Plant life

The dominant features of the vegetation are indicated by the terms forest, grassland, desert, and alpine tundra.

A coniferous forest of white and red pine, hemlock, spruce, jack pine, and balsam fir extends interruptedly in a narrow strip near the Canadian border from Maine to Minnesota and southward along the Appalachian Mountains. There may be found smaller stands of tamarack, spruce, paper birch, willow, alder, and aspen or poplar. Southward, a transition zone of mixed conifers and deciduous trees gives way to a hardwood forest of broad-leaved trees. This forest, with varying mixtures of maple, oak, ash, locust, linden, sweet gum, walnut, hickory, sycamore, beech, and the more southerly tulip tree, once extended uninterruptedly from New England to Missouri and eastern Texas. Pines are prominent on the Atlantic and Gulf coastal plain and adjacent uplands, often occurring in nearly pure stands called pine barrens. Pitch, longleaf, slash, shortleaf, Virginia, and loblolly pines are commonest. Hickory and various oaks combine to form a significant part of this forest, with magnolia, white cedar, and ash often seen. In the frequent swamps, bald cypress, tupelo, and white cedar predominate. Pines, palmettos, and live oaks are replaced at the southern tip of Florida by the more tropical royal and thatch palms, figs, satinwood, and mangrove.

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 meters) 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.

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.

Arrangement of the account of the distribution of the fauna according to the climatic and vegetal regions has the merit that it can be compared further with the distribution of insects and of other invertebrates, some of which may be expected to fall into the same patterns as the vertebrates, while others, with different modes or different ages of dispersal, have geographic patterns of their own.

The transcontinental zone of coniferous forest at the north, the taiga, and the tundra zone into which it merges at the northern limit of tree growth are strikingly paralleled by similar vertical zones in the Rockies, and on Mount Washington in the east, where the area above the timberline and below the snow line is often inhabited with tundra animals like the ptarmigan and the white Parnassius butterflies, while the spruce and other conifers below the timberline form a belt sharply set off from the grassland or hardwood forest or desert at still lower elevations.

A whole series of important types of animals spread beyond the limits of such regions or zones, sometimes over most of the continent. Aquatic animals, in particular, may live equally in forest and plains, in the Gulf states, and at the Canadian border. Such widespread animals include the white-tailed (Virginia) deer and black bear, the puma (though only in the remotest parts of its former range) and bobcat, the river otter (though now rare in inland areas south of the Great Lakes) and mink, and the beaver and muskrat. The distinctive coyote ranges over all of western North America and eastward as far as Maine. The snapping turtle ranges from the Atlantic coast to the Rocky Mountains.

In the northern coniferous forest zone, or taiga, the relations of animals with European or Eurasian representatives are numerous, and this zone is also essentially circumpolar. The relations are less close than in the Arctic forms, but the moose, beaver, hare, red fox, otter, wolverine, and wolf are recognizably related to Eurasian animals. Even some fishes, like the whitefishes (Coregonidae), the yellow perch, and the pike, exhibit this kind of Old World–New World relation. A distinctively North American animal in this taiga assemblage is the Canadian porcupine.

The hardwood forest area of the eastern and the southeastern pinelands compose the most important of the faunal regions within the United States. A great variety of fishes, amphibians, and reptiles of this region have related forms in East Asia, and this pattern of representation is likewise found in the flora. This area is rich in catfishes, minnows, and suckers. The curious ganoid fishes, the bowfin and the gar, are ancient types. The spoonbill cat, a remarkable type of sturgeon in the lower Mississippi, is represented elsewhere in the world only in the Yangtze in China. The Appalachian region is headquarters for the salamanders of the world, with no less than seven of the eight families of this large group of amphibians represented; no other continent has more than three of the eight families together. The eel-like sirens and amphiumas (congo snakes) are confined to the southeastern states. The lungless salamanders of the family Plethodontidae exhibit a remarkable variety of genera and a number of species centering in the Appalachians. There is a great variety of frogs, and these include tree frogs whose main development is South American and Australian. The emydid freshwater turtles of the southeast parallel those of East Asia to a remarkable degree, though the genus Clemmys is the only one represented in both regions. Much the same is true of the water snakes, pit vipers, rat snakes, and green snakes, though still others are peculiarly American. The familiar alligator is a form with an Asiatic relative, the only other living true alligator being a species in central China.

In its mammals and birds the southeastern fauna is less sharply distinguished from the life to the north and west and is less directly related to that of East Asia. The forest is the home of the white-tailed deer, the black bear, the gray fox, the raccoon, and the common opossum. The wild turkey and the extinct hosts of the passenger pigeon were characteristic. There is a remarkable variety of woodpeckers. The birdlife in general tends to differ from that of Eurasia in the presence of birds, like the tanagers, American orioles, and hummingbirds, that belong to South American families. Small mammals abound with types of the worldwide rodent family Cricetidae, and with distinctive moles and shrews.

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Most distinctive of the grassland animals proper is the American bison, whose nearly extinct European relative, the wisent, is a forest dweller. The most distinctive of the American hoofed animals is the pronghorn, or prongbuck, which represents a family intermediate between the deer and the true antelopes in that it sheds its horns like a deer but retains the bony horn cores. The pronghorn is perhaps primarily a desert mammal, but it formerly ranged widely into the shortgrass plains. Everywhere in open country in the West there are conspicuous and distinctive rodents. The burrowing pocket gopher is peculiarly American, rarely seen making its presence known by pushed-out mounds of earth. The ground squirrels of the genus Citellus are related to those of Central Asia, and resemble them in habit; in North America the gregarious prairie dog is a closely related form. The American badger, not especially related to the badger of Europe, has its headquarters in the grasslands. The prairie chicken is a bird distinctive of the plains region, which is invaded everywhere by birds from both the east and the west.

The Southwestern deserts are a paradise for reptiles. Distinctive lizards such as the poisonous Gila monster abound, and the rattlesnakes, of which only a few species are found elsewhere in the United States, are common there. Desert reptile species often range to the Pacific Coast and northward into the Great Basin. Noteworthy mammals are the graceful bipedal kangaroo rat (almost exclusively nocturnal), the ring-tailed cat, a relative of the raccoon, and the piglike peccary.

The Rocky Mountains and other western ranges afford distinctive habitats for rock- and cliff-dwelling hoofed animals and rodents. The small pikas, related to the rabbit, inhabit talus areas at high elevations as they do in the mountain ranges of East Asia. Marmots live in the Rockies as in the Alps. Every western range formerly had its own race of mountain sheep. At the north the Rocky Mountain goat lives at high altitudes—it is more properly a goat antelope, related to the takin of the mountains of western China. The dipper, remarkable for its habit of feeding in swift-flowing streams, though otherwise a bird without special aquatic adaptations, is a Rocky Mountain form with relatives in Asia and Europe.

In the Pacific region the extremely distinctive primitive tailed frog Ascaphus, which inhabits icy mountain brooks, represents a family by itself, perhaps more nearly related to the frogs of New Zealand than to more familiar types. The Cascades and Sierras form centers for salamanders of the families Ambystomoidae and Plethodontidae second only to the Appalachians, and there are also distinctive newts. The burrowing lizards, of the well-defined family Anniellidae, are found only in a limited area in coastal California. The only family of birds distinctive of North America, that of the wren-tits, Chamaeidae, is found in the chaparral of California. The mountain beaver, or sewellel (which is not at all beaverlike), is likewise a type peculiar to North America, confined to the Cascades and Sierras, and there are distinct kinds of moles in the Pacific area.

The mammals of the two coasts are strikingly different, though true seals (the harbour seal and the harp seal) are found on both. The sea lions, with longer necks and with projecting ears, are found only in the Pacific—the California sea lion, the more northern Steller’s sea lion, and the fur seal. On the East Coast the larger rivers of Florida are inhabited by the Florida manatee, or sea cow, a close relative of the more widespread and more distinctively marine West Indian species.