Plant-pollinator community assembly: What is the role of generalists?

Background/Question/Methods   Patterns of biodiversity are closely tied to community assembly, or the repeated colonization and extinction of species. A potentially useful approach to studying assembly is network theory, especially for plant-pollinator communities, because the mutualisms may play a strong role in determining community composition. In particular, network analyses of real plant-pollinator communities emphasize the importance of generalists for minimizing the cascading effect of random extinctions. Less understood, however, is how these generalists may function during introductions. To examine this relationship, we developed a simulation model. In this model, plants and pollinators constitute nodes in a bipartite network, with weighted edges between them representing the interdependence of plants and pollinators, as determined by the complementarity of their nectar depths and proboscis lengths, respectively. Candidate species are sourced from a regional species pool, and their inclusion in the new community depends on the persistence of its mutualists and the weight of their interactions. To identify stable community compositions (steady states), we employ a synchronous Boolean framework, wherein a state transition graph shows all possible assembly trajectories. Introducing new species forces the system away from a steady state, which allows for new dynamics and the possibility of reaching a new steady state.

Results/Conclusions   Preliminary results show that relatively few of the possible community compositions are stable. The inclusion of species in the steady states appears to be associated with a species' degree of generalization. Generalists (species with high in-degree in the regional species pool) are present in nearly all steady states, whereas specialists (species with low in-degree in the regional species pool) are generally absent. This difference suggests that the presence of generalists does not necessarily promote the ability of a specialist to join a community. Also, even when the regional species pool contains several generalists, assembly trajectories can lead to both species-rich and species-poor steady states, suggesting that the order of the introduction of generalists may be critical. Thus far, it appears that although generalists may play an important role in maintaining biodiversity in mutualistic communities, their contribution to species richness during assembly requires more study.