Ecoscience: The Serengeti Ecosystem

Biologists Anne and Paul Ehrlich walk MOTHER's readers through the fascinating Serengeti ecosystem.

| January/February 1985

  • Serengeti
    When the Serengeti wet season draws to a close, the zebras are the first to move off down the catena.

  • Serengeti

Paul Ehrlich (Bing Professor of Population Studies and Professor of Biological Sciences, Stanford University) and Anne Ehrlich (Senior Research Associate, Department of Biological Sciences, Stanford) are familiar names to ecologists and environmentalists everywhere. But while most folks are aware of the Ehrlichs' popular writing in the areas of ecology and overpopulation (most of us — for instance — have read Paul's book The Population Bomb), few people have any idea of how deeply the Ehrlichs are involved in ecological research (the type that tends to be published only in technical journals and college texts). That's why we're pleased to present this regular semitechnical column by these well-known authors/ecologists/educators.

An ecological system — or ecosystem — consists of all of the plants, animals and microorganisms in a given area together with the interactions of those organisms with their physical environments and with each other. Rather than introduce you to ecosystems through a general description of their properties, we'd like to describe a real one that we think is fascinating. It has been studied quite thoroughly and is probably familiar to you from nature films: the Serengeti ecosystem of northern Tanzania and southern Kenya in East Africa.

That system is more or less self-contained on a plateau encompassing some 10,000 square miles, which is about the size of Maryland. It's bounded on the east by hills and volcanic mountains (which include the famous Ngorongoro Crater), on the south and southwest by rocky woodlands and cultivated areas, on the west by Lake Victoria and on the northwest and north by cultivated land and an escarpment. The plateau lies just south of the equator, reaches an elevation of almost 6,000 feet in the east, and slopes downward to about 4,000 feet at Lake Victoria's shore. Low hills dotted with sparse Acacia woodland typify the western region, and from there stretches eastward a vast expanse of broad, grassy plains.

Actually, the ecosystem is best described not geographically but as an area strongly influenced by enormous migratory herds of one species of antelope: the wildebeest, or white-bearded gnu. It's also characterized by the presence of the last great assemblage of wild ungulate (hoofed mammal) species anywhere on this planet.

The Intertropical Convergence Zone

As with other ecosystems, the broad constraints within which the Serengeti system functions are set by its physical environment. In this equatorial setting, the major limiting factor on plant growth is not temperature but moisture, which is provided by an uneven and seasonal rainfall. The rain, in turn, is controlled by movements of a meteorological phenomenon called the Intertropical Convergence Zone, a semipermanent high-pressure belt around which converge the northeast trade winds from the Northern Hemisphere and the southeast trade winds from the Southern Hemisphere. The zone travels seasonally back and forth across the equator.

When it moves south, the convergence zone brings to the Serengeti relatively dry winds from the northeast and a small amount of rain — the "short rains" — starting around November. These rains, which end the dry season, sometimes last only until January and sometimes continue into March. Then the northward movement of the zone brings moisture-laden winds from the southeast. These, originating over the Indian Ocean, produce heavier rains — the "long rains" — from March to May. July through October is the dry season: The grass produced during the wet one can no longer grow.

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