Patterns and processes of compositional change in a California epibenthic community

As human modifications of the earth's systems have increased, so has interest in understanding past changes in order to predict future ecological trajectories. We compared historical (1969-1971) and contemporary (2005-2009) abundances of species in the marine epibenthic community of Bodega Harbor, California, USA. Between these 2 time periods, we found a decrease in the abundance of native species and an increase in non-native dominance, including of several species that were either rare or absent similar to 35 yr ago and whose introduction was likely human-mediated. This compositional shift was concurrent with an increase in local water temperature of similar to 1 degrees C over the same interval. To address the potential role of ocean warming in facilitating the increase of the new dominant species and maintaining compositional shifts, we evaluated the correlation between temperature and recruitment for 15 species. We found that recruitment timing and magnitude were positively related to temperature for non-native species but not for native species overall. Combined with previous results suggesting effects of ocean warming on the relative performance of native vs. non-native species in this community, our study indicates the potential for continued dominance of non-native species in Bodega Harbor due to local temperature increases. Simultaneously, anthropogenic transport has been responsible for several recent introductions of competitively dominant species, and shifts in contaminant loads or other factors between the 2 time periods could also contribute to compositional shifts, both historically and in the future. Our results highlight the need for studies of these additional factors, as well as the mechanisms underlying their effects on compositional shifts, in order to predict future changes.