Keystone intimidators in the intertidal:non-consumptive effects of a keystone sea star regulate feeding and growth in whelks

摘要

Predators can exert strong controls on community structure through both consumptive and non-consumptive effects. When one of these impacts on diversity is large, such predators have been labeled keystone predators and keystone intimidators respectively. Here we demonstrate that some predators play both roles simultaneously. We studied the species for which the term keystone predator was originally coined, the ochre sea star Pisaster ochraceus. We observed non-consumptive effects that were not mediated through its well-known invertebrate prey, the competitive dominant mussel Mytilus californianus, but instead through whelks Nucella emar-ginata, another important intertidal predator. Whelks exposed to effluent from stars in the laboratory consumed significantly less, grew significantly less, and developed less reproductive and digestive tissue. Star presence also altered the feeding preferences of whelks. Results suggest these responses are non-linear with respect to the number of predators and thus predator presence may be a larger determinant of non-consumptive effects than density. A second experiment found, in agreement with previous field surveys, that these large non-consumptive effects occurred even though whelk mortality due to consumption by stars is extremely low. These results offer a new mechanism to explain previous field experiments that found whelks have little impact on mussel mortality in the presence of stars yet large effects when stars are absent. They demonstrate the potential for non-consumptive effects to play a major role in community regulation by a keystone predator. They also demonstrate that non-consumptive interactions may have large effects on species even when rates of consumptive mortality are exceedingly low.