Disturbance, diversity and community dynamics in a southern Indian savanna-grassland ecosystem
Date of Award
Doctor of Philosophy (PhD)
S. J. McNaughton
Disturbance, Diversity, Indian, Savanna-grassland
Ecology and Evolutionary Biology | Life Sciences
Factors influencing structure and dynamics of savanna-grassland ecosystems were investigated at the Kalakad-Mundanthurai Tiger Reserve (KMTR), India, across spatial scales ranging from a few square meters to the entire reserve (900 km 2 ). Savanna habitats comprise a third of the reserve area, and are characterized by dominance of a few widespread species that turn-over across an elevation gradient. Most mid- and low-elevation grasslands are anthropogenic in origin, maintained by recurrent fires. Tall grasses dominate most low and mid elevations, short-grasses characterize high elevations. Plant richness is greatest at low elevations, and both point richness and pattern diversity are important determinants of site richness in the reserve. Herbivores primarily utilize short grasslands at low and high elevations, and avoid tall-grasslands due to the unpalatable nature of dominant plants.
In recent years, fire frequencies have increased and herbivore populations have decreased in the reserve. To understand causes, and potential consequences, of these changes, experimental manipulations were carried out to determine how burning and grazing influence plant and nutrient dynamics in three low-elevation grassland communities. Low diversity communities were more compositionally stable than high diversity communities to perturbations characteristic of their native environment, but less so when perturbations were novel. Grazing effects were more pronounced than burning. Increased colonization by annual species following high rainfall during the study period also induced compositional shifts in communities. Such shifts were greater in high diversity communities because of more free-space for colonization in these communities. When considering treatment effects on soil nitrogen dynamics, responses were complex and contingent on community type, disturbance history and season.
Herbivore densities at KMTR are low relative to other similar protected areas in India. It is hypothesized that herbivore declines and spread of tall-grasses at KMTR has resulted from disruption of facilitatory interactions between different body-sized herbivores. Reduction in large-bodied herbivores, the control agents for coarse tall grasses, has favored the spread of competitively superior tallgrasses in an environment where light competition is critical. High litter production by these species has increased fire frequencies and suppressed smaller palatable plants, subsequently reducing densities of selectively feeding smaller-bodied herbivores. This hypothesis awaits experimental testing.
Sankaran, Mahesh, "Disturbance, diversity and community dynamics in a southern Indian savanna-grassland ecosystem" (2001). Biology - Dissertations. Paper 40.