Role of catastrophic disturbance in mediating Nucella-Mytilus interactions in the Alaskan rocky intertidal

The mussel Mytilus trossulus is typically a dominant competitor and principal space-occupying species in the rocky mid-intertidal zone of south central Alaskan (USA). Its principal predator in the region is the whelk Nucella lima. We examined predator-prey interactions between M. trossulus and N. lima following a severe freeze which greatly reduced mussel abundance in the upper portion of their vertical range. N. lima in Kachemak Bay was locally abundant, but its spatial distribution was patchy, resulting in site-to-site variation in predation intensity. Following the freeze, mussel populations recovered to occupy 72% of primary space at a site where N. Lima was rare. Al sites where N. Lima occurred in high densities (>100 m(-2)), mussel cover remained significantly lower. At the latter sites, up to 81% of mussel mortality was due to snail predation. In field trials, average densities of N. Lima significantly reduced mussel cover within 2 wk. From published literature on Nucella sp. feeding rates and our data on in situ reductions in mussel beds, we estimate that N. lima can eliminate 60 to 90% of mussels at a given site in 1 season. In contrast to the general view that mussel recovery from disturbance is inevitable, our results indicate that local populations of young mussels were unable to increase space utilization because of intense N. lima predation. A stochastic, catastrophic disturbance event thus altered control of community structure by removing dense stands of large prey individuals that had grown to near-refuge size. This facilitated long-term N. lima control of what had been a mussel-dominated community by shifting the interaction toward smaller sized mussels that are susceptible to intense predation.