Field experiments predict ecological response to a large-scale epidemic of an anti-keystone pathogen.

Knowledge of how pathogens, in the context of a changing environment, shape the structure of ecological communities remains limited, and little information exists on the ability of small-scale field experiments to predict the consequences of large-scale environmental impacts. We addressed these issues using the emergence of a widespread epidemic (sea star wasting disease, SSWD) on rocky intertidal shorelines. Prior experiments demonstrated strong reductions in sessile species diversity in response to the local experimental removal of the sea star Pisaster ochraceus [R. T. Paine, Am. Nat. 100, 65-75 (1966); R. T. Paine, Oceologia 15, 93-120 (1974)] at our site. In surveys of sea star populations over 8 y spanning the onset of the SSWD epidemic, P. ochraceus populations declined by 90%, then subsequently recovered to about one third of pre-epidemic size under an endemic disease phase. Using four different types of temporal data, we documented large increases in populations of the competitively dominant mussel Mytilus californianus following P. ochraceus decline, which were of similar magnitude to those observed in prior field experiments. As in the field experiments, sessile species richness and diversity declined strongly following the onset of the SSWD epidemic as mussels were released from predator control. Our results reveal a strong effect of a pathogen in regulating ecological community structure by disrupting the role of a keystone species, a result anticipated by prior small-scale field experiments.