After the earth was created, soil formed slowly over geological time from the weathering of rocks. It began to support early plant life, which protected and enriched it until it became the topsoil that sustains the diversity of plants and animals we know today. Now the world's ever-growing herds of cattle, sheep, and goats are converting vast stretches of grassland to desert.
One indicator that helps us assess grassland health is changes in the goat population relative to those of sheep and cattle. As grasslands deteriorate, grass is typically replaced by desert shrubs. In such a degraded environment, cattle and sheep do not fare well. But goats -- being particularly hardy ruminants -- forage on the shrubs. Goats are especially hard on the soil because their sharp hoofs pulverize the protective crust of soil that is formed by rainfall and that naturally checks wind erosion. Between 1970 and 2009, the world's cattle population increased by 28 percent and the number of sheep stayed relatively static. Meanwhile, goat herds more than doubled.