MEPS 570:15-28 (2017)  -  DOI: https://doi.org/10.3354/meps12121

ABSTRACT: The recent outbreak of sea star wasting disease (SSWD) along the US West Coast, which has decimated the intertidal keystone predator Pisaster ochraceus, is predicted to change community structure by reducing predatory control of the competitively dominant mussel Mytilus californianus. However, keystone species effects can be variable, and the role of other predators without the keystone predator is not well understood. We were interested in the effects of subordinate whelk predators (Nucella ostrina, N. canaliculata) on prey following SSWD, and whelk population responses to declines of their predator and competitor P. ochraceus. We hypothesized that whelks would limit M. californianus establishment by reducing prey species facilitating its recruitment. We also predicted that whelk populations would increase and shift downshore with reduced predation threat. We addressed our questions using a whelk removal experiment started before the SSWD outbreak peak, and surveys of whelk distribution, abundance, and size structure. In the absence of P. ochraceus, whelks weakly facilitated, rather than limited, the establishment of the competitively dominant mussel but had few effects on other prey abundance. One year after SSWD onset, surveys indicated no change in whelk vertical distributions but revealed diverging patterns in abundance of whelk species. Whelk size structure shifted to smaller individuals, indicating a potentially lagged population response. Our results suggest that subordinate predators may not have strong short-term impacts on prey communities after keystone species decline. However, poor prey recruitment may have limited any whelk effects and population changes. Our study provides important context for understanding community dynamics following one of the largest marine disease outbreaks recorded.

KEY WORDS: Sea star wasting disease · SSWD · Rocky intertidal · Oregon · Subordinate predators · Nucella · Mytilus · Mesopredator release