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 semitechnical column by these well-known authors/ecologists/educators.
In our last column we discussed ecoscience and how the large herbivores in the Serengeti ecosystem divide their food resources. Such resource partitioning is not restricted to the herbivores, though: The predators that feed on those herbivores do it, too.
Predators of the Serengeti
Lions (which weigh 200 to 400 pounds) feed primarily on zebras and wildebeests when those migrating herbivores are within their prides' territories, which can be found in both the plains and the woodlands. The cats' prey among nonmigratory ungulates includes buffalo (which no other predator can kill) and giraffes, as well as warthogs and antelope. Lions stalk their prey mostly at night and sprint to catch their meals. Sometimes they hunt alone, sometimes in small groups.
Another large cat of the Serengeti, the leopard (75 to 130 pounds), is confined to the woodlands and generally takes smaller quarry than the lion does. At the uppermost end of the size scale, the leopard's choice of prey (Thomson's gazelles, topi and an occasional zebra) overlaps that of the lion, but the spotted cat feeds extensively on smaller antelope such as Kirk's dik-dik (only 15 inches high at the shoulder), small carnivores, hares and birds. The leopard is a solitary, nocturnal, stalk-and-sprint predator.
The cheetah is as large and heavy as the leopard, but is much more slender. It hunts in the daytime, taking small antelope and hares. After stalking, it can run down its prey at speeds up to 60 mph. This cat, the classic longdistance sprint predator, may chase its quarry as far as 350 yards, whereas a lion will sprint no more than 50.
Hyenas hunt the plains at night and in the early hours of the morning — in groups of one to three for wildebeests and other antelope, and in packs of four to twenty for the larger zebras. They are pursuit predators; that is, they may harry their prey for up to 2 miles before finally making the kill.
Wild dogs, the other pursuit predators of the Serengeti, take a similar array of prey. They hunt mostly by day, with an entire pack of two to 19 dogs participating in chases of a mile or more.
The pursuit predators concentrate more on attacking the sick, wounded or very old or young among their prey than do the sprint predators.
Serengeti Scavengers and Decomposers
Actually, there is relatively little competition for living prey among predators of the Serengeti, since each type has its own food preferences, territories and time of day to hunt. Competition for dead prey is another matter, however. Only cheetahs do not supplement their diet by scavenging. Hyenas get about a third of their food that way, lions 10 to 15 percent, leopards Solo to 10 percent and wild dogs about 3 percent.
Lions alone are not significantly interfered with by other predators of the Serengeti. But cheetahs lose 10 to 12 percent of their prey to hyenas and (occasionally) lions; wild dogs lose about half their kills to hyenas; and hyenas and leopards are thought to lose 5 percent or more to lions.
The final major element in the Serengeti ecosystem is the group of creatures that serve as decomposers. Many of these lifeforms are similar to the decomposers in other ecosystems: flies whose maggots help break down cadavers ... beetles that feed on hides or dung ... and bacteria and fungi that do much of the fine work of reducing organic compounds in dead plants and animals and in feces to inorganic compounds.
But the Serengeti ecosystem is unique in having a great variety of large decomposers, including scavenging predators — lions, leopards, hyenas and wild dogs. These animals (along with cheetahs, which eat only their own kills) devour a third or more of the roughly 40,000 tons of ungulates that die annually in the Serengeti.
Of the remaining two-thirds of dead animal matter, half or more is composed of soft tissues. Nearly all of this is consumed by the most abundant large decomposers in the system: the marabou stork and seven species of vultures. The remainder of the material is consumed by insects, other invertebrates and bacteria.
Thus we can see the major outlines of the Serengeti system. In a temporally and geographically varied, semi-arid physical setting, energy and nutrients enter the system in the form of grasses, herbs, shrubs and trees. This vegetation supports an array of herbivores that includes an unusual abundance and diversity of ungulates. And the herbivores, in turn, sustain an equally sizable number of large carnivorous predators. Finally, the decomposers make their living on what remains of the available energy ... and return the nutrients to the soil, to complete the cycle.
Serengeti Herbivores' Social Systems
Of course, this overview omits many important details that influence the functioning of the system. We'll briefly consider just one of them: How the combined selective pressures of getting food and avoiding predators has helped to shape the social systems of the hoofed herbivores.
Serengeti antelope, for instance, range in size from the 8-pound dik-dik to the 1,500-pound eland. Dik-diks live in pairs with their young on permanent territories (areas defended against other dikdiks) in woodland with thick cover. They communicate by scent marking, since calls or visual displays would reveal their positions to predators. Their territories, presumably, are large enough to contain sufficient food at even the leanest time of year.
The tiny dik-diks have higher metabolic requirements per unit of body weight than larger species have. As a result, these animals must feed very selectively on tender young leaves, buds, fruits and seeds. Both the dik-diks' small size and their need to search out special food items have apparently led to natural selection against forming herds for defense and have instead favored the practice of territoriality.
In contrast, larger antelope, such as impalas, cannot be quite so picky about their diets. A low-quality food intake is mandated by their greater total food requirements and by mouths too large to make fine discriminations among foods. Impalas cannot remain hidden; they must move about to seek the best grazing and browsing. Herds of up to a hundred or more impalas seasonally shift their habitat preference.
About a third of the male impalas establish temporary territories wherever the females are at the time; the rest of the males form a bachelor herd. Members of an impala herd benefit from group alertness and communication about predators. In an attack, the entire herd reacts as a coordinated unit.
Other antelope, such as waterbuck, kongoni, sedentary populations of wildebeests, Grant's gazelles and Thomson's gazelles, have social systems with many of the same features — especially the holding by males of spatially defined territories within the region occupied by the population at a given time, territories that are abandoned when the herd moves on.
Migratory wildebeests have no spatially defined territories. Instead, they set up tiny, temporary, sometimes mobile territories wherever the females are — defending them during the short rutting season. This allows the males to maintain exclusive sexual access to a group of females in spite of the need for constant traveling in search of food.
Among buffalo, and perhaps among eland — the largest of the antelope — a different system prevails. Both form large herds that move a great deal. Rather than defending territories, the males simply exercise individual dominance on the spot to keep subordinate males away from any female in estrus. Like the wildebeests, both the buffalo and the eland males solve the problem of access to females simply by staying with them. Neither of these species has a short rutting period, so a brief defense of a group of females would be to no avail; rivals for them must be perpetually excluded.
Both buffalo and eland herds cooperate in the defense of their young. Buffalo calves appear to be less able to fend for themselves than wildebeest calves, and attempts by males to cut out small groups of females from the herd would expose the calves to predation.
The formation of cohesive groups by buffalo and other large species is possible because of their ability to tolerate low-quality food. The herd can sustain itself on abundant, evenly distributed, relatively protein-poor food, such as a field of grass. Therefore, the animals can stay together, since they do not have to disperse to find food of higher quality, which tends to grow in patches.
Thus we can see that the differences in social behavior between different types of animals are linked to each kind of creature's unique dietary needs and requirements for self-preservation.
The Ehrlichs' work is supported in part by a grant from the Koret Foundation of San Francisco.