UC Davis

Understanding the effects of predator-induced “top-down” grazing regulation is critical to effective management of subtidal rocky reefs along the California coast, where macroalgae such as the giant kelp, Macrocystis pyrifera, are essential to ecosystem function (Dayton 1985, Graham 2004, Falkenberg et al. 2012). Previous studies in California have documented significant correlations between fishery-reduced predator densities, increased herbivore (i.e., grazer) abundance, and subsequent decreases in macroalgal cover, suggesting that predator removal has the potential to drive kelp forest decline by disrupting grazing regulation of benthic herbivores (Lafferty et al. 2004, Hughes et al. 2013). Thus, as coastal predatory assemblages are increasingly altered by fishing pressure and other anthropogenic stressors, identifying and preserving the essential components of top-down regulation is becoming ever more urgent (Tegner and Dayton 2000. Steneck et al. 2002).Existing research in California has focused almost exclusively on urchins, even though recent trophic interaction models identify mid-sized gastropods (e.g., Tegula spp.) as a critical and overlooked source of macroalgal consumption whose relative impact will likely increase as fishery-targeted grazers (i.e., urchins, larger invertebrates) are removed from the ecosystem (Sala and Graham 2002). Like urchins, subtidal Tegula are voracious and potentially destructive grazers in kelp forests (e.g., Chess 1983, Steinberg et al. 1995, Van Alstyne et al. 1999), and macroalgal preferences of these species appear to be driven by the presence of benthic predators (primarily sea stars). Top-down regulation of these grazers may also reflect a strong non-consumptive (i.e., trait-mediated) component (Watanabe 1984a; 1984b). Thus, evaluating the extent of top-down regulation for Tegula spp. could provide critical insight into the processes influencing the abundance of M. pyrifera off California, which have been insufficiently explained by urchin-based grazing models that typically consider only the direct, consumptive impacts of predation (Babcock et al. 2010, Guenther et al. 2012). Experimental field evaluations of behavioral, trait-mediated predator-grazer interactions have rarely been attempted in marine subtidal systems due to logistical constraints associated with monitoring/maintaining long-term predator exposure treatments. However, the 2013 outbreak of sea star wasting disease (SSWD) along the California coast presented a unique opportunity to conduct a natural experiment quantifying Tegula regulation by predatory sea stars on an ecologically relevant, reef-wide scale not otherwise achievable with conventional manipulative studies. Unlike experimental predator manipulations that are logistically limited in duration and scope, often precluding direct extrapolation of results to natural reefs, disease-induced removal of both main Tegula predators, the sea stars Pisaster spp. and Pycnopodia helianthoides persisted on many central California reefs for months-years following initial SSWD outbreak. Because the spatial distribution of disease occurrence was regionally widespread but locally patchy, I was able to use a paired experimental design to compare Tegula macroalgal association, dietary preference, and life history characteristics at reefs with severe disease outbreaks (i.e., SS- sites; complete or near predatory sea star absence) vs. low-moderate disease severity reefs (i.e., SS+ sites), where predatory sea stars retained some degree of local presence. This provided a rare opportunity to examine the long-term impacts of large-scale predator removal on Tegula grazing behavior and population dynamics in an ecologically realistic field setting. With this in mind, the overarching goal of my proposed research was to characterize predator-prey interactions and evaluate the ecological significance of top-down regulation for Tegula species in giant kelp forests. These largely understudied grazers could increase our understanding of the ecological interactions influencing kelp forest productivity and stability off the California coast (e.g., Babcock et al. 2010, Guenther et al. 2012). Through a series of natural field experiments and subsequent modeling approaches developed based on my empirical research results, I attempt here to address the following research questions: (1) do Tegula spp. alter their grazing behavior, dietary preferences, and/or algal utilization patterns in the presence of predatory sea stars?, (2) does long-term predator exposure history influence morphometric growth or energy allocation patterns (e.g., reproductive investment) in subtidal Tegula at a local population level, and are potential responses consistent across the three Tegula species commonly found in kelp forests throughout central California?, and (3) what are the potential ecosystem-level impacts of predatory sea star removal on Tegula population dynamics and productivity and stability of lower trophic levels in kelp forest food webs?