Year-to-year variation in the topology of a plant-pollinator interaction network

Vigorous discussion of the degree of specialization in pollination interactions, combined with advances in the analysis of complex networks, has revitalized the study of entire plant-pollinator communities. Noticeably rare, however, are attempts to quantify temporal variation in the structure of plant-pollinator networks, and to determine whether the status of species as specialists or generalists is stable. Here we show that network structure varied through time in a montane meadow community from southern California, USA, in that pollinator species did not form the same links with plant species across years. Furthermore, composition of the generalized core group of species in the network varied among summers, as did the identity of those species involved in relationships that appeared to be reciprocally specialized within any one summer. These differences appear to be related to severe drought conditions experienced in the second summer of the 3 year study. In contrast to this variation, the pollinator community remained similarly highly nested in all three summers, even though species were packed into the nested matrix differently from year to year. These results suggest that plant-pollinator networks vary in detail through time, while retaining some basic topological properties. This dynamic aspect of community-scale interactions has implications for both ecological and evolutionary inferences about pollination mutualisms.