Spatial Variation in Predator-Prey Interactions on Subtidal Rocky Reefs

Almost all large-scale studies in marine systems demonstrate variability in the patterns found between species and communities at multiple spatial scales and the spatial scale of research can have drastic effects on the interpretation of these patterns. While ecologists have traditionally used experimental methods to describe community level processes that determine patterns of species abundance at local scales, biogeographers seek to explain these patterns over much larger scales using more observational methods. Macroecological research has attempted to unite these ecological and biogeographical perspectives by combining observations of patterns of abundance and distribution over larger spatial scales with controlled manipulative experiments at a number of locations spanning hundreds to thousands of kilometers along biogeographic gradients.;Grazing by sea urchins is part of a complex combination of physical and biotic factors that leads to geographic variation in community structure on temperate subtidal rocky reefs worldwide. The relative strength and community level implications of species interactionsparticularly the grazing of sea urchins and predation upon these grazers- can vary across the interacting ranges of these species. While in some systems changes in interaction strength occur along a latitudinal gradient (often in response to gradual changes in abiotic factors), variation in species interactions is often spatially heterogeneous and context-dependent, driven by changes in species abundances, community structure, and environmental conditions. For my dissertation, I utilized both observational and experimental methods at multiple spatial scales to better understand patterns of abundance and distribution of a suite of interacting species that are hypothesized to induce top-down control on temperate nearshore subtidal rocky reefs south of Point Conception, California.;In the first chapter I used a biogeographical approach to describe patterns of abundance and distribution of sea urchins and their predators over the majority of their interacting range on subtidal rocky reefs along the coastlines of California and Baja California, Mexico. Rather than latitudinal gradients of abundance of each species, we found much of the variation in species abundances occurred at site and transect level scales. This local scale variation did not appear to be determined by latitudinal changes in sea surface temperature, but more likely is driven by other abiotic metrics and recruitment and dispersal dynamics. Despite this high site level variability, broad scale correlative trends suggest region-wide opportunity for top-down control in which higher densities of predators (primarily spiny lobster) lead to increased abundances of the kelp Macrocystis, presumably via reduction of sea urchin densities. These results help provide context to studies of the ecological processes that lead to variability in spatial structure and function of these communities, while cautioning against making broad scale generalizations based on inferences drawn from local-scale studies.;In the second chapter, I explored the predator-urchin relationship across a latitudinal gradient within the Southern California Bight (SCB) using a macroecological comparativeexperimental approach. Mortality rates of sea urchins were derived using field tethering experiments. Experimental urchin mortality was compared to population demographics of both urchins and predators to assess factors influencing mortality across space. Mortality rates varied significantly among sites but were generally higher in the southern region of the SCB. Tethered urchin mortality decreased in areas of increased urchin abundance. Predator density and size were not predictive of urchin mortality in logistic models although sites with highest predator densities did have the highest mortality rates. These results indicate that the abundance and mortality of sea urchins within the SCB varies significantly among sites and this variation is driven in some locations by predators and in others by factors that may overwhelm or obscure the relative importance of trophic interactions.;In the third chapter I used a more traditional local scale, experimental approach to assess the relative impact of predators on altering sea urchin behavior, rather than direct mortality, that might impact rocky reef community dynamics. Incorporating these non-consumptive effects (NCE) into trophic dynamic models can be particularly important in systems such as nearshore rocky reefs where targeted prey species are strong grazers of primary producers. We measured spatial variation in urchin behavior across the SCB, then compared this to predator densities at each site to assess possible correlation and inferred causation of NCE. We then used laboratory mesocosm and field enclosure experiments to measure the NCE induced by each of the predator species, using grazing rates on kelp fronds as a proxy for urchin behavior. We found evidence from the field surveys that predator density was negatively correlated to the percentage of exposed, and thus actively foraging, sea urchins. In laboratory mesocosms the presence of both lobster and sheephead significantly reduced foraging behavior of sea urchins. These findings indicate that predators can induce changes in urchin behavior over broad spatial scales despite high site level variability in the densities of these interacting species. While the debate regarding the generality of top-down forcing in these systems is primarily driven by research into the role of consumptive effects in shaping community structure, this work provides additional insight into these predator-prey interactions and the complex hierarchy of factors that determine community structure and function in kelp forest systems.;Overall, this dissertation demonstrates the importance of scale in studying and interpreting patterns and processes in ecological systems. While broad scale research cannot always determine the mechanism by which patterns emerge, it does provide context for more detail oriented studies within similar systems. This dissertation should facilitate continued discussion about the relative importance of top-down verses bottom-up forcing in these subtidal rocky reef communities while cautioning against making broad scale generalizations based on inferences drawn from local-scale studies, particularly as it pertains to management and conservation planning.