Rangelands Must Be Grazed to Stay Healthy
Over much of the area that is now public land in the West, native plant communities evolved largely in the absence of grazing herd animals. Between the Sierra Nevada-Cascade crest and the Rocky Mountains lies the arid Intermountain West, composed of areas such as the Great Basin, the Palouse prairie, and the deserts of the Southwest, where bison were mostly absent and even herds of pronghorn antelope, bighorn sheep, elk, and other herbivores tended to be small and widely distributed. Consequently, the plant species of this region are not adapted to continual heavy grazing and trampling, as occurs with domestic livestock.
Yet some livestock proponents argue that although no large herds of grazing or browsing animals occurred in the Intermountain West in historic times, during the last Ice Age great numbers of wild horses, mastodons, giant sloths, and other herbivores roamed these lands. Thus, livestock advocates claim, cattle are merely filling a niche left empty since the extinction of these Pleistocene mammals. The problem, however, is that climatic conditions were very different during the Ice Age-precipitation was higher, for example-and plant communities were much different in composition, as well as generally more productive than today. Cattle are not filling some long-vacant ecological niche but are, in fact, exotic animals that have dramatically altered the native plant communities of the arid West.
Even where large herds of bison, elk, and pronghorn were common, such as on the Great Plains, plants do not need to be grazed. Rather, many Great Plains grasses tolerate grazing by compensating for losses in leaf and stem materials through additional growth. However, when plants move carbohydrates up from their roots to produce new leaves, root growth may slow, and seed production may be inhibited. Only plants with unlimited access to water and nutrients and with no competition (conditions found only in a growth chamber) can withstand repeated cropping without harm. In nature, plants repeatedly munched by livestock suffer from diminished root mass-a potentially lethal situation for the plant during a drought. Of course, drought occurs commonly in the West, including the Great Plains.