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.
Physical variations on a smaller scale, such as the different water-holding capacities of various soils, also come into play. The soils along the ridgetops can store relatively little water, and nutrients leach out quickly, washing downhill. The hillside soils are linked by a series of intermediate steps that form a soil catena leading down the slopes to nutrient-rich earth deposited by flowing water in the valleys.
The term catena is also used to describe the broader soil trends in the Serengeti. The plains were formed by airborne deposits of volcanic material from eastern-range eruptions. The fallout was coarsest in the plains near the base of the volcanoes and formed porous, easily leached soils there. Farther west in the plains and in the northwest, where the finer materials were blown, a more closely packed soil that holds water better was formed. So an animal can "move down the catena" either by traveling from a ridgetop to a valley or by migrating north and west — in either case moving toward increasingly moisture-retentive soils.
Gnus on the Go
Today, more than a million wildebeests graze the Serengeti, moving from one area to another at various times of the year in order to find the highest-quality forage. During the dry season, they are concentrated in thorn woodland in the relatively moist north-western section, which is infested in the wet season with mosquitoes and tsetse flies. Within that area, the animals may travel as much as 50 miles a day if food and water are widely separated, as they will not go more than five days without drinking water.
At the beginning of the wet season, the wildebeests move toward thunderstorms (which they can detect from 15 miles away by sound and 60 miles away by sight of the anvil-topped storm clouds) to take advantage of the grass that sprouts after the rains. The animals soon move eastward into the plains, and from January to May they travel in a generally clockwise direction through the central and eastern plains of the Serengeti. At the end of the wet season, as the grass on the plains dries up, the wildebeests return to the moist northwest.
In their migrations, the wildebeests ordinarily are accompanied by zebras and Thomson's gazelles. These three species, however, do not all follow precisely the same routes and timetables, and they do not use exactly the same food plants. During the wet season, the animals are all concentrated on the upper (southeastern) catena of the plains, feasting on freshly sprouted grasses that have a high protein content. When the wet season draws to a close, the zebras are the first to move off down the catena. By preference, they eat the relatively tough, protein-poor flower stems of the drying grasses. (Grazing animals make little use of the protein-rich seeds on the stems.)
After the zebras come the wildebeests, which feed primarily on leaves and the sheaths at the bases of leaver — plant parts that contain more protein than the stems, and that are made more accessible by the zebras' removal of the taller stems. Then it's the turn of the Thomson's gazelles to occupy the now-short sward.
The "Tommies" feed heavily on protein-rich forbs (non-grass herbs) — which the previous grazers have exposed — in addition to the grass sheaths and leaves. Their slender muzzles allow them to feed much more selectively than either the zebras or wildebeests. Tommies avoid areas of tall grass, perhaps because their small size (they're around 26 inches high at the shoulder — about half the height of wildebeests and zebras) makes it hard for them to spot predators.
Thus, each migrant changes the structure of the Serengeti vegetation in ways that benefit those that follow.
The total impact of the community of herbivores on the Serengeti grassland has been a subject of intensive research. Studies using fenced areas to exclude the large herbivores have shown that grazing increases the diversity of grasses while reducing the average height of the foliage. Grazing also causes individual plants to produce more shoots than roots, thus raising aboveground productivity.
In addition, the pressure of the herbivores affects the competitive relationships among the Serengeti's various plants. For example, grasses are generally more resistant to grazing than forbs are. The permanently embryonic tissue responsible in herbs for the growth of shoots is at the ends of the shoots, where they are vulnerable to removal by grazers. But grasses have those tissues just above each node of the stem, where they are much less likely to be eaten and are protected and supported by sheaths at the bases of the leaves. The blade of the leaf also has an embryonic area at its base so that, unlike the leaves of forbs, it can continue to grow if the end of the leaf is eaten. Furthermore, branching in grasses occurs primarily at ground level, and the resulting rosette is often extended and reproduced by stems growing horizontally underground or spreading along the surface of the ground.
If the grass plant is smashed flat, differential growth of the embryonic tissue occurs and once again aims the shoot upward. This, in combination with the ability of the chewed leaves to continue to elongate and spread by horizontal stems, makes grasses extremely resistant to trampling and grazing by ungulates. The presence of these animals in large numbers actually favors the continued dominance of grasses and the perpetuation of grasslands.
In fact, plant populations exposed to grazing on the Serengeti have proved to be genetically different from those permanently protected in fenced-in areas. The grazed plants are somewhat dwarfed and grow more closely to the ground as a result of the selection pressures applied by the grazers. Indeed, the Serengeti system appears to be one of the few remaining great reservoirs of grazing-resistant plant genotypes — a "genetic library" of potentially incalculable value to humanity.