The Serengeti ecosystem: species richness patterns, grazing, and land-use

Abstract

The Serengeti National Park is a natural laboratory for many reasons, particularly because it is a large, continuous stretch of land that harbors very diverse and dense biological fauna that has not been modified through human-induced changes. The ecosystem spans a precipitation, nutrient availability, and productivity gradient and is contained within areas managed under different land use practices.

Using a multi-scale plot design, vegetation species richness was sampled along this gradient to determine the relationships between species richness, primary productivity and scale. It was found that species richness was positively correlated with water availability and primary production and negatively correlated with nutrient availability. Examination of the slopes of the species area curves for each plot determined that species richness accumulated at a faster rate with increased water availability and primary production. Species richness was lowest in areas with limited water availability and heavy grazing and increased when released from these constraints.

The short-grass plains, which are critical habitat for the wildebeest migration, falls into two land use areas, the Serengeti National Park (SNP) and Ngorongoro Conservation Area (NCA). The SNP is a nature reserve while the NCA is a multi-use area allowing settlement and livestock grazing. This study examines vegetation structure and diversity patterns on the short-grass plains where changes in land-use have resulted in a portion of the plains that is grazed historically only during the wet season but is now grazed throughout the year by livestock. Vegetation diversity was not found to be different between these two areas, but changes in vegetation structure were correlated to dry season grazing densities, indicating that changes in the season of grazing could be influencing vegetation structure.

Woody vegetation structure and woody species composition is described along the precipitation gradient. Below approximately 500 mm mean annual precipitation, woody species are essentially absent from the landscape and herbaceous vegetation coverage is high. Above this threshold, woody species begin to appear on the landscape. As precipitation increases, density of trees increase and trees become taller. This increase in woody species is accompanied by a decrease in herbaceous vegetation coverage. Species composition is also described. The family Fabaceae is the most dominant species of trees under 900 mm mean annual precipitation, but above 900 mm of rainfall four families are equally represented.