UC Santa Cruz

Empirical evaluations of the ecological processes that enhance or dampen the likelihood of shifts between top-down (i.e., predator-driven) and bottom-up (i.e., resource-driven) forcing are essential to understanding the potential for cascading effects that can underpin community functioning, productivity, and stability. This suite of research used an extraordinary herbivore-outbreak in kelp forests along the central coast of California as a natural field setting to disentangle: (1) how alternations in the foraging behavior of a primary consumer drives patch state transition dynamics, (2) whether predation or resource abundance are the predominant drivers of community regulation, and (3) community-wide consequences to the formation of alternative ecosystem states. This dissertation was motived by a rapid and dramatic decline in the abundance of a sea star predator (Pycnopodia helianthoides) of sea urchins, and a decline of a primary producer (Macrocystis pyrifera, ‘kelp’) that initiated a fundamental change in purple sea urchin (Strongylocentrotus purpuratus) foraging behavior and condition, resulting in a spatial mosaic of remnant kelp forests interspersed with patches of sea urchin barrens. In Chapter 1, I demonstrate the important role of grazer behavior in mediating switching among patch states. I show that the 2014 sea urchin outbreak along the Monterey Peninsula, California, USA is explicable by a shift in sea urchin grazing behavior, not by a demographic (i.e., recruitment or survivorship) response. During this six-year study, kelp forests recovered to an area that was once an expansive sea urchin barren. I show that this remarkable recovery of kelp forests in 2019 to an area in deep water was evidenced by sea urchin movement to shallow water. These results highlight the role of grazer behavior in facilitating patch transition dynamics. In Chapter 2, I demonstrate how the behavioral response of an apex predator to changes in prey behavior and condition can dramatically alter the role and relative contribution of top-down forcing, depending on the spatial organization of ecosystem states. I show that the mosaic of adjacent alternative ecosystem states led to an increase in the number of sea otters (Enhydra lutris nereis) specializing on urchin prey, a population-level increase in urchin consumption, and an increase in sea otter survivorship. I further show that the spatial distribution of sea otter foraging effort for urchin prey was not directly linked to high prey density, but rather was predicted by energetically profitable prey patches. Finally, in Chapter 3 I examine whether the spatial mosaic of sea urchin barrens interspersed with remnant patches of kelp forest resulted in a departure of community structure from the long-standing configuration that proceeded the formation of the mosaic. I found that beginning in 2013, many sites across the study region departed from a common multivariate (“forested”) state, which had persisted for the previous six-years, and drifted into a new multivariate configuration (“urchin barrens”). Although sites trended toward a common reconfiguration, community trajectories were highly variable, and sites exhibited regional cohesion in their trajectories (Carmel, Monterey Bay). These results suggest that outbreaks of grazers associated with punctuated environmental (e.g., marine heatwaves) and biotic (loss of predators, sea urchin outbreaks) perturbations can drive apparently stable kelp forest communities to alternative potentially stable states. Collectively, the results of this dissertation highlight how patch dynamics, behavioral responses, and biotic and environmental perturbations underpin the structure and stability of ecosystems.