Herbivores on the Serengeti Plain

Herbivores (plant- eating animals) living in the same environment often compete for food plants. But in some cases, herbivores may cooperate in the harvesting of plant matter. The grazing system of large herbivores on the Serengeti Plain of east Africa is an excellent illustration of how these animals may interact over their food supply. The Serengeti Plain contains the most spectacular concentration of large mammals found anywhere in the world. Approximately 1,000,000 wildebeests, 600,000 Thomson’s gazelles, 200,000 zebras, and 65,000 buffalo occupy an area of 23,000 square kilometers (9,000 square miles), along with undetermined numbers of 20 other species of grazing animals.

The dominant grazers of the Serengeti Plain are migratory and respond to the growth of the grasses in a fixed sequence. First, zebras enter the long-grass communities and remove many of the longer stems. Zebras are followed by wildebeests, which migrate in very large herds and graze and trample upon the grasses until they are close to the ground. Wildebeests are, in turn, followed by Thomson’s gazelles, which feed on the short grass during the dry season.

The various grazing species in the Serengeti system do not each select different species of grasses but instead select different parts of the grass plant during different seasons. Zebras eat mostly grass stems and sheaths and almost no grass leaves. Wildebeests eat more sheaths and leaves, and Thomson’s gazelles eat grass sheaths and a large fraction of herbs not touched by the other two herbivores. These feeding differences have significant consequences for grazing species because grass stems are very low in protein and high in lignin, an organic substance that strengthens the cell walls of some plants. Grass leaves, on the other hand, are relatively high in protein and low in lignin, which means leaves provide more energy per gram of dry weight than stems. Herb leaves typically contain even more protein and energy than grass leaves. So zebras seem to have the worst diet and Thomson’s gazelles the best.

How can zebras cope with grass stems as the major part of their diet during the dry season? Most of the grazing animals in the Serengeti are ruminants, animals that have a specialized stomach containing bacteria and protozoa that break down the cellulose in the cell walls of plants. But the zebra is not a ruminant and is similar to the horse in having a simple stomach. Zebras survive by processing a much larger volume of plant material through their gut than ruminants do, perhaps roughly twice as much. So even though a zebra cannot extract all the protein and energy from the grass stems, it eats more and thus compensates by volume. Zebras also have an advantage of being larger than wildebeests and Thomson’s gazelles, and larger animals need less energy and less protein per unit of weight than smaller animals do.

It would seem that competition for food would occur between wildebeests and Thomson’s gazelles because they eat the same parts of the grass. Wildebeests have what appears to be a devastating effect on the grassland as they pass through in migration. Green biomass was reduced by 85 percent and average plant height by 56 percent on sample plots. By establishing fenced areas as grazing enclosures, one researcher was able to follow the subsequent changes both in grassland areas subject to wildebeest grazing and in areas protected from all grazing. Grazed areas recovered after the wildebeest migration had passed and produced a short, dense lawn of green grass leaves. As gazelles entered the area during the dry season, they concentrated their feeding on areas where wildebeests had previously grazed and they avoided areas of grassland that the wildebeest herd had missed.

Grass production was reduced by both wildebeests and gazelles, but no signs of competition were found. The Serengeti grazing populations instead show evidence of grazing facilitation, in which the feeding activity of herbivore species improves the food supply available to a second species. Heavy grazing by wildebeests prepares the grass community for subsequent exploitation by Thomson’s gazelles in the same general way that zebra feeding improves wildebeest grazing.

1.Herbivores (plant- eating animals) living in the same environment often compete for food plants. But in some cases, herbivores may cooperate in the harvesting of plant matter. The grazing system of large herbivores on the Serengeti Plain of east Africa is an excellent illustration of how these animals may interact over their food supply. The Serengeti Plain contains the most spectacular concentration of large mammals found anywhere in the world. Approximately 1,000,000 wildebeests, 600,000 Thomson’s gazelles, 200,000 zebras, and 65,000 buffalo occupy an area of 23,000 square kilometers (9,000 square miles), along with undetermined numbers of 20 other species of grazing animals.