We pursue questions about the causes and consequences of biodiversity, from genes to ecosystems. Current research interests in the lab center on geographic diversity gradients, community and ecosystem genetics, global climate change and species distributions, and the structure and function of ant and temperate tree communities. Generally speaking we ask three broad questions: (1) What processes underly the assembly of ant communities? (2) What factors govern broad-scale patterns in the distribution of biodiversity?, and (3) Do trophic dynamics limit local community structure and mediate ecosystem processes?

:: Biodiversity :::::::::::::::::::::::::::::::::::::::::::::::::::::::::

Elevational gradients as tools for understanding the determinants of biodiversity. Dozens of mechanisms have been suggested to explain geographical variation in species richness. In our view, elevational gradients are a nearly ideal tool to uncover the causes of broad-scale patterns of biodiversity because climate, area, stochastic factors, and history can vary systematically along numerous elevational gradients. In addition, it is possible to manipulate experimentally potential causal mechanisms along elevational gradients. My approach is largely macroecological: I collect field data at along environmental gradients and amass datasets that are global in extent to test theory.

Global diversity gradients as tools for understanding how historical and ecological factors determine biodiversity. To understand the determinants of global-scale diversity gradients, we have compiled a dataset consisting of the number of ant species occurring at nearly 3,000 sites from every continent except Antarctica. Our primary goal is to model potential biodiversity as a function of climatic and topographical variation and a variety of ecological drivers. We have also begun to examine the extent to which multiple drivers at local and regional scales interact to shape ant communities. For instance, our analyses indicate that the impact of invasive species on ant community structure varies with climatic temperature: the impact of invasive species on native communities is greater at higher temperatures than at lower temperatures. These results suggest that, with climatic warming, the impact of invasive species will be intensified. This global dataset will also let us examine the relative influence of current climatic conditions and history on community structure at a variety of spatial grain sizes.

Modeling the future distribution of biodiversity and experimentally assessing the effects of climatic change on communities and ecosystems. My work examining the determinants of diversity along elevational gradients and at global scales has indicated the importance of climate in determining geographic variation in ant biodiversity. Of course, global climate is changing. To understand the potential effects of climatic change on patterns of ant diversity, we have used Maximum Entropy (MaxEnt) and niche models to project the distributions of key species (e.g., the Red-Imported Fire Ant) and, more generally, total biodiversity in multiple biogeographic provinces. We were also recently awarded two major grants from the Department of Energy to model the response of ant biodiversity to changing climate and to manipulate climate experimentally to understand whether climatic change will affect ant communities and the processes they mediate.

:: Linking communities and ecosystems ::::::::::::::::::::

The bottom-up and top-down controls on plant community structure and invasibility. In an ongoing manipulative experiment at Oak Ridge National Lab, I am examining the combined and relative effects of soil nutrient availability, herbivory by insects, and propagule supply rate on the dynamics of a plant invasion, and more generally on plant community structure. The focus of this experiment is on the exotic plant species Lespedeza cuneata. After four years, this experiment has uncovered a complicated series of interactions. Adding propagules of Lespedeza cuneata increases its density, cover, and biomass, but only in plots in which nitrogen availability has been reduced and insect herbivores are reduced. Taken together, this multifactorial experiment provides evidence that native insect herbivores mediate the interactive effects of propagule supply and resources on invasion by a widespread invasive plant species and ultimately shape plant community structure.

The community- and ecosystem-level consequences of intraspecific diversity. Theory and field studies have shown that the diversity of plant species often influences number of insect herbivores and predators associated with the plant community, affects ecosystem processes, and limits biological invasions. My students and I are conducting a series of experiments examining how within-population diversity affects the structure and dynamics of old-field communities and ecosystems. Beginning in 2005, in a project led by my PhD student Greg Crutsinger, we manipulated the number of Solidago altissima genotypes in experimental plots at Oak Ridge National Lab. To date, we have found that population genotypic diversity contributes to arthropod diversity and community structure, affects the distribution of a keystone herbivore, increases above-ground net primary productivity, and limits invasion by non-native plant species. In most cases, the effects of genotypic diversity were comparable to the effects of plant species diversity measured in other studies.

The consequences of species loss for the functioning of forest ecosystems. Though considerable research in ecology has focused on how diversity affects ecosystem processes, it is clear that some species are more important to the functioning of ecosystems than are others. For instance, hemlock trees provide the underlying structure for many forests in the eastern US and modulate a variety of ecosystem processes. However, hemlock is being lost rapidly from eastern forests owing to infestation by an exotic pest, the hemlock woolly adelgid. We recently initiated series of experiments to determine rates and mechanisms by which loss of hemlock, and cascades of changes in arthropod species composition attendant to its loss, lead to changes in core ecosystem processes. In short, we are manipulating the density of ants and other arthropods in hemlock and deciduous forests to assess the direct and indirect effects of ants and the loss of hemlock trees on ecosystem processes. The ultimate goal of this research is to increase the ability of decision makers and land managers to make credible forecasts.
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