A recent outbreak of sea star wasting syndrome (SSWS) has devastated populations of many sea star species, including the original keystone predator Pisaster ochraceus throughout its entire range. Because P. ochraceus plays an essential role in intertidal communities, SSWS is expected to lead to significant changes in coastal marine community structure and functioning. The degree of these changes will depend on the resistance (i.e., the ability of the community and ecosystem to remain unchanged) and resilience (i.e., the ability of the system to recover without collapsing into a different state) of this system to perturbation. In the years after the onset of SSWS, surveys have shown an unprecedented number of newly settled P. ochraceus recruits. We used a manipulative field experiment to determine the role of adult P. ochraceus in determining recruit success. We manipulated abundances and population size structure of P. ochraceus on isolated rocks using factorial removals of adults and recruits. We predicted that density dependent effects of adults would inhibit the growth and survival of recruits. With lowered adult density, we expected P. ochraceus recruits would survive and grow at higher rates.
Results/Conclusions
Response of P. ochraceus recruits to adult presence was variable. However, on outcrops where adults were removed, there tended to be higher abundance of recruits and more recruits in the larger size classes (6-15mm arm length) by the end of the 13-month experiment, indicating a higher growth rate. This suggests that that the loss of adults resulted in lowered intraspecific, density-dependent competition between P. ochraceus at the recruit and adult stages, resulting in the observed enhanced survival of recruits. In this way, the SSWS disease outbreak may have indirectly facilitated recruit success, indicating the presence of a resilience mechanism by which populations are able to rebound from low numbers. The success of these recruits will be crucial to rebuilding P. ochraceus populations while reproductively active adult density remains low, and to the resistance and resilience of the intertidal system to SSWS perturbation.