Ecoscience: Grazing Ecosystems?From Theory to Practice

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 Ehrliehs' 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 authorsecologists-educators.

by Anne and Paul Ehrlich

In the last three columns, we've seen how resources are partitioned among the wild herbivores of the Serengeti ecosystem. Kenyan wildlife biologist David Hopcraft, along with his wife, Carole, has used such knowledge to launch a successful "game ranch."

Though cattle are viewed as a source of wealth by many African peoples, such as the Masai, the animals are—from an ecological point of view—actually a source of poverty in hot, semiarid climates. Cattle (and goats and sheep) must walk daily to a water supply to drink. This passage consumes a good deal of energy and slows the rate at which the animals gain weight. It also results in the trampling of valuable grasses and compacting of the soil surface.

On the other hand, wild herbivores have much less need to drink. Some, such as eland, oryx, and Grant's gazelles, may even obtain all the water they require from the vegetation they eat. Others drink some water but still need much less than do cattle. This is because most native African herbivores conserve water much more efficiently in digestion. For example, nearly all of the moisture is extracted from the intestinal contents of gazelles, and dry feces are released.

Cowpats, in contrast, are produced moist, and rapidly lose ammonia (and thus the vital nutrient nitrogen) to the atmosphere. They then dry in the sun and heat up, killing the bacteria and fungi that might speed their decomposition. The flat, dried cowpat even kills the grass beneath it!
The dry fecal pellets of antelope, however, are roughly spherical. They fall between the grass blades, do not heat up, and retain their nitrogen. Rather than tending to create a "fecal pavement," as cattle droppings do, they break down readily and return nutrients to the soil.