COS 181-1 - Geographic variation in keystone predation

Abstract: The keystone predation hypothesis is a fundamental ecological theory, but whether its importance is widespread or geographically variable is poorly known. The original keystone predator, the sea star Pisaster ochraceus, is widely thought to exert top-down control of intertidal communities by eating the competitively dominant mussel Mytilus californianus, thereby allowing weaker competitors to invade and increasing biodiversity. This concept was developed from classic field experiments that were mostly limited to the Pacific Northwest. The coast-wide sea star wasting disease (SSWD) epidemic in 2013-2014 caused widespread declines in this keystone predator, presenting a unique opportunity to test the keystone predation theory on a coast-wide scale for the first time. We surveyed 33 rocky intertidal sites along the Oregon and California coasts since 2015 to track change in low zone community structure. We surveyed the abundance of all space-holding species using transects and quadrats, measured changes in mussel bed zonation using shore leveling equipment, and tracked sea star decline and recovery using surveys of density and size. While there was substantial site-to site variation, our findings suggest that the decline in sea stars was followed by a slow increase in competitively dominant mussels, declines in mussel competitors like gooseneck barnacles, algae and sea anemones, and a decline in biodiversity over several years. These trends were detected primarily Oregon and Southern California, but were weak or absent in much of Northern and Central California. Both results contrast to those of earlier experiments that found mussel responses to sea star absence occurred relatively rapidly, within about three years. We conclude that the role of Pisaster as a keystone predator is widespread, but the strength of the effect is locally or regionally controlled. Further, it appears that some communities (primarily in Northern and Central California) were resilient to the effects of the trophic downgrade caused by sea star wasting disease. We speculate that the mechanism underlying the slow change or absence of change after sea star loss is variable strength of bottom-up processes, including environmental conditions and prey production. Related research suggests that these factors can influence the strength of top-down keystone predation and the resilience of communities to a trophic downgrade.