Title

Determinants of plant species diversity across spatial scales in Serengeti National Park, Tanzania

Date of Award

2004

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Advisor(s)

Samuel J. McNaughton

Keywords

Serengeti National Park, Tanzania, Plant diversity, Soil nitrate

Subject Categories

Botany | Ecology and Evolutionary Biology | Life Sciences | Plant Sciences

Abstract

The determinants of plant species and functional-type diversity were studied in savanna-grasslands of Serengeti National Park, Tanzania. Experiments and observational studies spanned multiple spatial scales, from 1 m 2 plots to large areas of similar climate, topography, and geomorphology classified as Land Regions. Scale-dependence was a universal and important ecological property of Serengeti grasslands; small scale diversity (1 m 2 ) was associated with climate factors while larger scale diversity (1000 m 2 ) was associated with landscape factors. A measure of among-plot similarity in species composition vs. distance varied among Land Regions, with eastern regions having no variation among plot similarity with distance and western plots showing a strong decay in compositional similarity as plot distance increased.

The spatial distribution of soil nitrate was related to plant species diversity and plant size distributions. Small-scale variation in soil nitrate was positively correlated with 1 m 2 plant species diversity (H ' ), while 1000 m 2 plant species richness was a log normal function of soil nitrate patch size. Moreover, between scales of 10 m 2 and 1000 m 2 , where the spatial distribution of soil nitrate was fractal-like, plant species diversity was greater and plant size distributions were more left-skewed than where the spatial distribution of soil nitrate was random. Computer simulation models suggested that the fractal dimension of the spatial distribution of resources affects plant functional-type diversity when plants are of different sizes and their nutrient requirements are size-specific. Plant diversity in simulated landscapes was greater when the fractal dimension of the resource was lower (more aggregated and patchy) compared to when resources had a higher fractal dimension (more diffuse and grainy).

Finally, a series of laboratory and field experiments conducted at multiple spatial scales suggested that the dominant plant species in Serengeti, Digitaria macroblephara and Themeda triandra , coexist by exploiting different topographic hillslopes positions that differ in water availability and grazing frequency. Isotopic tracer studies conducted in the field and physiological studies conducted in the laboratory showed that the species have opposite responses to water availability and grazing, but that grazing may promote species coexistence by producing different responses to water availability.

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