Date of Award


Document Type

Open Access Dissertation


Biological Sciences

First Advisor

Blaine D. Griffen


Ecological communities and the biological interactions that regulate community structure are notoriously complex. To make these systems more tractable, ecologists traditionally measure and model communities at the population level, treating individuals as functionally equivalent. While this approach has yielded tremendous insight into the factors governing communities, it remains unclear whether accounting for individual-level variation could improve our capacity to predict the responses of communities to perturbation, a major goal in the midst of unprecedented rates of environmental change.

The objective of this study was to examine the magnitude of individual-level phenotypic variation in predatory crabs (family Xanthidae), and the effects of this variation on crab trophic behavior and the strength of their interactions with bivalve prey in oyster reef communities. Specifically, I measured individual variation in crab body size, behavioral traits and parasite infection. A main aspect of this work was testing how each of these factors affected the crab functional response, i.e. the per capita rate of prey consumption depending on prey density. This response is important in scaling up prey consumption rates to the population level, and to larger spatial scales. I also explored how oyster reef habitat structure and threat from toadfish, a predator of crabs, can mediate the ecological effects of crab phenotype.

The results of this work support the importance of individual-level variation for species interactions that influence the structure of reef communities. The body size distribution of crabs, which is in part dependent on the presence of structurally complex reef habitat, determined their top-down effects on the bivalve prey community. Furthermore, individual behavioral traits scaled with crab body size and were consistent over time in the field. Individual crab behavior also varied independently of crab body size, but could not be predicted by individual metabolic rate. Individual-level variation in crab body size, behavioral traits and parasite infection all influenced the crab functional response to bivalve prey density in different ways. This work provides a general pathway (modification of the functional response) by which the effects of individual phenotypes can scale up to influence predator-prey population dynamics.