Trophic interactions are a potential structuring force for community dynamics in subtidal marine ecosystems, and predator responses to gradients in prey density have important implications for population regulation. Similarly, ecological resilience can be strongly dependent on species interactions across trophic levels, and maximizing resilience is one goal of ecosystem-based fisheries management. For example, predator-herbivore interactions on rocky reefs can potentially drive shifts between kelp forests and urchin barrens, and fishery harvest can play an important role in mediating these interactions. On rocky reefs in the Southern California Bight, where the archetypal sea urchin predator (the sea otter) is no longer present, top-down control of urchins by spiny lobsters, Panulirus interruptus, and California sheephead, Semicossyphus pulcher, has been hypothesized, but rarely tested experimentally. We conducted manipulative experiments to test for density-dependent predation mortality of sea urchins via the functional and aggregative responses of predators. We then developed a dynamic mathematical model to scale-up these empirical results and investigate the effects of simultaneous fishery harvest of predators and prey in an ecosystem subject to alternative stable states.
Results/Conclusions
In mesocosm-based feeding assays, spiny lobsters demonstrated a saturating, type II functional response to urchin prey (Strongylocentrotus purpuratus & Mesocentrotus franciscanus), whereby prey mortality decreased with increasing prey density. In field experiments on rocky reefs near San Diego, CA, predators (mostly the fish, California sheephead) inflicted positive density-dependent mortality on purple urchin prey, with mortality peaking at an urchin density of ~10 m-2. However, when larger, refuge-providing red urchins were available as an alternative prey, no density dependence was observed. Underwater videography revealed a positive relationship between purple urchin density and both the number and diversity of fish predators, but no correlations were observed when red urchins were present. These results demonstrate that predator regulation of urchins can occur only under limited circumstances which depend on both predator and prey community composition. Within our theoretical model, changes to predator fishing mortality allowed for alternative community states to exist, while manipulations of herbivore fishing mortality did not. However, fishing for herbivores interacted with predator harvest to influence ecological resilience by reducing the range of predator harvest rates at which alternative states were possible. Therefore, even if increasing prey harvest does not alter community structure directly, prey harvest can be important to understand ecosystem resilience and therefore the sustainability of both predator and prey populations.