Animal Distribution, the geographic distribution of animal life on the Earth, the study of which is called zoogeography. Animals vary widely in their tolerance of environmental conditions. Some can survive in several types of habitat, whereas others perish when removed from their natural surroundings. No animals other than humans can create sufficient artificial changes to enable them to exist in a totally strange environment without evolving through many generations of adaptation. The specific interactions of animals with their environment are the subject matter of ecology. The zoogeographer studies the distribution of animal life, using ecological principles to explain the patterns of that distribution.
Animal environments can be roughly classified as either water or land habitats. Winged creatures are classified according to the land or water bases to which they ultimately return. Distribution of aquatic animals is subdivided into salt-water and freshwater habitats.
II SALT-WATER ANIMALS
Many species of whales and predatory fish have been observed in all salt-water seas. Most aquatic animals, however, are limited to relatively definite climatic areas. In general, animals do not leave their climatic zone, and in a zone that is divided by land masses, the animals are prevented from passing to other bodies of water in the same zone.
The environmental conditions in deep waters are markedly different at each level of depth. The water temperature decreases and pressure increase with increasing depth. Feeding conditions, which depend on the number and types of plants and animals present, also vary greatly with depth. The depth of the ocean floor is much more variable than is the elevation of land. Consequently, the few widely distributed salt-water forms are those that live at or near the surface. An aquatic animal that can live only at depths of 6,000 to 7,500 m (20,000 to 25,000 ft) cannot cross a ridge in the ocean floor if the crest of the ridge is only 3,000 m (10,000 ft) below the surface.
Assuming relative uniformity of temperature, pressure, and feeding conditions, salt-water habitats may be divided into three zones: littoral, pelagic, and abyssal. The littoral zone includes the coastal regions of oceans and seas, from the shoreline seaward to a depth of about 180 m (600 ft). The animal population includes the crowded life of the shore region, such as corals, mussels, higher arthropods, and fish. The pelagic zone includes the surface waters of the open sea to the same depth as the littoral zone. Many pelagic forms, such as jellyfish and true fish equipped with air bladders, are adapted for floating, but most of the inhabitants of this zone are swimming forms. The abyssal zone is the deep, unilluminated region of the ocean. This region contains almost no plant life, but the abyssal inhabitants, such as crabs, feed on dead organisms that sink from the surface. Unique in this setting is the hydrothermal vent communities of plants and animals, where the food chain is based on sulfur-digesting bacteria.
III FRESHWATER ANIMALS
The composition of freshwater communities is much more dependent on climate than is that of salt-water communities. The oceans of the world cover vast areas and to a great extent flow into each other; bodies of fresh water do not. Consequently, dissemination of freshwater species is much more limited than is that of salt-water species. Variation in chemical composition is greater for inland waters than for ocean waters because minerals dissolved in fresh water cannot dissipate over broad areas to the extent that oceanic minerals can. Within these limits, however, two large subdivisions of inland fresh waters exist flowing waters and standing waters. Flowing waters are usually connected with the sea, and the many oceanic species that enter the rivers form an important part of the animal population. The swiftness of currents in flowing water requires that animals be strong swimmers (such as salmon), bottom dwellers (such as crayfish), or forms that can attach themselves to rocks, water plants, or debris (such as leeches). Standing waters have little movement, so slow-swimming and sedentary forms are abundant in such areas. Basins of standing water collect organic debris more than flowing water, making possible plant populations large enough to furnish an abundant food supply for the animal population.
IV LAND ANIMALS
Like aquatic animals, land animals are limited in distribution by environmental conditions, of which the most restrictive is the separation of land masses by water, high mountain ranges, and extensive desert areas. An isolated island may have fauna strikingly different from the fauna of a nearby continent. On the island of Madagascar, for example, no large mammals are found, but several families of primates are wholly or partly confined to the island, and more than 100 species of native birds occur nowhere else.
The land area of the world is divided into six zoogeographic regions, each having a different fauna: (1) the Palaearctic region, including Europe, Asia north of the Himalaya, and the northern coasts and deserts of Africa; (2) the Oriental region, including India, Myanmar, Thailand, Indonesia, and the Philippines; (3) the Ethiopian region, including Africa south of the Sahara; (4) the Australian region, including Australia, New Zealand, New Guinea, and the islands of Oceania; (5) the Nearctic region, including Greenland and all of North America north of Mexico City; and (6) the Neotropical region, including southern Mexico, the Caribbean, Central America, and all of South America. Within these boundaries, animals are grouped according to the particular habitat that they occupy, such as plains, deserts, or forests. The habitat of a land animal is determined by such variables as the type of food available and the amount of protection from predatory animals that the habitat affords.