Research

Phylogenetic community assembly across the Serengeti ecosystem

Reconstructing phylogenetic relationships help scientists understand evolutionary processes such as speciation, convergence, and adaptive radiation. But how does knowledge of phylogenetic relationships contribute to understanding the processes that form natural extant communities? In other words, do evolutionary relationships among species tell us anything about community structure, i.e. the types and abundances of species in communities? These questions are part of a controversial issue currently facing ecology, namely, whether natural communities are assembled according to species interactions, or are whether communities are neutral in their species compositions.

This aspect of my research seeks a deeper understanding of the relationship between phylogeny and the assembly of natural plant communities. Theory suggests that the association between phylogenetic distance and community structure depends on the relative strength of environmental factors versus competition, and whether trait conservatism or trait convergence predominates. I am currently conducting a study in which I will attempt to test these predictions by studying plant trait evolution, community structure, and DNA sequence similarity of species at grassland sites across the Serengeti environmental gradient. Parallel studies inside and outside permanent fences established at the sites in 1999 will determine top-down consumer effects on phylogenetic plant community assembly. My hope is that the combined study of phylogenetic relationships, species traits, and ecological patterns may provide insight into mechanisms that generate and maintain biodiversity in Serengeti, while achieving greater integration between the fields of ecology and molecular evolution.

Ecology of African savanna ecosystems

Determinants of plant diversity in savanna ecosystems

Grassland plant communities are a critical components of African savanna foodwebs. As a result, they are important for the stability and diversity of ungulate and predator populations that depend on them. But are African grasslands themselves important storehouses of biodiversity in African savanna ecosystems? Moreover, what factors are responsible for generating and maintaining plant diversity in African savanna ecosystems? Part of my research is aimed at quantifying plant diversity in the Serengeti and understanding what factors are responsible for variation in diversity across the ecosystem. For example, Beta-diversity (β-diversity), or variation in plant species composition from one site to another, is an important property of Serengeti grasslands. Yet no clear theory exists for what generates and maintains the high degree of β-diversity across Serengeti grasslands. Theory suggests that if niche processes play a strong role in β-diversity, then plant communities should differ along gradients of environmental variation and sites with similar environmental conditions should be compositionally similar regardless of their distance. In contrast, if neutral processes predominate, then β-diversity will be explained by distance and limitations to dispersal, regardless of environmental variation. Of course the answer probably lies somewhere in the middle, with both niche and neutral processes playing important roles; the interesting part becomes understanding which processes predominates under which set of conditions and why.

Analyzing comples, multivarate interactions in savanna ecosystems

Why are some grasslands nutrient rich and others nutrient poor? Why do some grasslands contain dense assemblages of ungulate grazers while others are impoverished with respect to herbivores? Moreover, how do we tease apart the multiple correlated factors that appear to be controlling the properties of savanna ecosystems (such as forage nutritive quality and ungulate densities) and identify those factors that are most important? These questions have important implications for the management and conservation of grazing ecosystems, i.e. systems dominated by ungulate herbivores, such as those in Yellowstone National Park (USA), Serengeti National Park (Tanzania), and the Alaskan-Yukon-Northern Territories ecosystem (USA & Canada). 

All natural biological systems are composed of constellations of strong and weak and positive and negative forces that interact to determine the processes and properties that characterize the system, such as species diversity, primary and secondary production, rates of nutrient cycling, coexistence of herbaceous and woody vegetation, etc. But how do we study such complex interactions in natural multiple variable systems? To understand the complex interactions that characterize savanna ecosystems, my collaborators and I employ structural equation modeling (SEM). SEM is a data analytic technique that relies on the structure of covariance among variables to ask whether a system behaves as predicted from an a-priori model. As an example, we used an SE model to ask how fire and herbivory related to plant nutritive quality inside and outside of the boundary of Serengeti National Park. Our results suggest that fire and herbivory play important roles in determining the nutrient quality of forage that is available to ungulates. In particular, herbivory and fire influence the concentrations of nitrogen, phosphorus, and sodium in ways that may determine ungulate carrying capacities across the landscape. While these are essential dietary elements to maintain metabolism for all ungulates, they are especially important to sustain lactation in pregnant females.

COLLABORATORS

Joey Shaw, University of Tennessee, Chattanooga (http://oneweb.utc.edu/~Joey-Shaw/)

Mahesh Sankaran, Leeds University (http://www.fbs.leeds.ac.uk/staff/profile.php?tag=Sankaran_M)

Jim Grace, USGS NWRC (http://www.nwrc.usgs.gov/about/web/j_grace.htm)

Mark Ritchie, Syracuse University (http://biology.syr.edu/ritchie/)

William Starmer, Syracuse University (http://biology.syr.edu/starmer/index.html)