Predator identity dominates non-consumptive effects in a disease-impacted rocky shore food web

Predicting the effects of predator diversity loss on food webs is challenging, because predators can both consume and induce behavioral responses in their prey (i.e., non-consumptive effects or NCEs). Studies manipulating predator diversity and investigating NCEs are rare, especially in marine systems. Recently, a severe outbreak of sea star wasting syndrome (SSWS) on the west coast of North America resulted in unprecedented declines of the sea star Pisaster ochraceus. We investigated the consequences of Pisaster loss on an abundant grazer, the black turban snail Tegula funebralis, through NCEs. We combined a laboratory experiment and field surveys to examine the importance of identity vs. diversity in a predator assemblage (Pisaster, crabs, and octopuses) on Tegula behavior, feeding, and growth. Laboratory and field results indicated that predator identity, not diversity, drives Tegula behavior and causes NCEs. Mesocosm treatments with Pisaster caused greater NCEs on Tegula than assemblages without Pisaster. Tegula’s distribution in the field, which is driven primarily by anti-predator behavior, was strongly associated only with Pisaster abundance, and not with the abundance of crabs, octopuses, and other predatory sea stars (Leptasterias spp.). We conclude that Pisaster primarily drives Tegula vertical distribution and may be having strong NCEs on Tegula on northern California rocky shores. Furthermore, predator diversity in northern California does not provide functional redundancy, in terms of NCEs on Tegula, to buffer the system from Pisaster loss. Thus, predator-induced vertical distributions and grazing suppression may not be maintained in areas where Pisaster populations are reduced or slow to recover from SSWS.