Climate driven disruption of transitional alpine bumble bee communities

Pollinators at high elevations face multiple threats from climate change
including heat stress, failure to phenological match advancing flower
resources and competitive pressure from range-expanding species of lower
elevations. We conducted long-term multi-site surveys of alpine bumble
bees to determine how phenology of range-stable and range-expanding
species is responding to climate change. We ask whether bumble bee
responses generate mismatches with floral resources, and whether these
mismatches in turn promote community disruption and potential species
replacement. In alpine environments of the central Rocky Mountains,
range-stable and range-expanding bumble bees exhibit phenological
mismatches with flowering host plants due to earlier flowering of
preferred resources under warmer spring temperatures. However, workers of
range-stable species are more canalised in their foraging schedules,
exploiting a relatively narrow portion of the flowering season.
Specifically, range-stable species show less variance in phenology in
response to temporally and spatially changing conditions than
range-expanding ones. Because flowering duration drives the seasonal
abundance of floral resources at the landscape scale, we hypothesize that
canalisation of phenology in alpine bumble bees could reduce their access
to earlier or later season flowers. Warmer conditions are decreasing
abundances of range-stable alpine bumble bees above the timberline,
increasing abundance of range-expanding species, and facilitating a novel
and more species-diverse bumble bee community. However, this trend is not
explained by greater phenological mismatch of range-stable bees. Results
suggest that conversion of historic habitats for cold-adapted alpine
bumble bee species into refugia for more heat-tolerant congeners is
disrupting bumble bee communities at high elevations, though the precise
mechanisms accounting for these changes are not yet known. If warming
continues, we predict that the transient increase in diversity due to
colonization by historically low-elevation species will likely give way to
declines of alpine bumble bees in the central Rocky Mountains.