Prey size refugia and trophic cascades in marine reserves

After the establishment of marine reserves, trophic cascades, with an increase in top predators, decrease in herbivores and increase in producers, are often expected but not consistently observed. Recent empirical results suggest that the lack of cascades in a Caribbean coral reef reserve may be due to larger herbivores escaping predation. To explore the potential for such prey size refugia to prevent trophic cascades after reserve establishment, I construct a simple trophic model with and without herbivore size refugia and determine the conditions necessary for herbivorous fish to decrease after the elimination of harvest mortality. Generally, cascades do not occur and herbivores increase if the effect of harvest on herbivores before reserve establishment is greater than the effect of predation after reserve establishment. The parameter space where herbivores increase is much greater when accounting for size refugia. The potential for prey size refugia to prevent cascades makes it an important dynamic to consider in community-level approaches to reserve design and monitoring.