Diversity, Generalization, and Specialization in Plant-Pollinator Networks of Montane Meadows, Western Cascades, Oregon

Despite the importance of plant-pollinator interactions for ecological communities, few long-term observational studies have been conducted of plant-pollinator networks. Using four years of plant-pollinator interaction data from 18 meadows in the Willamette National Forest, Oregon, this thesis examined how meadow size, the amount of nearby meadow habitat, weather, degree days, and soil moisture are related to the frequency of plant-pollinator interactions. Spatial and temporal turnover of plant and pollinator assemblages (based on the Jaccard dissimilarity index) were compared between meadows in a given year and within a meadow in different years. The degree distributions of these plant-pollinator networks were constructed, and analyses were conducted to determine the prevalence of generalization in the component species, and the possible presence of modularity in some of the meadows. Pollinator assemblages varied little in space: pollinator communities were very similar in the 18 study meadows in a given year, but pollinator communities differed dramatically between years. In contrast, plant assemblages had high turnover: plants differed dramatically between nearby meadows in the same year, and plant turnover was also high within meadows between years. Network structures were dominated by species that were very well connected, and they contained more well-connected species than would occur in a randomly-assembled network. Plant and pollinator assemblages in interaction networks also were dominated by generalist species. Some evidence of modularity occurred in small meadows in the network. The results of this study are consistent with many published studies that have found that generalization in plant-pollinator networks promotes their resilience over time in spite of the high component species turnover occurring between growing seasons. These results provide little support for the hypothesis that pollinator networks in fragmented habitats are fragile and highly sensitive to the loss of individual species. However, the high spatial heterogeneity among the meadows in this study, shown by dissimilarity in flowering plants, and the high permeability of the forest separating the meadow habitats, shown by the similarity among pollinators, both contribute to the generalization and resilience of these networks. Future work on plant-pollinator networks should focus on naturally fragmented, heterogeneous habitats, and continue to observe long-term changes in pollinator assemblages.