Species interactions in intertidal food webs: Prey or predation regulationof intermediate predators?

Most natural food webs have more than one predator species, and many have t rophic interactions among these predators. When a top predator feeds on an intermediate predator and they both feed on a shared basal resource, a phen omenon labeled intraguild predation (IGP), the potential exists for complex food web dynamics due to predation and competitive effects. Here we invest igate the relative importance of direct predation vs. competition by a top predator on an intermediate predator. The study system is the rocky intertidal interaction web formed by the pred atory seastar Pisaster ochraceus, the predatory whelks Nucella emarginata a nd N. canaliculata, and a shared resource species, the mussel Mytilus tross ulus. Previous experiments documented strong negative effects of Pisaster o n mussels and whelks, but the mechanisms responsible for the effects on whe lks, whether competition or predation, were not identified. Here we report results of a field experiment that manipulated both Mytilus and Pisaster to determine the short- and longer-term changes in whelk populations. Using a simplified dynamic model for changes in abundance over the initial stages of the experiment, we separated and quantified the top-down effect of direc t predation by seastars vs. the bottom-up effects of competition and food l imitation. Short-term results were in agreement with longer-term responses. Results suggest that direct and indirect bottom-up influences of mussels w ere far stronger than predation, and thus, whelk increases in the absence o f seastars were due to reduced competition with Pisaster. Large differences between the body sizes of seastars and whelks make it difficult to determi ne the ultimate nature of the resource under competition between predators. Small mussels may constitute only a food resource for seastars, but to som e extent, they also represent a microhabitat for whelks. Differences in the magnitude of the response to mussel manipulations between Nucella species might be due to slight differences in the way the species utilize mussel be ds. Some of the predictions of theoretical IGP models regarding coexistence and stability of species may not apply to this interaction web because it includes species with both "open" and "closed" populations, rather than jus t closed populations as assumed by the models.