Effects of the starfish Patiria miniata on the distribution of the sea urchin Lytechinus anamesus in a southern Californian kelp forest.

At two stations in the San Onofre kelp bed near San Clemente, California the abundance of sea urchins, Lytechinus anamesus, and starfish, Patiria miniata, were negatively correlated. At one station, urchins were more abundant outside compared to inside the kelp bed, a pattern which generally occurs in southern California. In contrast, at the other station urchins were more abundant inside the bed. This exception to the general rule indicates that the distributions were not simply determined by something associated with the presence of kelp. Samples of substrates, sea urchins and sea stars suggested that the distribution of Patiria but not Lytechinus was controlled by the availability of suitable substrates. Further experiments showed that: 1) Patiria elicit an escape response from Lytechinus, the strength of which is positively related to the size of the urchins, 2) small urchins, which react less strongly to Patiria, are preyed on more heavily than are larger ones, and 3) escape responses elicited by Patiria cause local reductions of Lytechinus in the field. We conclude from these results that predation by Patiria controls the distribution of Lytechinus at our study sites, and may account for this species distribution in kelp forests throughout southern California. The possible effects of this predator-prey interaction on the structure of kelp forest communities are discussed.