Moving on up: Upslope migration of subalpine bees alters the structure and dynamics of an alpine bumblebee network

Link To Share This Poster: https://cdmcd.co/v9DKrG

Valerie Martin, Biology, Utah State University, Logan, UT, Cassidy E Bishop and Nicole E. Miller-Struttmann, Department of Biological Sciences, Webster University, St. Louis, MO

Background/Question/Methods
With warming, the climate of some bumble bee populations is exceeding their observed climate tolerances, resulting in populations declines and shifts in altitudinal ranges. Bumble bee behavior varies substantially with altitude, influencing plant-bumble bee interaction networks. Historically in the Colorado Rock Mountains, bumble bee visitation networks were more highly nested in the alpine than in the subalpine, reflecting lower niche partitioning in the alpine. As subalpine bees have moved up slope in response to warming, we predict that alpine bumble bee foraging networks have become less highly nested and less robust to perturbations. To test these predictions, we compared visitation networks of bumble bees collected in 2012-14 to those collected by L.W Macior’s at the same locations (Mt. Evans and Niwot Ridge, Colorado, U.S.A.) in 1966-69. We quantified network structure (i.e., weighted nestedness, connectance, and modularity) and robustness to extinction for the “past” and “modern” networks pooled across locations as years, and calculated z-scores standardized against a null model to control for network size. We then resampled the past and modern visitation networks to estimate confidence limits for each metric and tested for a difference between the past and modern network structure and robustness.
Results/Conclusions
The structure of the modern bumble bee visitation network was unique relative to past alpine or subalpine networks. The modern alpine community had lower nestedness than the past alpine network but higher than the past subalpine network. Connectance and robustness to extinction (regardless of the type of extinction cascade modeled) of the modern network was lower than either past network. On the other hand, modularity of the alpine network was unchanged and similar to that of the past subalpine network. Based on a space-for-time substitution, we had predicted that the structure and dynamics of this modern alpine community would appear more “subalpine-like”, as subalpine bees have migrated upslope. While nestedness of the modern network was intermediate to that of past alpine and subalpine networks, connectance and robustness were lower in the modern network relative to either past network. These results suggest that this modern alpine visitation network has a unique structure that may be more susceptible to future perturbations. We discuss how differences in the life history traits of the interacting organisms (i.e., mobile, short-lived pollinators vs. long-lived plants with short dispersal distances) may mediate the effects of climate change on network structure and stability.