The sea star wasting disease (SSWD) outbreak devastated populations of the intertidal predator Pisaster ochraceus along most of the North American West Coast starting in 2013. The keystone predation hypothesis predicts that decreases in Pisaster should allow their mussel prey Mytilus californianus to overtake space in the intertidal and drive down biodiversity of other space holders. We examined whether the degree of takeover by mussels among 14 sites from Central Oregon to Central California was correlated with mechanisms that may promote change or confer resistance and resilience to change. These include (a) the severity of decline or the recovery Pisaster, (b) the growth and recruitment of mussels, (c) the abundance of alternate predators or (d) environmental parameters including temperature and wave height. We also mapped variation in each variable along the coast to visualize how each was correlated with hotspots and coldspots of takeover by mussels.
Contrary to the expectations of the keystone predation hypothesis, the degree of mussel takeover 2-3 years post-SSWD was highly variable and formed a mosaic along the coast. This suggests that some resistance or resilience mechanisms are operating to limit mussel takeover in many locales. However, our analyses have not yet revealed discernable relationships between the potential mechanisms and the degree of takeover by mussels. We believe that not enough time has elapsed for community shifts to take place, and we are examining the importance of interactive effects between resistance and resilience mechanisms. By examining which ecological and environmental variables influenced mussels along the coast, we capitalized on this natural experiment by taking the keystone concept “out of the cage” and scaling up to test coast-wide patterns. The finding that many communities are thus far resistant or resilient to change may inform our understanding of other systems that have experienced a similar trophic downgrade.