The Effects of Grazing and Trampling Behaviors of Large-Sized Livestock on the Formation and Weathering Patterns of Soils
Walter Coppinger, a Professor of Geosciences at Trinity College in San Antonio and long-time observer of Montana geology, was the first person to describe to me the many problems of the western rangelands that have developed out of the over-grazing of cattle. From a hilltop among the upland slopes of Whitehall, Montana, he pointed out a few patches of bare earth on the horizon and some gullying out of trails across the rangelands in the distance. Rangelands are areas of land on which livestock are left to roam and graze. Traditionally the great plains and rolling hills of the Western States have been dominated by rangelands left to cattle and bison, and though it has long been acknowledged that cattle grazing and roaming can alter the features of the land, the extent and depth to which they can do this has been underestimated and at times ignored. Privately owned pastures and rangelands in the United States have suffered a more than 15% decrease in number of acres since 1940, but despite figures like this one and a multitude of essays lamenting the "shrinking of the great plains," the number of cattle in the Western United States has more than doubled in the last 60 years (Trimble and Mendel, 1995; U.S. Census of Agriculture). With the numbers of grazing animals growing and the lands for them to occupy getting smaller, a better understanding of how and to what degree these animals affect the soil is needed.
Most changes in processes of soil formation and soil erosion are indirectly affected by the presence of livestock and more directly associated with the geomorphic changes these animals generate. Changes in geomorphology are controlled by the local topography, rainfall and hydrology, and groundcover, as well as the type and size of the animal population. There are many different animal species that graze the rangelands of the world: bison in the grasslands of the Western United States, mountain goats in the Rocky Mountains, and herds of Caribou in Northwest Canada (Butler, 1995). In many cases, the actual effects that different animals have depends greatly on biological factors characteristic of their particular species, such as how large their populations tend to be, their average weight and how they distribute their weight when they walk or climb, feeding patterns, migration patterns, and other differences that depart too far from the focus of this essay to be considered in any great detail. For the purposes of this paper, unless otherwise specified, I will be considering the effects of cows, bison, and other "cattle-like" livestock animals that tend to have relatively similar physical characteristics.
The degree to which an area has been grazed is often loosely categorized as an area of "light," "moderate," or "heavy" grazing, with "light" indicating rangelands with the greatest surface area...