Resource-dependent dispersal promotes spatial food-web persistence in a rock-pool metacommunity
In fragmented landscapes, the dispersal of individuals among local habitat patches connects isolated populations and food webs at a larger, regional spatial scale, often with important dynamical consequences (e.g., spatial synchrony). As we know from optimal foraging theory, however, many mobile organisms do not disperse randomly among patches, but move in response to the availability of local food resources, which they also directly affect via consumption. As a result, dispersal may both influence and be influenced by local food-web dynamics, creating a feedback across spatial scales. This local-regional feedback has been theorized to permit locally unstable trophic interactions to persist regionally, as long as within-patch dynamics are not synchronized across space, but to date has received little empirical attention. Here, I combine observational, experimental, and theoretical approaches to explore this local-regional feedback and its dynamical consequences in a system of freshwater rock pools on Appledore Island, Maine. The pools contain a relatively simple food web dominated by a tri-trophic food chain of Chlorophyte algae, zooplankton grazers (primarily Moina macrocopa), and a predatory insect (Trichocorixa verticalis).
Lab and mesocosm experiments demonstrate that Trichocorixa is indeed a highly voracious apex predator that can strongly reduce local Moina densities. This predation may be high enough to drive local Moina populations extinct, particularly when Trichocorixa populations are primarily comprised of adults (the most voracious life-history stage). However, Trichocorixa adults are also capable of flying among pools, and a mesocosm experiment shows that their emigration rates decrease as a function of Moina densities (i.e., negative resource-density-dependent dispersal or NRDD dispersal; as predicted by optimal foraging theory). I used a Monte Carlo modeling approach to combine the predation and emigration data sets, and show that Trichocorixa’s NRDD dispersal may contribute to the spatial persistence of Moina by creating temporary refuges from predation where prey densities are low (effectively converting a local Type II functional response into a regional Type III functional response). These dynamics appear to occur in the field, as shown using observational data from three pools, where the apparent emigration of adult Trichocorixa prevented the extinction of local Moina populations by relaxing predation pressure. Taken together, these studies suggest that the dynamics and persistence of food webs in spatially fragmented landscapes are driven by a feedback between local trophic interactions and regional dispersal.