Spatial processes have a major influence on the persistence of species interaction networks and their resilience to environmental fluctuations. Using the framework of metacommunity theory, we sought to understand how the balance between local dynamics and dispersal may promote local and regional species persistence, in a network composed of the flowering plant snapdragon (Antirrhinum majus), its cohort of pollinators, and a specialist seed-predator and its parasitoid.
The interactions taking place within this system were investigated by an innovative approach consisting of in situplant observations, by determining the fate of flowers and fruits on flowering and fruiting stems at 16 study sites in its natural area, the Pyrenees Mountains. We then used this information to estimate spatial and temporal variation in the pollination rate, parasitism rate and hyperparasitism rate. Finally, we modeled community dynamics with a tritrophic Nicholson-Bailey model, and we tested three different combinations of local versus spatial processes on the empirical data.
Results/Conclusions
We found that the plants were pollinator-limited, with relatively variable fruit-to-flower ratios across sites. On almost all sites, plants were both parasitized and hyperparasitized, at a low to moderate rate. Comparing our field observations with the outputs of community dynamics simulations, we found that empirical data are not always consistent with the conditions for local tritrophic persistence in absence of spatial processes. This suggests that other mechanisms such as random disturbances and recolonizations (patch dynamics) or inter-site migration through metacommunity dynamics (source-sink dynamics) play a role in this system. Model simulations showed that dispersal could contribute to increasing tritrophic persistence in this system, and that source-sink structure, not just environmental stochasticity, may cause the observed pattern of spatial variation.
We suggest that the functioning of the snapdragon metacommunity is more consistent with source-sink than patch metacommunity dynamics, highlighting the extent to which dispersal explains the persistence of the system at a regional scale. We recently published our results in Journal of Ecology (2016: 104, 456-468).