Components of predation intensity in the low zone of the New England rocky intertidal region.

Predation intensity often increases along local gradients of decreasing environmental rigor. Assuming factors such as productivity and heterogeneity do not change along such gradients, potential factors responsible for this pattern include: (1) increased effectiveness per individual predator, (2) increased numbers of individual predators, (3) increased numbers of predator species of the same general morphological attributes, and (4) increased numbers of predator species of different general morphological attributes. I term these factors the components of predation intensity.In relatively protected low rocky intertidal regions of northern New England, community structure depends in part on the foraging activities of up to six species (three general types) of predators (Lubchenco and Menge 1978). These include three species of crab, two species of seastars, and one thaidid gastropod. This predator guild prevents mussel and barnacle populations from outcompeting Chondrus crispus. This red alga dominates space when predators are present, but is outcompeted by mussels when predators are excluded.Prey consumption rates (mg per predator individual per hr or mg g hr) in field experiments indicate that the rank from most to least effective predator type is crabs, seastars, and the gastropod. Statistically significant variations occur between predator types (largest differences), species of a given type, and individuals of a species (smallest differences). Estimates of the relative contribution of each species to total predation intensity in the low zone at several sites indicate that each predator species is a major predator at one or more sites. Thus, if one predator species in this guild becomes scarce, the other predators may increase their effects and reduce variation in the total predation intensity exerted by the guild. Comparisons with other systems suggest that increased diversity of types of foraging characteristics in predatory guilds is an important component of increased predation intensity along gradients of decreased environmental rigor.