Population genetics of museum specimens indicate decreasing genetic diversity in Bombus occidentalis, a species of conservation concern

Authors: ROHDE, ASHLEY - Utah State University; Branstetter, Michael; MOCK, KAREN - Utah State University; Knoblett, Joyce; PILLIOD, DAVID - Us Geological Survey; EVERETT, JEFFREY - Department Of Fish And Wildlife; GALPERN, PAUL - University Of Calgary; STRANGE, JAMES - The Ohio State University

Submitted to: Biological Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Bombus mckayi and B. occidentalis are two important species of montane bumble bees in western North America that, until recently, were treated as a single species. Populations of B. occidentalis (Western bumble bee) have decreased in abundance in portions of the range since the mid 1990’s, while populations of B. mckayi (McKay’s bumble bee) appear to be stable. Updated population genetic analyses to assess the genetic resiliency of both species are needed to inform conservation decisions. Researchers used museum specimens collected between 1960 and 2020 to examine genetic structure and diversity in populations of B. mckayi and B. occidentalis using microsatellite markers. They found evidence of decreasing genetic diversity through time in B. occidentalis, but not B. mckayi, and that the rate of change for B. occidentalis did not correlate with the dramatic decrease in abundance observed in the 1990’s. Although both species have significant remaining genetic diversity, decreases in genetic diversity through time may compound the challenges of recovery for B. occidentalis. This work will help inform decisions regarding whether or not B. occidentalis should be listed under the U.S. endangered species act.

Technical Abstract: Studies have noted decreases in abundance and range within bumble bee species around the world. Bombus mckayi and B. occidentalis are two species of montane bumble bees in western North America that, until recently, were treated as a single species. Populations of B. occidentalis (Western bumble bee) have decreased in abundance in portions of the range since the mid 1990’s, while populations of B. mckayi (McKay’s bumble bee) appear to be stable. A need for updated population genetics analyses to assess the genetic resiliency in both species has been identified by bumble bee experts. Studies of the genetic characteristics of populations of B. occidentalis and B. mckayi have been previously published, but none treated the taxa as separate species, and none directly measured changes in population genetic characteristics through time. We used museum specimens collected between 1960 and 2020 to examine genetic structure and diversity in populations of B. mckayi and B. occidentalis using microsatellite markers. We found evidence of decreasing genetic diversity through time in B. occidentalis, but not B. mckayi. The rate of change of genetic diversity through time for B. occidentalis does not correlate with the dramatic decrease in abundance observed in the 1990’s, but decreases in genetic diversity through time may compound the challenges of recovery for B. occidentalis. Geographic patterns of genetic structure were also different between the species. This study disentangles the population genetic structure of two widely distributed and closely related North American bumble bee species, one declining in abundance and one stable.