Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Emmett Duffy


The rapid and global rise in species extinctions has prompted much research into the causes and consequences of biodiversity loss. In the past two decades, efforts have expanded beyond characterizing diversity through numbers of species -- or species richness -- and integrated additional information on how species interact with one another and their environment via functional traits. Functional traits permit a more nuanced exploration of patterns in community structure and composition, and provide a mechanistic basis to link community diversity to ecosystem processes. In this dissertation, I apply functional traits to observational surveys and a small-scale experimental manipulation to understand and explain patterns in diversity, and to link functional diversity to ecosystem functioning. In all three cases, I show that functional traits yield substantial additional insight into ecological patterns and processes beyond what can be gained via richness alone.;In the first chapter, I use functional traits and two newly-derived phylogenies to understand the role of biotic interactions in determining how local communities of reef fishes assemble from the available pool of species. to address this question, I utilized data from the Reef Life Survey network, a global citizen science program that has conducted visual censuses of reef fish communities at nearly 2,000 sites across the globe. to rigorously disentangle the biotic and abiotic drivers of assembly, I aimed to factor out the effect of environment by grouping species based on their fine-scale habitat requirements, then tested for significant patterns in functional and phylogenetic diversity of local communities relative to the regional species pool. I found that most communities were functionally and phylogenetically clustered relative to the regional pool, meaning that species found in these communities were more functionally- and phylogenetically-similar than expected by chance. This clustering increased with increasing latitude independent of several major axes of environmental variation. I propose several non-mutually exclusive explanations for this pattern, including: (1) increased competition at higher latitudes, potentially driven by variability in resources; (2) higher mobility of fishes at high latitudes reducing trait and evolutionary composition at any given site relative to what could be observed there (i.e., high turnover), and; (3) reduced richness at high latitudes reducing the probability of capturing functionally and phylogenetically unique species. This chapter is one of the first studies to unite a macroecological perspective on assembly with functional biogeography across global gradients, particularly for vertebrates.;In the second chapter, I utilized data from a 15-year observational survey of an eelgrass Zostera marina L. bed in the York River Estuary, Chesapeake Bay, USA to test the relative strength of top-down and bottom-up control and the role of species richness and functional diversity in mediating trophic processes. I united biological data on eelgrass, microalgal epiphyte, and invertebrate grazer biomass, and predator abundances with physical data on temperature, light, turbidity, and nutrients using structural equation modeling. Across spring, summer, and fall seasons, biological variables appeared to be largely controlled by temperature and turbidity. However, there was weaker but statistically significant evidence for top-down control in the spring and summer, changing over to bottom-up control in the fall. In contrast to evidence from small-scale experiments, there was no effect of diversity on ecosystem properties such as standing stock biomass of eelgrass, grazers, and predators, which may have been a consequence of the overall low diversity and high functional redundancy present in this system. This chapter reveals a small but significant role for biology in the face of strong, long-term natural variation in abiotic parameters in a temperate eelgrass bed.;In the third and final chapter, I experimentally manipulated functional trait diversity of estuarine mesograzers and predators within multiple levels of species richness to test the relative predictive ability of functional diversity and species richness on ecosystem functioning. I found that multivariate functional diversity based on 8 traits was a better predictor and explained more variation in standing stock biomass of predator, grazer, and recruiting invertebrates than did species richness. Aggregating across all 8 traits in a multivariate index of functional diversity improved prediction accuracy relative to any individual trait. I then used structural equation modeling to show that the positive effects of community-level functional diversity were a consequence of both predator and grazer functional diversity, although predator effects were much stronger. I also modeled the contributions of each individual species to show that different functions were driven by different species with unique combinations of traits, suggestive of functional complementarity. Together, these results suggest that functional diversity is a powerful alternative to species richness in predicting the ecosystem consequences of species loss. This chapter is one of the first studies to conduct an a priori manipulation of functional traits using consumers, and the first to manipulate traits across multiple levels of a realistic food web.



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