Climate-related change in an intertidal community over short and long timescales

Changes in the abundance of macroinvertebrate species documented in a rocky intertidal community between surveys in 1931-1933 and 1993-1996 are consis tent with the predicted effects of recent climate warming. We resampled 57 0.84-m(2) plots of an intertidal transect first surveyed by W. G. Hewatt at Hopkins Marine Station (HMS), Pacific Grove, California, between 1931 and 1933. Replicating precisely the location of the plots and methodology used by Hewatt, we documented changes in the abundances of 46 invertebrate speci es, indicating that this intertidal community changed significantly during the 60 yr between surveys. Changes in abundance were related to geographic ranges of species. Most southern species (10 of 11) increased in abundance, whereas most northern species (5 of 7) decreased. Cosmopolitan species sho wed no clear trend, with 12 increasing and 16 decreasing. Although Hewatt d id not record algal species as thoroughly as invertebrates, we were able to document a massive decline in cover of Pelvetia compressa, a cosmopolitan fucoid alga that is typically more common in the southern part of its range . Shoreline ocean temperature, taken daily at HMS, warmed by 0.79 degrees C during this 60-yr period, with average summer temperatures up to 1.94 degr ees C warmer in the 13 yr preceding our study than in the 13 yr preceding H ewatt's. The hypothesis that climatic warming drove the observed range-rela ted community shifts is supported further by historical records and data fr om other investigators. Several alternative hypotheses to explain changes i n the invertebrate community at HMS, including habitat changes, anthropogen ic effects, indirect biological interactions, El Nino-Southern Oscillation (ENSO) events, and upwelling are considered to be less important than clima te change. Changes in species' abundances over a short period (3 yr) were r elatively small compared to large species shifts over 60 yr and were unrela ted to geographic range of the species, indicating that short-term populati on fluctuations play a relatively minor role in the long-term community cha nges that we observed.