGWALA, IMTIAZ - University Of Colorado; RICHARDSON, LEIF - The Xerces Society; Rohde, Ashley; STRANGE, JAMES - The Ohio State University; TRONSTAD, LUSHA - University Of Wyoming; GRAVES, TABITHA - Us Geological Survey (USGS)

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2022
Publication Date: 1/23/2023
Citation: Janousek, W.M., Douglas, M.R., Cannings, S., Clement, M.A., Delphia, C.M., Everett, J.G., Hatfield, R.G., Keinath, D.A., Koch, J., McCabe, L.M., Mola, J.M., Ogilvie, J.E., Rangwala, I., Richardson, L.L., Rohde, A., Strange, J.P., Tronstad, L., Graves, T.A. 2023. Recent and future declines of a historically widespread pollinator linked to climate, land cover, and pesticides. Proceedings of the National Academy of Sciences (PNAS). 120(5). Article e2211223120. https://doi.org/10.1073/pnas.2211223120.
DOI: https://doi.org/10.1073/pnas.2211223120

Interpretive Summary: One dramatic impact of the global biodiversity crisis is the rapid loss of once-common species and the potential cascading effects of their absences on ecosystems. We investigate the mechanisms of decline in a formerly common pollinator, the Western bumble bee (Bombus occidentalis), including the cumulative effects of climate, land cover, and pesticide use. Using 23 years of data with 14,457 surveys across 2.8 million km2 in the western conterminous United States, we demonstrate negative impacts of increasing temperatures and drought and identify neonicotinoids as the most likely pesticide type impacting B. occidentalis. Even in the best-case future scenario, occupancy declined in 44% of ecoregions by the 2050’s with projected declines in all ecoregions under more severe scenarios, ranging from 51% to 97%. The precipitous decline of this generalist species is a bellwether for potential loss across many taxa less well suited for ongoing environmental changes around the globe.

Technical Abstract: The global biodiversity crisis includes the loss of rare or formerly uncommon species, but also the rapid decline of common species across many different taxa. The loss of pollinating insects is of particular concern because of ecological and economic values these species provide. The Western bumble bee (Bombus occidentalis) was once common in western North America, but this species has become increasingly rare through much of its range. To understand the potential mechanisms driving these declines, we used Bayesian occupancy models to investigate the effects of climate, land cover, and pesticide use on this species from 1998 to 2020 and projected expected occupancy under three different future scenarios. Using 14,457 surveys across 2.8 million km2 in the western United States, we found strong negative relationships between increasing temperature and drought on occupancy and identified neonicotinoids as the likely pesticide of influence across our study region. Mean predicted occupancy declined by 57% from 1998 to 2020, ranging from 15% to 83% declines across 16 ecoregions. Even under the best-case scenario, we found continued declines in nearly half of the ecoregions by the 2050s and, under the most severe scenario, mean declines of 93% across all ecoregions. This assessment underscores the tenuous future of B. occidentalis and demonstrates the scale of stressors likely contributing to rapid loss of related pollinator species throughout the globe. To address these large-scale mechanisms, increased, international, species-monitoring schemes and integrating data, including formal surveys and community science, will support future assessments.