The relationship between species diversity and ecosystem function has stimulated a large body of research in the last 15 years. However, less attention has focused on the ecological importance of genetic diversity within a species. Genetic variation in dominant estuarine and marine primary producers may have important effects on the overall productivity of these systems. My research to date concentrates primarily on genotypic (i.e., clonal) diversity in eelgrass (Zostera marina), a habitat-forming seagrass species that is found in shallow coastal systems throughout the northern hemisphere. Eelgrass can reproduce clonally as well as sexually, generating variation in clonal diversity within and among beds. To evaluate experimentally the importance of genotypic diversity in Z. marina for benthic productivity, I genotyped individual shoots using DNA microsatellites and transplanted them into the field to produce plots of equivalent initial shoot density but varying from one to eight genotypes (Hughes and Stachowicz 2004). This experiment revealed strong and lasting effects of clonal diversity on the resistance of eelgrass and the community it supports to seasonal grazing. I am currently using a combination of field and laboratory experiments to identify the mechanisms leading to this increased disturbance resistance.

To determine whether eelgrass genotypic diversity influences the productivity of natural eelgrass populations, I quantified the clonal diversity of twelve 1m² plots in each of seven eelgrass sites in Bodega Bay and Tomales Bay, CA. Clonal diversity varies by site, tidal level, and position in the bed (edge versus interior). Interestingly, although genetic diversity is not correlated with eelgrass shoot density in the productive summer months, there is a positive relationship between genetic diversity and shoot density in the winter, when densities are lower due to factors such as grazing by geese and lower light levels. This natural pattern suggests that the positive effect of genotypic diversity on disturbance resistance is sufficiently strong to impact patterns of shoot density and perhaps primary production in the field, despite the presence of many confounding factors.

As a complement to my empirical research on eelgrass genotypic diversity and ecosystem function, I have worked on several projects to examine the role of herbivore and/or predator diversity on food web dynamics. For example, interactions among multiple predator species and their herbivore prey in a kelp food web can strengthen the effects of a trophic cascade in this system, leading to increased primary production (Byrnes et al. 2006). In seagrass systems, a diverse suite of grazer species can have contrasting effects on seagrass productivity due to their opposing impacts on epiphyte biomass, often resulting in no net effect (Hughes et al. 2004). Despite an increasing emphasis on the effects diversity at higher trophic levels, few studies have examined how habitat complexity mediates these biodiversity effects. In collaboration with Jonathan Grabowski (Gulf of Maine Research Institute), I investigated the individual and combined effects of two predators when foraging on two common bivalve species in habitats of varying structural complexity, oyster reefs and sand flats (Hughes and Grabowski 2006). Consumption rates of each predator in isolation from the other were lower in the structurally complex oyster reef, yet consumption from combined predation surprisingly did not differ between the two habitats due to variation in predator interactions. This context dependency suggests that an understanding of how habitat characteristics like physical complexity influence predator interactions may be critical to predicting the effects of predator diversity on resource capture.

Both overfishing and eutrophication have been associated with the decline of seagrasses worldwide. However, despite the strong experimental tradition in seagrass biology, the relative importance of nutrients and grazing to seagrass growth rate and biomass remains unclear. In collaboration with Dr. Susan Williams and fellow UC Davis graduate students Jun Bando and Laura Rodriguez, I conducted a meta-analysis seagrass manipulations that confirmed strong negative effects of water column nutrients on seagrass biomass and growth (Hughes et al. 2004). In addition, epiphyte grazers have positive effects on seagrasses that are of comparable magnitude to the impacts of nutrients, suggesting that these two factors should not be considered in isolation. I am currently participating in a working group at the National Center for Ecological Analysis and Synthesis to assess the global status of seagrass populations. Our first meeting resulted in a review of the contemporary crisis faced by seagrasses (Orth et al. 2006), and we have begun a quantitative analysis of seagrass population trajectories worldwide.