THE DISTRIBUTION OF ANIMALS
Technical Note.—The larger aspects or phenomena of the distribution of animals over the earth on land and in sea cannot be studied personally in the field by the student, but many local features of distribution can be so observed and studied. The restriction of certain kinds of animals to certain kinds of habitat, the presence and character and effectiveness of barriers, some of the modes of distribution, the presence and successful life of introduced foreign species such as the black and brown rats, the English sparrow, the German and Asiatic cockroaches, the gradual change of range or distribution of certain kinds of animals through the influence of a change in environment (caused by man in cutting off forests, cultivating heretofore wild pastures, etc.) all offer favorable and profitable opportunities for personal observation.
An excellent and feasible piece of field-work in distribution is the making of a zoological survey of the locality in which the school is situated. A map of the locality should be made on a generous scale, which should include all prominent physical features of the region, such as streams, ponds, hills, woodlands, marshes, etc., and on this map should be indicated the places where the various animals of the local fauna occur. Some of the animal species will have a limited range, and the limits of this range should be shown. This map and faunal list can be added to and perfected by successive classes. For fuller discussions of the geographical distribution of animals see Jordan and Kellogg's "Animal Life," Beddard's "Zoogeography," Heilprin's "The Distribution of Animals," and Wallace's "Geographical Distribution."
Geographical distribution.—It is a matter of common knowledge with all of us that there are no wild lions or camels or kangaroos or monkeys or ostriches or nightingales in North America. Ostriches are found only in Africa and South America, kangaroos only in Australia, lions only in Asia and Africa. On the other hand there are no opossums in Europe or grizzly bears or rattlesnakes anywhere else in the world than in this country. That is, certain kinds of animals have a certain limited range of occurrence or distribution. It is obvious, too, that certain animals live only on land, while others live only in water, and of these latter some are restricted to the ocean, while others live only in fresh water. All of the facts regarding the dispersion or diffusion of animals on land and in water make up the science of the geographical distribution of animals, or, as it is sometimes called, zoogeography. Under this subject are included not only the facts of the present actual distribution or occurrence of animals over the world, but the facts concerning the reasons for this distribution, the modes of travel and dispersion, the character and influence of barriers to the spread, and the results, in the adaptation of old forms and the production of new forms, of the phenomena of distribution.
Just as maps are made to show graphically the facts of political geography, which concerns the position and extent of the various powers and States which claim the allegiance of the people, and the facts of physical geography, which concerns the physical character of the earth's surface, so maps are made to show the geographical distribution of animals. Because of the great numbers of animal species no one map can show the distribution of all species, but a series of maps of the world or of a continent or of a State or county or more limited region could be made (and many such have been made) showing the distribution of selected species. In a map of a limited locality, say of a few square miles, the occurrence and distribution of most of the commoner and more familiar animals can be shown, and each high school should possess such a map (see technical note at beginning of this chapter).
Laws of distribution.—The laws governing the distribution of animals over the earth's surface have been recently[20] expressed in a simple statement as follows: Every species of animal is found in every part of the earth unless (a) its individuals have been unable to reach this region on account of barriers of some sort; or (b) having reached it, the species is unable to maintain itself, through lack of capacity for adaptation, through severity of competition with other forms, or through destructive conditions of environment; or (c) having entered and maintained itself it has become so altered in the process of adaptation as to become a species distinct from the original type.
Modes of migration and distribution.—That animals should be continually trying to extend their range is obvious from what we know of their rapid increase by multiplication. In a region which can provide food for but one thousand wolves, there is a production each year of several times one thousand. These new wolves must struggle among themselves for food, or migrate, if possible, to new regions as yet not inhabited by wolves. The wolf's mode of migration or distribution is walking or running, and so with other mammals except the bats and aquatic forms. Birds and bats can fly, and can thus migrate more swiftly, farther, and over barriers which would stop mammals. Most insects can fly. Worms can only crawl and very slowly at that. Fishes can swim, but if they are in a landlocked sheet of water, they cannot go beyond its confines. Marine animals can migrate from ocean to ocean, and land animals from continent to continent unless checked by barriers (see next paragraph).
But besides such voluntary and independent modes of distribution long journeyings may be made involuntarily, or by a passive migration as it may be called. Parasites, for example, are carried by their hosts in all their travels; the tiny Tardigrada and Rotifera, which can be desiccated and yet restored to active life by coming again into water, are carried in the dried mud on the feet of birds or other animals. On floating objects in rivers or in ocean currents land-animals may be carried long distances. Man, the greatest traveller of all, is responsible for the widened distribution of many animals. Thus have come to us in ships from Europe the black and brown rats, the English sparrow, the Hessian fly, the commonest cockroaches of our houses and many other forms. And these animals have been carried involuntarily all over the United States in railway-cars and wagons.
Barriers to distribution.—As is indicated in the paragraph on the modes of migration, a considerable sheet of water is obviously a barrier to the further travelling of a walking or crawling land-animal, although no barrier to a winged form. Similarly a strip of land is a barrier to a strictly aquatic animal as a fish. Or a high fall in the stream may serve as an insuperable barrier, making it impossible for any fish below the fall to reach the upper part of the stream. Numerous cases of this kind are known in the Rocky Mountains and Sierra Nevada, where a stream may be well supplied with trout below a fall, and quite bare of these fish above the fall. In the Yosemite Valley in California trout live in the Merced River below the great Vernal and Nevada falls, but above these falls the Merced contains no trout. To fresh-water swimming animals salt water may be a barrier; thus some kinds of fresh-water fishes are limited to one of two near-by streams although the mouths of these streams empty near each other into the ocean. The amphibious batrachians, at home in fresh water and on land, are killed by contact with sea-water. Earthworms also are killed by salt water. Thus the narrowest ocean strait is as effective a barrier to these animals as a whole sea. High mountain ranges and broad deserts are barriers to many land-animals, partly because of the physical obstacles, partly because of the differences in temperature and in food-supply.
Temperature and climate (as distinct from temperature) and the ocean are the three great barriers when we consider the animal kingdom as a whole, and look for the causes which determine the chief zoogeographical divisions of the earth's surface. Most of the tropical animals cannot endure frost, hence the isothermal line of frost is a line across which few tropical animals venture. Most arctic animals are enfeebled by heat, and the isothermal line which marks off the region in which frost occurs the year round is another great zoogeographical boundary. But while these lines are limits for localized species, some animals, as birds, especially, keep within a relatively uniform temperature by migrations across these lines. It should be borne in mind that the gradual decrease in temperature met with in going north or south from the tropics is also met in the ascent of high mountains. The summits of lofty peaks, even in the tropics, are truly arctic in character; they are snow-covered, and the animals and plants on them are truly arctic. Thus in the ascent of a single mountain a whole series of life-zones from tropical to arctic can be traversed.
Climate, as distinct from temperature, establishes limits of distribution. The animals of Eastern North America accustomed to a humid atmosphere cannot live in the dry plains and deserts of the West. Closely associated with climate is the nature of the plant-growth covering the land; here are forests and luxuriant meadows, there are sparse tough grasses of the dry plateau. The limits of a special kind of plant-growth often are the limits of distribution of certain animals.
The third great barrier, the ocean, is perhaps the most obvious of all in its influence. It is only in rare cases that any land-animal can independently cross a great ocean. Thus the land-animals of Australia differ from those of all other countries, and those of Africa and South America have developed almost independently of one another. The ocean is, as already mentioned, also a barrier for fresh-water aquatic animals, and even marine fishes which live normally in shallow waters along the shore rarely venture across the great depths of mid-ocean.
The obstacles or barriers met with determine the limits of a species. Each species broadens its range as far as it can. It attempts unwittingly, through natural processes of increase, to overcome the obstacles of ocean or river, of mountain or plain, of woodland or prairie or desert, of cold or heat, of lack of food or abundance of enemies—whatever the barriers may be. The degree of hindrance offered by any barrier differs with the nature of the animal trying to pass it. That which forms an impassable obstacle to one species may be a great aid to the spread of another. "The river which blocks the monkey or the cat is the highway of the fish and turtle. The waterfall which limits the ascent of the trout is the chosen home of the ouzel."
Faunæ and zoogeographic areas.—The term fauna is applied to the animals of any region considered collectively. Thus the fauna of Illinois includes the entire list of animals found naturally in that State. The fauna of a schoolyard comprises all the animals found living naturally in the yard. The fauna of a pond includes all the animal inhabitants of the pond. (Flora is used similarly of all the plants in a given region.) The relation of one fauna to another depends on the character and effectiveness of the barriers between, and the physical character of the two regions. Thus the fauna of Illinois differs but little from that of Indiana or Iowa, because there are no barriers between the States, and they are alike physically. On the other hand the fauna of California differs much from that of the Eastern States because of the great barriers (the desert and the Sierra Nevada Mountains) which lie between it and these States, and because of the great differences in the physical and climatic conditions of the two regions.
The land-surface of the earth has been divided by zoogeographers into seven great realms of animal life, based on the distributional characters shown by these various regions. These realms are separated by barriers of which the chief are the presence of the sea and the occurrence of frost. These realms are named, from their geographical region, the Arctic, the North Temperate, the South American, the Indo-African, the Madagascar, the Patagonian, and the Australian. Of these the Australian alone is sharply defined. Most of the others are surrounded by a broad fringe of debatable ground that forms a transition to some other zone.
Habitat and species.—The habitat of a species of animal is the region in which it is found in a state of nature. It is currently believed that the habitat of any animal is the whole of that region for which it is best adapted. But this is not necessarily true. In fact in most cases it is not true. The trout naturally debarred from the rivers in Yellowstone Park by the waterfalls could live there well if the barrier could be passed. In the case of one stream it has been passed and the trout flourish above the fall. The success of the black and brown rats and the English sparrow in America, of the rabbit in Australia, of bumblebees and house-flies in New Zealand, all of which animals had a natural habitat not including these regions, but by artificial means have been carried over the barriers and into the new territory, prove that "habitat" is not necessarily coincident with "only fit region." Shad, striped bass, and catfish from the Potomac River have been introduced into and now thrive in the Sacramento River in California. In fact the whole work of the introduction and diffusion of valuable food-animals in territory not naturally included in the habitat of the species is based on our knowledge that the habitat of a species is often determined by physical barriers rather than by exclusive fitness of environment. Within the natural habitat the environment is fit for the species' existence, outside of it the environment may be fit.
But there occur numerous instances where a species successful in leaving its original habitat is unsuccessful in attempting to maintain itself on new ground. Man has introduced various animals from one country to another. The English sparrow (naturally debarred from this country by the ocean barrier), brought to America from Europe, has covered its new territory rapidly and maintains itself with brilliant success. But the nightingale, the starling and skylark which have been repeatedly introduced and set free are unable to maintain themselves here.
Species-extinguishing and species-forming.—Accompanying the constant slow migrating of species into new habitats and the constant slow changing of environment and conditions everywhere is to be seen a constant modification of the fauna of any region due to the inability of some species to maintain their ground, the predominating increase of others, and the modifying or adaptive changing of others into new forms. In 1544 the black rat of Europe was introduced into America and it soon crowded out the native rats, being in its turn crowded out by the European brown rat (the present common rat in buildings), introduced about 1775. Here we have the original native species unable to maintain itself in competition with introduced forms.
With a change of environing conditions, certain species are unable to maintain themselves. With the destruction of the forests going on in parts of our country the great host of wood-creatures, the bears, squirrels, the wood-birds and insects, can no longer maintain themselves, and grow rare and disappear. Man often also influences the status of a species by checking its increase either by actual slaughter, as with the bison and passenger-pigeon, or by making adverse changes in its environment, as by destroying forests, or putting the plains under cultivation.
In the discussion of "species-forming" (see p. [408]) it was shown that adaptation may lead to the altering of species, and to the formation of new ones (under the influence of natural selection). With the gradual change of conditions, or with the facing of new conditions because of an unusual migration to or invasion of new territory, those individuals of the species exposed to the new conditions must adapt themselves in structure and habit in order to meet successfully the new demands. By the cumulative action of natural selection these adaptive changes are emphasized; and this emphasis may come to be so pronounced that the part of the species represented in this newly acquired territory, if isolated from the original stock, is so altered as to be quite distinct in appearance from the old. If these changed individuals are also physiologically distinct from the old stock, i.e. are unable to mate with them, a new species is established. As already mentioned, the peopling of islands from mainlands is an excellent and readily observable example of the phenomena referred to in the third law of distribution.