Serengeti grassland ecology: the role of composite environmental factors and contingency in community organization.

Abstract

Canopy interception data from 105 grassland stands in the Serengeti National Park in Tanzania and the Masai Mara Game Reserve in Kenya were used to evaluate community organization within an ecosystem defined by the annual movements of large herds of nomadic ungulates. Other studies tested hypotheses about factors influencing species composition and diversity of the communities. The herbaceous vegetation was principally C 4 grasses, which commonly contributed over 90% of total abundance. Ninety—nine stands were classified into 17 communities, using a numerical clustering method. Sixteen communities were principally perennial grasses; one was dominated by an annual species. Short grasslands predominated below 700 mm mean annual rainfall and in heavily grazed sites on hilltops at higher—rainfall locations. Much of the savanna understory at intermediate rainfall levels was medium—height Themeda triandra grassland; six variants were recognized by clustering. Tall grasslands dominated by Hyparrhenia filipendula occurred as a savanna understory in locations with >900 mm of annual rainfall. Two ordination axes revealed a shortgrass continuum that graded into a medium—to tallgrass continuum, and an outlying floodplain tall grassland dominated by Echinochloa haploclada. The latter and the short grasslands dominated by Andropogon greenwayi were very different in species composition from the other communities. The first ordination axis was related to grazing intensity, and the second to soil texture. Spatial heterogeneity was an important attribute of the grasslands from the level of local stands to the level of landscape regions. Spatial pattern, rather than point diversity, was the major vegetation property correlated with α—diversity. Higher α—diversity was accompanied by low dominance concentration and more clumped species distributions. Individuals of rare species were more aggregated than members of common species. β—diversity also was a significant feature of the vegetation, leading to pronounced γ—diversity and a variety of different grassland types in local areas. Coenoclines were evident only in more arid locations. γ—diversity was associated with topographic and geological heterogeneity. Animals, soils, the overlying tree canopy in savannas, and fire influence α—diversity through their contribution to creating spatial pattern. Animals have particularly complicated effects, in addition to defoliating areas. Grazing is a major environmental factor affecting the grasslands, but it is a composite factor interacting in complex ways with other environmental factors.