Plant-pollinator networks under drought perturbation alters generality of interactions

Connor N. Morozumi, Program in Population Biology, Ecology, and Evolution, Emory University, Atlanta, GA, Beth M. L. Morrison, Biology, Stanford University, Stanford, CA and Berry J. Brosi, Department of Environmental Sciences, Emory University, Atlanta, GA

Background/Question/Methods
We lack clarity on how ecological networks change under perturbation. Diet theory predicts foraging niche expansion under resource reduction. If realized, this could have profound impacts on ecological network structure, yet whether this prediction plays out at the community level is unclear. To test this, we leveraged a plant-pollinator system with long-lived perennial plants that vary dramatically in flower and floral reward production in drought versus non-drought years. We assessed whether drought effects on available floral resources altered pollinator dietary niche breadth to drive higher network-level generalization, by comparing the structure of plant-pollinator networks in two drought years relative to two non-drought years. To understand the effect of reduced flower abundance in driving network structure we resampled the interactions present in non-drought years repeatedly until the networks equaled the number of total interactions found in drought years. We compared metrics of generality in these downsampled networks to those calculated in the raw drought networks. In drought years, we hypothesized that interactions would tend to become generalist, as niches broaden due to adaptive foraging. Additionally, we hypothesized that if drought reshapes networks independent of abundance, then generality metrics of downsampled networks would be statistically different from that of drought years.
Results/Conclusions
While we continue to analyze our results, our preliminary findings indicate that plant-pollinator networks in drought years were more generalized than networks in the non-drought years for metrics based on presence-absence but, intriguing, more specialized when assessing quantitative network structure. These network differences were statistically different from expectations if drought networks were just subsets with fewer total interactions of the non-drought network, indicating that reduced abundances of floral resources in drought years were not solely responsible for network pattern changes observed.

The implications of our study shine a light on how increased drought, due to climate change, may impact plant-pollinator mutualisms. Increased generalism might have wide ranging consequences for plant fitness and the pollination services plants derive from floral visitors. Increased generalism could lead to more interspecific pollen transfer and reduced plant fitness. Alternatively, broadened foraging niches within these systems might allow plants that otherwise have low pollinator visits to receive increased pollination services. When using interaction frequencies to assess network change we conversely found increasing specialization which might also have profound impacts to species persistence under perturbation. Under increased climatic variability, specializing intensively on a limited number of partners may make species more vulnerable when a partner fails.