UC Santa Barbara

The strength of species interactions shapes the structure and function of ecological communities, with profound implications on the ecosystem services these communities provide, such as maintenance of biodiversity, carbon sequestration, cultural heritage, and viable food production. However, we, as humans, are altering the strength, direction, and variability in species interactions through global climate change, habitat loss, and harvest. By altering how species interact, these anthropogenic impacts are shifting both consumptive and non-consumptive ecosystem services. Therefore, understanding why species interaction are changing and what the consequences of these changes are on ecological communities is an important component of effectively managing ecosystems in a dynamic future. In this dissertation, I explore two different mechanics that underscore variation in species interactions across space and through time: variability in body size among individual predators and their prey and contingencies associated with historic population fluctuations in a marine foundation species.

In Chapter 1, I combined mesocosm experiments and long-term ecological data to test to what extent individual variation in predator body size, prey body size, and prey density drove spatiotemporal variation in interaction strength. I then tested the efficacy of established body size-scaling relationships at predicting variation in interaction strength. My results demonstrate that the majority of variation in how strongly California spiny lobster (Panulirus interruptus) interact with their purple sea urchin (Strongylocentrotus purpuratus) prey can be attributed to variation in body size. Furthermore, utilizing established size-scaling relationships from the literature failed to accurately predict our experimental estimates of interaction strength by more than an order of magnitude.

In Chapter 2, I sought to uncover the physiological mechanisms driving the relation between a predator’s body size and its consumption rate. Specifically, I tested between alternative theoretical hypotheses for the relationship between an animal’s size, metabolism, and consumption rate to better understand the connection between a predator’s ecology and physiology. Contrary to prevailing theoretical expectations, I demonstrate that larger lobster can consume disproportionately more than smaller conspecifics, despite declining metabolic requirements, which could have implications on how body size is incorporating into models of community and ecosystem dynamics.

Finally, in Chapter 3, I examine how historic variability in the foundation species, Macrocystis pyrifera, alters non-trophic interactions between functional groups on the seafloor. My results suggest that, while the current biomass of M. pyrifera has the strongest impact, metrics of historic variability in the foundation species have strong effects on benthic community structure that ameliorate with time.

A pressing issue in managing ecosystems is understanding what causes variation in how strongly species interact, what the implications of this variation are for communities, and how to predict shift in species interactions in the future. My research suggests that incorporating historical contingencies and individual variation in body size could bolster management and restoration efforts that aim to increase the resilience of marine communities in a dynamic future.