The importance of interannual variation and bottom up nitrogen enrichment for plant pollinator networks

Striking changes in food web structure occur with alterations in resource supply. Like predator prey interactions, many mutualisms are also consumer resource interactions. However, no studies have explored how the structure of plant pollinator networks may be affected by nutrient enrichment. For three years, we enriched plots of subalpine plant communities with nitrogen and observed subsequent effects on plant pollinator network structure. Although nitrogen enrichment affects floral abundance and rates of pollinator visitation, we found no effects of nitrogen enrichment on the core group of generalist plants and pollinators or on plant pollinator network structure parameters, such as network topology (the identity and frequency of interactions) and the degree of nestedness. However, individual plant and pollinator taxa were packed into the nested networks differently among nitrogen treatments. In particular, pollinators visited different numbers and types of plants in the nested networks, suggesting weak, widespread effects of nitrogen addition on individual taxa. Independent of nitrogen enrichment, there were large interannual differences in network structure and interactions, due to species turnover among years and flexibility in interacting with new partners. These data suggest that the community structure of small-scale mutualistic networks may be relatively robust to short-term bottom up changes in the resource supply, but sensitive to variation in the opportunistic behavior and turnover of plant and pollinator species among years.