United States

The grasslands occur principally in the Great Plains area and extend westward into the intermontane basins and benchlands of the Rocky Mountains. Numerous grasses such as buffalo, grama, side oat, bunch, needle, and wheat grass, together with many kinds of herbs, make up the plant cover. Coniferous forests cover the lesser mountains and high plateaus of the Rockies, Cascades, and Sierra Nevada. Ponderosa (yellow) pine, Douglas fir, western red cedar, western larch, white pine, lodgepole pine, several spruces, western hemlock, grand fir, red fir, and the lofty redwood are the principal trees of these forests. The densest growth occurs west of the Cascade and Coast ranges in Washington, Oregon, and northern California, where the trees are often 100 feet (30 metres) or more in height. There the forest floor is so dark that only ferns, mosses, and a few shade-loving shrubs and herbs may be found.

The alpine tundra, located in the conterminous United States only in the mountains above the limit of trees, consists principally of small plants that bloom brilliantly for a short season. Sagebrush is the most common plant of the arid basins and semideserts west of the Rocky Mountains, but juniper, nut pine, and mountain mahogany are often found on the slopes and low ridges. The desert, extending from southeastern California to Texas, is noted for the many species of cactus, some of which grow to the height of trees, and for the Joshua tree and other yuccas, creosote bush, mesquite, and acacias.

The United States is rich in the variety of its native forest trees, some of which, as the species of sequoia, are the most massive known. More than 1,000 species and varieties have been described, of which almost 200 are of economic value, either because of the timber and other useful products that they yield or by reason of their importance in forestry.

Besides the native flowering plants, estimated at between 20,000 to 25,000 species, many hundreds of species introduced from other regions—chiefly Europe, Asia, and tropical America—have become naturalized. A large proportion of these are common annual weeds of fields, pastures, and roadsides. In some districts these naturalized “aliens” constitute 50 percent or more of the total plant population.

Paul H. Oehser Reed C. Rollins The Editors of Encyclopaedia Britannica

Animal life

With most of North America, the United States lies in the Nearctic faunistic realm, a region containing an assemblage of species similar to Eurasia and North Africa but sharply different from the tropical and subtropical zones to the south. Main regional differences correspond roughly with primary climatic and vegetal patterns. Thus, for example, the animal communities of the Dry West differ sharply from those of the Humid East and from those of the Pacific Coast. Because animals tend to range over wider areas than plants, faunal regions are generally coarser than vegetal regions and harder to delineate sharply.

The animal geography of the United States, however, is far from a natural pattern, for European settlement produced a series of environmental changes that grossly altered the distribution of animal communities. First, many species were hunted to extinction or near extinction, most conspicuously, perhaps, the American bison, which ranged by the millions nearly from coast to coast but now rarely lives outside of zoos and wildlife preserves. Second, habitats were upset or destroyed throughout most of the country—forests cut, grasslands plowed and overgrazed, and migration paths interrupted by fences, railroads, and highways. Third, certain introduced species found hospitable niches and, like the English sparrow, spread over huge areas, often preempting the habitats of native animals. Fourth, though their effects are not fully understood, chemical biocides such as DDT were used for so long and in such volume that they are believed at least partly responsible for catastrophic mortality rates among large mammals and birds, especially predators high on the food chain. Fifth, there has been a gradual northward migration of certain tropical and subtropical insects, birds, and mammals, perhaps encouraged by gradual climatic warming. In consequence, many native animals have been reduced to tiny fractions of their former ranges or exterminated completely, while other animals, both native and introduced, have found the new anthropocentric environment well suited to their needs, with explosive effects on their populations. The coyote, opossum, armadillo, and several species of deer are among the animals that now occupy much larger ranges than they once did. Peirce F. Lewis