Research interests

My research interests span two broad areas of ecology: grazer-plant-soil interactions and biodiversity-ecosystem function-stability relationships. Within this overarching context, I am interested in questions of both ecological and evolutionary significance, employing both theoretical and empirical approaches. Wherever possible, it is my goal to design the empirical part of my research such that it leads to insights on strategies for the conservation of biodiversity and the ecosystems in which it is conducted. For the most part, my research thus far has concentrated on savanna and grassland ecosystems; systems where rainfall, herbivores and fires are important regulators of plant community structure and function.


Zebra at KNPShola-grassland at KMTR
 
1. Determinants of savanna stucture and function
At the current time, I work in conjunction with a multi-disciplinary team of savanna biologists, ecosystem ecologists and mathematical modelers on a biocomplexity study of African savannas. Besides being economically important biomes, savannas are also perfect model systems for the kinds of research questions that I am interested in. They can be abstracted into a few components: trees, grasses, grazers, browsers and fires, whose interactions are mediated by climate, soil and human use. However, despite this apparent simplicity, understanding how different components interact to function as an integrated whole remains a challenge. Using both synthetic and modeling approaches, we are investigating the combinations of environmental factors that allow trees and grasses to coexist in savannas, and how interactions and feedbacks between climate, biogeochemistry, fire and herbivory influence savanna tree-grass ratios, savanna stability, resilience and bifurcation (open versus closed, eutrophic versus dystrophic) dynamics.

Our results suggest that across a gradient of increasing mean annual precipitation (MAP), African savannas switch from being water-limited 'stable' systems to disturbance-mediated 'unstable' systems. Above 650 mm MAP, rainfall is sufficient to support a closed woody canopy and disturbances such as fire and herbivory are required to maintain the tree-grass mix that characterizes savannas. Below 650 mm MAP, rainfall limits woody cover such that grasses can coexist in these systems, and fire and herbivory, while they influence savanna structure, are not necessary for tree-grass coexistence.  Importantly, maximum realizable woody cover in these arid and semi-arid systems is primarily determined by water availability, and is not related to soil characteristics, grazing or fire regimes. These data indicate that woody encroachment may be a bounded process in arid and semi-arid savannas, ultimately limited by water availability. In these systems, changes in precipitation regimes that lead to increased water availability, as predicted by many climate change models, may be a greater cause for concern for woody encroachment than increased levels of cattle grazing per se. Our analyses have also provided critical insights into the functional forms of the dependence of mean woody cover on climate, soil properties and disturbance regimes. We are now developing spatial models that explicitly couple N & P cycles with water availability to predict continent-wide patterns of savanna NPP and structure.

2. Grazer-plant-soil interactions
My research also addresses central questions in ecosystem ecology concerning the role of large herbivores in influencing patterns of energy and nutrient cycling in savannas. In particular, I am interested in understanding spatial and temporal feedbacks between grazers and soil microbial populations, and how this in turn relates to nutrient availability for plant growth. This work involves field studies in savanna-grasslands of Africa and India.

Savanna at MRC In collaboration with Dr. David Augustine, I looked at how producer-decomposer interactions were mediated by nutrient availability and grazing in a semi-arid savanna at the Mpala Research Centre in Kenya. We conducted replicated exclosure experiments at both high and low levels of soil fertility to investigate the effects of large grazers on decomposer biomass. Results support previous ecosystem-level studies showing that microbial biomass and growth are constrained by plant production and soil C availability. In addition,our findings demonstrate that decomposer abundance can be influenced by an ecosystem’s trophic structure, with significant reductions in microbial biomass occurring as a result of herbivores diverting plant carbon away from soils. Future work will investigate conditions under which native ungulates reinforce and maintain nutrient hot-spots, and the subsequent impacts of these processes for the sustainability and carrying capacity of the grazing ecosystem as a whole. Additionally, by quantifying microbial community composition,functional group representation and microbial substrate use efficiency we hope to gain a mechanistic understanding of how grazing interacts with soil fertility to influence patterns of carbon,nitrogen and phosphorous cycling in this system.





3. Diversity-stability relationships
Understanding consequences of changes in biodiversity that result from species loss, introduction of exotic species and alteration of biogeochemical cycles by humans represents one of the most challenging problems facing ecologists today.Does biodiversity act as a buffer to counteract large-scale anthropogenic alterations of the global environment such as increased levels of atmospheric CO2 or nitrogen deposition? In the face of these changes, will species rich systems necessarily fare better than species poor ones? How does species loss within communities affect their subsequent ability to function, recycle nutrients and water, and withstand future disturbances?

Tall grass communities at KMTR

I explored some of the above questions using savanna-grassland communities along a natural gradient of diversity at the Kalakad-Mundanthurai Tiger Reserve (KMTR) in southern India. Results from experimental perturbations (imposed in the form of burning and grazing treatments) indicate that extrinsic determinants of biodiversity such as disturbance regimes and site history, rather than species diversity per se, may be the primary determinants of certain measures of plant compositional stability in these grasslands. Results also highlight the disproportionate influence of dominant species in determining community level responses.  Future work will explore how the interplay between intrinsic and extrinsic determinants of biodiversity influence the stability properties and functioning of communities.