SYMP 13-2 - Quantitative specialization by pollinators favors rare plants and plant coexistence

Investigating the effects of animal pollination on plant populations and communities has gained much recent attention due to ongoing global pollinator decline. One active avenue of research has focused on the effects of pollinators on plant coexistence. Some studies have found that pollinators increase positive density dependence among plants, favoring abundant over rare plant species which, in turn, may result in abundant species excluding rare ones over longer timescales. Other studies have found that pollinators favor rare plants, supporting the idea that pollinators enhance negative density dependence among plants. Here, we use data-theory integration to investigate the effects of pollinators on plant coexistence in the context of plant-pollinator networks. We first used a consumer-resource dynamic model on thousands of algorithm-generated networks with a wide range of varying structures to evaluate how network structure and pollinators’ adaptive foraging affect pollen deposition rate and, therefore, plant fecundity. We found that networks exhibiting adaptive foraging and structures like the ones observed in nature (i.e., nested, moderately connected) exhibited the highest pollen deposition rate. This result was driven by two mechanisms. First, pollinators quantitatively specialize on specialist plants, preferring them over generalist plants. Second, this quantitative specialization (fraction of visits to a plant species over all visits performed by a pollinator species) increases conspecific pollen deposition rate and, therefore, plant fecundity. We then used empirical plant fecundity data (seed set) and visitation network data from the Colorado Rocky Mountains, in which plant density covaries with plant generality (degree or number of interactions), to test these two model predictions. Using generalized linear mixed models and controlling for plant abundances, we found that pollinators prefer rare over abundant plant species and that increased quantitative specialization by pollinators enhances plant fecundity. These results support the hypothesis that pollinators promote plant coexistence by enhancing negative density dependence among plants, favoring rare over abundant species. Our work also shows that binary nested but quantitatively anti-nested network structures produce the highest ecosystem services (in terms of plant fecundity), which is driven by the niche partitioning caused by adaptive foraging. Our contribution shows how data-theory integration can elucidate mechanisms maintaining the biodiversity of natural ecosystems.