Landscape heterogeneity drives population structure in four western bumble bee species

Authors: SAKULICH, ELIZABETH - The Ohio State University; Koch, Jonathan; STRANGE, JAMES - The Ohio State University

Submitted to: Insect Systematics and Diversity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/2025
Publication Date: 2/15/2025
Citation: Sakulich, E.M., Koch, J., Strange, J.P. 2025. Landscape heterogeneity drives population structure in four western bumble bee species. Insect Systematics and Diversity. https://doi.org/10.1093/isd/ixaf003.
DOI: https://doi.org/10.1093/isd/ixaf003

Interpretive Summary: This study evaluates the effects of habitat suitability on gene flow across a broad landscape scale for four western species of bumble bees: Bombus flavifrons, B. melanopygus, B. mixtus, and B. sylvicola. Previous research found evidence of genetic structure and isolation between populations at a regional scale in B. mixtus and B. sylvicola in the Pacific Northwest. Here, we investigated whether these patterns existed at a broader geographic scale while incorporating habitat suitability into isolation models. We found varying degrees of genetic structure and isolation in all four of our study species. While the structure patterns varied between species, we identified potential conservation units with notable genetic differentiation across the landscape and provided regions of potential cryptic speciation. Our findings highlight the need to analyze populations further to understand phylogenetic relationships and how changing landscapes can further drive genetic differentiation.

Technical Abstract: Bumble bees are critical pollinators in wild, agricultural, and urban ecosystems—providing the necessary ecological services for food and crop production. In western North America, mountain ranges serve as areas of high bumble bee species richness. However, as climate change increases temperatures and restricts montane populations to higher elevational spaces, their ability to disperse and maintain genetic diversity decreases. This genetic isolation could lead to the extirpation of local pollinator communities and an overall loss of pollinators. We analyzed the genetic diversity of four broadly sympatric species of bumble bees across the Rocky and Cascade Mountains of western North America to assess habitat isolation's impact on population genetic structure. We expected species restricted to higher elevation habitats to display higher population structure and less genetic diversity across the landscape. We sampled approximately 150 bees per species from seven to eight sites across each species' range. We genotyped bees with an average of 10 loci and used FST and Bayesian Structure analysis to quantify population differentiation. Using isolation by distance and isolation by resistance analyses, we found evidence of habitat suitability restricting gene flow in species occupying both narrow and broad elevation gradients at varying degrees. Although each species showed varying degrees of genetic structure, knowing how habitat heterogeneity drives genetic structure and isolation can help guide conservation efforts and determine focal regions for bumble bee conservation in the face of climate change.