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

The mussel Mytilus trossulus is typically a dominant competitor and pnncipal spaceoccupying specles in the rocky mld-intertidal zone of south central Alaskan (USA) Its pnncipal predator in the region is the whelk Nucella hma We examined predator-prey lnteractlons between M trossulus and N l ~m a following a severe freeze which greatly reduced mussel abundance in the upper portion of their vertical range A T l m a in Kachemak Bay was locally abundant but its spatial distnbution 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 llma was rare At sites where N lima occurred in high densities ( > l 0 0 m-2], mussel cover remained significantly lower At the latter sites up to 81 % of mussel mortality was due to snail predation In field tnals average densities of N lirna s~gnificantly reduced mussel cover within 2 w k From published literature on Nucella sp feeding rates and our data on in situ reductions In mussel beds, we estimate that N lima can ehmlnate 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 hma 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 Thls facilitated long-term N hma control of what had been a mussel-dominated community by shifting the interaction toward smaller sized mussels that are susceptible to intense predation