The moderately dry climate of grasslands carries the constant risk that fluctuation in rainfall will create drought. Twenty-three separate droughts have been recorded in the Russian steppes along during the past century and a half. But the grassland community survives, because the plants are adapted to the ravages of drought.
Overgrazing reduces the resistance of grasses to drought, and plowing the sod can lead to erosion that will nearly destroy the entire ecosystem. Let us examine the history of American grasslands and the effect of humans and drought on them.
The Dust Bowl. The late 1920s and early 1930s were marked with increased rainfall that permitted tall grasses to flourish on the American prairie. The seven-year drought that struck in 1933 drastically altered the species composition of the unplowed parities, most of which had been subjected to heavy grazing. Dry-adapted grasses quickly replaced those requiring more moisture.
The big bluestem grass that could grow nearly eight feet high almost disappeared; only a deep root system and underground food reserves permitted some plants to survive.
Even more drastic changes occurred in the normally drier Great Plains to the west. Grazed areas were badly damaged, but ranchers were reluctant to reduce their herds. One study in western Kansas showed less than half of the ground covered with vegetation in 1935 and only five percent in 1936.
Great dust stroms were both the cause and the result of the decreased vegetation. High temperatures and low rainfall prevented crop growth on plowed fields. Dry winds raised doused of dust from these fields and from the poorly covered grassland; when the dust fell from the air, it drifted like a horrible dark snow. A deposit of only an inch was often sufficient to smoother short grasses and produces another field of unstable soil.
Stressful as the great drought was, no species is known to have been completely eradicated. All survived in favourable habitats or as ungerminated seeds! Such survival would be expected, for only a small portion of the seeds of wild plants will sprout upon encountering favourable conditions. Not will all the remainder sprout in any one season. So after several years in which all germinating grass seedlings shrivel from lack of water and die, there will still be live seed left in the ground. When a normal year finally comes, a few seeds will sprout and begin to replenish their kind.
Although no species were lost in the drought, the effect on the soil was another story. The rains that followed the drought fell on loose, naked soil that eroded into steep gullies.
Turning Grasslands into Desert. The drought of the 1930s was certainly not the first one Americans had known, nor was that the first time dry weather and overgrazing had damaged the grasslands and hurt the ranchers. Increasing herds of sheep and cattle during the latter part of the nineteenth century had culminated in starvation of stock and disastrous economic losses when drought struck in the 1890s.
Following the Spanish pattern of migratory sheep grazing, some regions of the West have been overgrazed for several centuries. By moving the animals continuously, mainly to higher elevations int he spring and back downwards in the fall, ranchers can maintain large herds on very poor land. Arid western lands often show the effect of this practice. The hills are terraced by continuous rows of sheep paths. Except on the highest peaks of the Sierra Nevada and the Rockies, the vegetation has been altered, perhaps permanently, by such practices.
In some regions natural grasslands have been replaced by weedy desert vegetation. In western North America sagebrush now dominates many of what were once cool, short grasslands. To the south, creosote bush, cacti, and mosquito have replaced other grasses. It is important to recognize that these "new deserts" lack the diversity and complexity of old "true deserts."
But once overgrazing has produced bare sports, the mesquite thrives. It sends roots out literally, as well as deep into the soil, sometimes as far as 8 meters (25 feet). Overgrazed grasses with poor root systems cannot compete for water with mesquite. Soon the grasses die back further, and the mesquite expands. Hungry cattle feed on the fleshy mesquite fruit pods, but mature seeds pass unharmed through the cattle.
Thus cattle scatter mesquite seeds about, especially in the trampled areas where they congregate. Fertilized by the cattle dung, the mesquite seeds are well planted in the barren but hospitable soil.
American cattlemen have no monopoly on the conversion of grasslands to deserts. It has long been said that the Sahara Deserts marches farther south each year. During the early 1970s drought coupled with high human and cattle populations led to loss of herds and massive starvation among the people of the sub-Saharan or Sahel grasslands. Some climatologists trace this disaster to major weather trends, but drought, overgrazing, and disaster are not strangers to these or most other arid grasslands.