When attention deficit pays: Bumble bee foraging breadth expands to compensate for declining flower resources under climate change

Background/Question/Methods: Climate change is shifting the resource landscape in which pollinators forage, potentially leading to novel interactions and selection regimes. Flower resource gaps and temporal mismatches between pollinators and their historic food plants have emerged as plants respond directly to climate change. For instance in response to drying soils and warming temperatures, arctic and alpine flowering plants exhibit advancing flowering schedules and declining flower abundances. While much has been speculated concerning the impacts of shifting flower resources for pollinators, few studies address effects on individual pollinator behavior. Often considered ‘canaries in the coal mine’ for their sensitivity to anthropogenic disturbance, alpine ecosystems may forecast potential impacts of climate change on plant-pollinator interactions. Here, we explore the effects of flowering declines in historically preferred food plants on bumble bee foraging behavior in alpine ecosystems of the Rocky Mountains. Optimal foraging theory (OFT) posits that, in order to maintain energy intake, foragers will compensate for declining flower abundances by incorporating new hosts, which will likely be more variable in floral traits and less well-matched morphologically.  We used behavioral observations and an OFT based-model to predict impacts of declining flower densities on bumble bee host choice and pollination services for alpine meadow plants.

Results/Conclusions: In response to a 2°C increase in mean minimum summer temperature, flower densities of historically preferred bumble bee host plants have declined by 22% over 35 years in the alpine meadows of Pennsylvania Mountain, CO (USA). In congruence with OFT, bumble bees (Bombus balteatus) foraging freely on Pennsylvania Mountain are more likely to switch to less-preferred, shallow flowers when flower densities are low. Additionally, alpine bumble bee foragers at Niwot Ridge Long Term Ecological Research station and Mount Evans Wilderness Area have become more generalized over 40 years, foraging from a greater diversity of plant species with shorter, more variable flower tube depths. Results from a foraging model that describes the energetic tradeoffs between specialist and generalist strategies suggest that these patterns may not be unique to alpine bumble bees. As resources decline and search times increase, the advantage of being a generalist forager increases. For example, the decline in flower densities on Pennsylvania Mountain has decreased the advantage of specializing by up to 14% in lower alpine zones. Both theoretical and empirical data indicate that pollinators will expand their foraging niches when resource density declines in response to climate change generating functional mismatches and reducing pollination quality for host plants.