Contrasting patterns of richness, abundance, and turnover in mountain bumble bees and their floral hosts

Environmental gradients generate and maintain biodiversity on Earth.
Mountain slopes are among the most pronounced terrestrial environmental
gradients, and the elevational structure of species and their interactions
can provide unique insight into the processes that govern community
assembly and function in mountain ecosystems. We recorded bumble
bee-flower interactions over three years along an 1400 m elevational
gradient in the German Alps. Using nonlinear modeling techniques, we
analyzed elevational patterns at the levels of abundance, species
richness, species β-diversity, and interaction β-diversity. While floral
richness exhibited a mid-elevation peak, bumble bee richness increased
with elevation before leveling off at the highest sites, demonstrating the
exceptional adaptation of these bees to cold temperatures and short
growing seasons. In terms of abundance, though, bumble bees exhibited
divergent species-level responses to elevation, with a clear separation
between species preferring low vs. high elevations. Overall interaction
β-diversity was mainly caused by strong turnover in the floral community,
which exhibited a well-defined threshold of β-diversity rate at the tree
line ecotone. Interaction β-diversity increased sharply at the upper
extreme of the elevation gradient (1800-2000 m), an interval over which we
also saw steep decline in floral richness and abundance. Turnover of
bumble bees along the elevation gradient was modest, with the highest rate
of β-diversity occurring over the interval from low- to mid-elevation
sites. The contrast between the relative robustness bumble bee communities
and sensitivity of plant communities to the elevational gradient in our
study suggests that the strongest effects of climate change on mountain
bumble bees may be indirect effects mediated by the responses of their
floral hosts, though bumble bee species that specialize on high-elevation
habitats may also experience significant direct effects of warming.