Range-wide genetic analysis of an endangered bumble bee (Bombus affinis) reveals population structure, isolation by distance, and low colony abundance

Authors: MOLA, LOHN - Colorado State University; PEARSE, IAN - Us Geological Survey (USGS); BOONE, MICHELLE - University Of Minnesota; EVANS, ELAIN - University Of Minnesota; HEPNER, MARK - Metamorphic Ecological Research & Consulting; JEAN, ROBERT - Environmental Solutions & Innovations, Inc; KOCHANSKI, JADE - University Of Wisconsin; NORDMEYER, CALE - Minnesota Zoological Garden; RUNQUIST, ERIK - Minnesota Zoological Garden; SMITH, TAMARA - Us Fish And Wildlife Service; STRANGE, JAMES - The Ohio State University; WATSON, JAY - Wisconsin Department Of Natural Resources; Koch, Jonathan

Submitted to: Conservation Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Rusty patched bumble bee (B. affinis) is now extirpated in ~70-90% of its former range. Our population genetic study shows that there is current population structure in this species, roughly along an east-west gradient and with three major clusters, representing Minnesota, the rest of the upper Midwest, and Appalachia. At local scales, there is evidence for a loss of genetic diversity, and many population sizes, based on genetic mark-recapture, are very small compared to those of other bumble bee species where this has been quantified. These results are not entirely surprising for a species that has undergone a recent and rapid decline. However, they serve as a baseline for understanding the local and regional status for this species. Likewise, they point to clear information gaps about dispersal, demography, and genetic health where novel research may be of particular value. The conservation and recovery of B. affinis depends on population and genetic health, and our study provides the first evidence defining those factors and providing a baseline for this endangered bumble bee.

Technical Abstract: Declines in bumble bee species are well documented across multiple continents and have prompted the need for informed management plans to aid in species recovery and conservation of bumble bee communities. The rusty patched bumble bee (Bombus affinis) is the first federally-listed bumble bee species in North America. To inform conservation efforts for this species, we report the results of the first range-wide population genetics study of B. affinis from across all extant conservation units. To understand the species vulnerability to ongoing environmental change and help establish informed conservation units, we report results on population structure, patterns of genetic diversity, and population differentiation. Additionally, we conducted site-level analysis of colony abundance to aid in prioritizing areas for conservation and inform potential translocation and other recovery programs. We find substantial evidence of population structuring along an east-to-west gradient within the species range. Putative populations show evidence of isolation by distance and high inbreeding coefficients (FIS) in some populations and a range wide male diploidy rate of ~15%. Our results suggest the Appalachians represents a genetically distinct cluster with high levels of private alleles and substantial differentiation from the rest of the extant range. Site-level analyses suggest low colony abundance estimates for B. affinis when compared to similar datasets of stable, co-occurring species. Together, these results lend genetic support to trends from occupancy and observational studies suggesting that B. affinis has undergone a recent decline and exhibits substantial spatial structure. The low colony abundances observed here suggest caution in overinterpreting the stability of populations even at sites where B. affinis is regularly detected year-to-year. These results should help delineate more informed management units and provide context for the potential risks of translocation programs. More targeted work on subpopulations or with clear hypotheses stemming from this initial work will be needed to further advance our findings.