Abstract: Forecasting how communities will respond to rapid climate change is a grand challenge for modern ecologists. In generalized consumer-resource communities, it is expected that climate warming will reshuffle pairwise interactions and subsequently alter community structure. Biodiversity loss driven by competition-induced local extinction is a potential consequence of altered community structure. It is therefore critical to quantify resource use among competing species across environmental gradients so that we can make better predictions about the fate of ecological communities under climate change. Alpine pollinators are highly biodiverse and provide important ecosystem services. As climate change alters pollinator phenology and range dynamics, we can expect rewiring of pairwise interactions with plants, which also experience climate-driven phenological and range shifts. Pollinator species persistence will depend on their ability to acquire suitable resources under shifting community dynamics. In an observational field study across an elevation gradient in the Swiss Alps, we asked whether bumblebee dietary profiles change in response to seasonal inter- and intraspecific competition intensity (co-forager density). We tested the hypothesis that bumblebee dietary profiles (pollen load species diversity) would become narrower in response to increased interspecific competition, which would indicate community-level resource partitioning. At the same time, diet breadth should increase under high intraspecific competition. In this case, we would expect population persistence across species, indicating a potential for long-term coexistence. Alternatively, a failure to partition floral resources could lead to population declines due to competitions, or novel diets could be nutritionally insufficient for population growth. We sampled flower-visiting bumblebees from alpine sites above and below tree line and identified their dietary profiles through DNA metabarcoding of corbicular pollen loads. Preliminary results indicate that diet breadth (species richness) increased for all focal species in response to increased conspecific density, as we predicted. However, species’ responses to heterospecific density were mixed. There were also site-level differences in the diet breadth – co-forager relationship. Future models will explicitly consider seasonality and floral resource abundance, which may shed light on our more puzzling results. We will also link diet breadth with bee fitness in future seasons through genetic mark-recapture techniques.
