Meeting Abstract

7.10  Jan. 4  When keystones meet: sea stars and sea otters in central California VICKNAIR, K.E,; PEARSE, J.S.**; MCCLINTOCK, J.B.; FEDER, H.M.; Univ. of California, Santa Cruz; Univ. of California, Santa Cruz; Univ. of Alabama,Birmingham; Univ. of Alaska, Fairbanks pearse@biology.ucsc.edu

Ochre sea stars (Pisaster ochraceus) and sea otters (Enhydra lutris) are both recognized keystone species that profoundly influence community structure. However, there are no data on whether the two species influence each other. Sizes, densities, and prey of ochre sea stars were documented at three sites adjacent to Hopkins Marine Station in Monterey Bay, California in the mid-1950s, a decade before sea otters returned to the area. Data were collected again from the same sites 2, 3, and 4 decades after the sea otters returned. Sea-star mean size was reduced more than 50% at the exposed, mussel-dominated site in the mid-1980s, remained low in the mid-1990s, then slowly increased. However, density changes were reciprocal to size changes so biomass at the mussel-dominated site remained about half what it was in the 1950s. Mussels at the site increased in size and the bed expanded into the low zone until it was torn out by storms or sea otters in 2004. Heavy recruitment in 2005 resulted in extensive mussel cover throughout the intertidal in 2006 when sea-star density was very low. Sea-star densities declined steadily at the two more protected sites, and patches of mussels now occur at one of those sites. We propose that sea-otter predation on sea stars, particularly larger individuals, allows mussels to increase in size and mussel beds to extend into the low zone, becoming more susceptible to storms. The decline in sea-star abundance at the more protected sites probably is related to other factors, perhaps the development of a major harbor-seal haul out in the area, gull predation, or climate change.