Ecological consequences of sea star wasting disease: Non-consumptive effects and trait-mediated indirect interactions from Pisaster ochraceus

Consumptive effects (CEs) of predators are an important factor in structuring biological communities, but further work is needed to understand how the interaction between spatial and temporal differences in predator density affects non-consumptive effects (NCEs) on prey. NCEs can cause indirect effects on food resources, known as trait-mediated indirect interactions (TMIIs), and thus can also affect community structure. However, few studies have considered the relationships between spatial and temporal predator density variation and the strength of NCEs and TMIIs in the natural environment. The ochre star Pisaster ochraceus is common predator of the herbivorous black turban snail Tegula funebralis, imposing both CEs, but also NCEs and TMIIs by inducing Tegula avoidance behavior and suppressing Tegula grazing. Pisaster density differences along the eastern North Pacific have been exacerbated by the onset of Sea Star Wasting Disease (SSWD), resulting in a gradient of Pisaster abundance along California’s North Coast. I hypothesized that Tegula growth and grazing would be increased at sites with decreased Pisaster density via release from NCEs, and that temporal Pisaster density variation would elicit stronger anti-predator responses from Tegula at sites with lower background levels of Pisaster density. To test these hypotheses, I used a cage-exclusion experiment at sites comprising a gradient of Pisaster density, introduced temporal Pisaster density variation in experimental plots, and measured Tegula growth and grazing in cages at unmanipulated and experimental plots. I also used a laboratory experiment to confirm the association between Tegula growth and grazing across field-relevant concentrations of Pisaster cue. My results indicate that decreased Tegula soft tissue growth and grazing were associated with increased Pisaster density, and that Tegula anti-predator responses to temporal variation in Pisaster density were strongest at sites with low background Pisaster density. These results suggest that NCEs and subsequent TMIIs were predator density-dependent, highlighting the interactive effect of spatial and temporal variation in predator density on NCE and TMII strength. My lab experiment suggested that the NCEs and TMIIs observed in my field experiment were induced by Pisaster cues, and also highlighted the importance of designing realistic laboratory experiments. Finally, my thesis indicates that variation in Pisaster density associated with Sea Star Wasting Disease (SSWD) could affect NCEs and resulting TMIIs in a site and context-specific manner, contributing a novel finding to the growing body of literature considering the ecological impacts of SSWD in intertidal communities