A factorial field manipulation reveals direct and indirect effects of ocean acidification and warming on community composition

Ecosystems are experiencing changes in multiple environmental factors, and these changes have the potential to influence species both directly and indirectly via altered species interactions. We conducted a factorial field manipulation of CO 2 (reducing pH) and temperature in twenty tide pools over a six-month growing season on the coast of Alaska, USA. Our manipulations resulted in unexpected increases in community evenness due to reduced abundance of dominant mobile herbivores, which reflected relationships with basal species. The climate factor most strongly related to population changes differed by species, and we observed greater changes upon exposure to single rather than multiple environmental factors. Total taxon richness did not differ between treatments over the time scale and magnitude of our experimental manipulations. These findings highlight the importance of long-term experiments in natural, dynamic ecosystems to account for environmental variation, community interactions, and behavioral responses of mobile species.