Diversity, heterogeneity and consumer pressure in a tropical rocky intertidal community

Summary

Previous studies indicated that at Taboguilla Island (Gulf of Panama), persistence of many intertidal organisms depended on holes and crevices in the rock as refuges from both vertebrate (fishes) and invertebrate (crabs, gastropods, chitons) consumers. Here, we evaluate the influences of substratum heterogeneity and consumers on patterns of diversity of sessile organisms in this habitat. Local substratum topography is highly variable, ranging from smooth to irregular surfaces. Algal crusts typically dominate all low zone rock surfaces, and most other sessile spegies (invertebrates and foliose algae) occur in holes and crevices. Number (S) and diversity (H′) of sessile species is lower on homogeneous surfaces than on heterogeneous surfaces. Rate of increase in S with area sampled is positively correlated with substratum heterogeneity; number of species sampled per transect at a homogeneous site would be about 10 vs 30 to 60 on a heterogeneous site. Large fishes and crabs forage intensively over both substratum types, but cannot enter holes and crevices to eat prey. Gastropods, chitons, limpets, and small crabs feed on both substrata but vary in abundance from hole to hole. Prey mortality is thus intense and constant on open surfaces, but variable in space and time in holes and crevices. When consumers are excluded from the general rock surface, algal crusts are settled upon and overgrown by foliose algae, hydrozoans, and sessile invertebrates, particularly bivalves. Both S and H′ first increase, as sessile species invade and become more abundant, and then decrease as the rock oyster Chama echinata begins to outcompete other species and dominate primary space. Hence, consumers normally keep local diversity low by removing most sessile prey from open surfaces.

In these experiments, a consumer pressure gradient was established by removing 0, 1, 2, 3, and all of 4 distinct groups of consumers. As predicted by the intermediate disturbance hypothesis, lowest diversity occurred at lowest (total exclusion) and highest consumer pressure (normal condition). Highest diversity occurred at intermediate consumer pressure. Unexplained variation in this relationship is probably due to quantitative and qualitative differences in consumer regime, variation among plots in substratum heterogeneity, and insufficient time for competitive dominance by Chama to be fully expressed. On a small (0.25 m²) spatial scale, consumers maintain low diversity by keeping prey scarce and causing local extinctions. On larger spatial scales, they may maintain and even produce high diversity through their interaction with substratum heterogeneity and possibly low dispersal rates of sessile species.