As climate extremes become more frequent, bumblebees in North America and Europe are declining. However, some populations may be buffered through microclimate refugia. High elevation alpine environments have considerable microclimate variation and may act as refugia for cold-adapted species. Contrary to patterns seen in other regions, subalpine bumblebees in the Colorado Rocky Mountains are migrating upslope. Historically, competition for nesting sites restricted subalpine bumble bees from alpine habitats, but competition might be easing as the climate gets warmer and drier. Alternatively, the phenological match between bumblebees and their host plants may mediate effects of climate on bumblebee spring emergence times. Specifically, when late season floral resources are limited, gynes emerge earlier the following spring. We test direct and indirect effects of climate on the phenology of two resident alpine bumblebees and four subalpine bumblebees that have recently migrated upslope (hereafter “immigrants”). Weekly surveys documented bumblebee and flowering phenology on three mountains over five years that varied in winter precipitation and summer temperature. We tested (1) whether climate predicted the phenological match between bumblebee activity and flowering for resident and immigrant bumblebees, and (2) whether climate and/or a phenological match with flowers the previous year predicted the date of queen.
Results/Conclusions
The phenological match between bumblebees and flowers was influenced by both climate and origin (resident vs. immigrant). Emergence (date of first collection) was more likely to align with first flowering date for resident queens than immigrant queens. On average, immigrant queens emerged nine days later than resident queens. However, immigrant and resident queen emergence times were more closely aligned during warm, dry years. If nesting sites are limiting, earlier emergence may provide a competitive advantage. Origin did not mediate effects of climate on the phenological match during peak worker foraging, suggesting faster accrual of workers by immigrant bumblebees may compensate for later colony initiation. Regardless of origin, phenological disparity between peak worker activity and peak flowering increased with warmer summer temperatures. Males exhibited similar patterns, though this could only be tested in resident bees. A late season phenological disparity may have long-term impacts on population dynamics. Indeed, after controlling for the effects of climate, immigrant queens emerged earlier in years following summers with high worker phenological disparity resulting in synchronized emergence with resident queens. Our results suggest that a warmer, drier alpine environment may improve the odds for colonization by subalpine bumblebees competing with alpine residents for limited nest sites.