Recovery of an alpine Bombus community following drought-mediated population contractions

Background/Question/Methods

Apparent global declines in native pollinators have generated impetus to monitor their populations and understand the environmental pressures they face. There are a variety of natural and anthropogenic factors implicated in pollinator decline, and increases in global temperature and extreme weather events as forecast by climate models are predicted to exacerbate the problem. Experimentally testing hypotheses related to impacts of extreme events on population and community dynamics is practically difficult and/or unethical.  Instead, we took advantage of natural population contractions that occurred in a semi-annually monitored alpine Bombus community during the 2012 extreme drought in the Western U.S. to probe impacts to and resilience of native pollinator populations. The relative isolation of these mountaintop populations, located within the Pennsylvania Mountain Natural Area (Park County, CO), predicted that carry-over impacts from the drought would be observed in 2013.

We lethally sampled Bombus workers during peak forager abundance in 2013 to compare with past surveys, testing for pre- and post-drought differences in community composition, species diversity and richness, and tongue length.  To assess population genetic impacts, we also genotyped 2013 foragers of one species, the long-tongued alpine bumble bee, B. balteatus, following PCR amplification of microsatellite markers.  

Results/Conclusions

Results showed faster-than-expected recovery of the alpine Bombus community following drought-mediated population contractions. There were no pre-post drought differences in proportion of alpine vs. non-alpine species, mean worker tongue length, or Bombus species diversity or richness. B. balteatus effective population sizes (N_e) following “normal” and drought years were similar, and allele frequencies and distributions from 2013 showed no evidence of a genetic bottleneck.  Instead, one test (Sign Test for heterozygosity excess, a measure of genetic variability) revealed heterozygosity deficiency, indicating recent expansion and addition of alleles into the B. balteatus population. Findings from this study refute that mountaintop Bombus populations are isolated and are suggestive of active dispersal and gene flow among populations. Source-sink dynamics may promote regional stability of alpine species and communities and buffer them from detrimental impacts of short-term extreme weather events.