Status and Trends of the Rocky Intertidal Community on the Farallon Islands

Abstract

The Farallon Islands in the Gulf of the Farallones National Marine Sanctuary (GFNMS) is a 7-island chain located 48 km west of San Francisco, California. Since 1993, GFNMS biologists and associates have monitored algal and invertebrate species abundances on the intertidal shores of the 2 South Farallon Islands. The monitoring occurred 1-3 times yearly in 6 study areas. In each study area, 3-4 permanent, 0.15-m2 quadrats located between the upper and midintertidal zones were sampled for algal and sessile invertebrate cover and invertebrate counts. Taxonomic surveys were also completed to document other species in the vicinity of the sampling quadrats and to further characterize the sampling areas. Here we report monitoring results for the period 1993 to 2011. While species richness has remained relatively stable and high compared to the nearest mainland sites (Sonoma County through San Mateo County), there has been a slow, long-term net decline in the abundance of algal species and mussels at various sites on the islands. Causes for the declines remain unknown, but increased trampling from rising numbers of pinnipeds and increased waste from pinnipeds and seabirds are among the influences suspected to be important.