Where Animals Live
Generally speaking, each animal is adapted for existence in a certain environment and may be unable to survive or reproduce in other environments. Environment includes such factors as temperature; light; moisture; atmospheric and water pressure; and gas and mineral content of air, water, and soil. It also includes other animals and plants. For example, a fish cannot live on land. It also cannot live in water that has the wrong temperature or lacks the kind of food the fish eats, and it cannot live at a depth where its body cannot withstand the water pressure.
The various factors of the environment in which any particular animal lives may not remain constant at all times. Most animals are adapted to withstand certain environmental variations. For example, birds and mammals maintain a fairly constant internal temperature in spite of limited external temperature changes. Such animals are popularly termed warm-blooded. However, some warm-blooded animals go into an inactive state called hibernation during cold seasons. Invertebrates, fish, amphibians, and reptiles have no internal means of regulating their temperatures, and cannot survive wide temperature variations. They are popularly called cold-blooded.Amphibians are cold-blooded animals.
Many birds, and some insects, fish, and mammals, travel regularly from one location to another to accommodate themselves to environmental variations, or to their needs for certain changes in the environment. These periodic movements, which are often seasonal, are called migrations. Some animals apparently migrate for feeding purposes. As winter comes on, for example, many plant-eating or insect-eating animals of the Northern Hemisphere go south, where growing plants and insects are plentiful. Deer, elk, and some birds that spend summers high in the mountains spend winters at much lower elevations.
Some animals migrate to find conditions that are suitable for breeding. These conditions may include availability of nesting sites and materials and proper temperatures for breeding and caring for their young. Several species of whales migrate as far as 11,000 miles (18,000 km) from cold waters to warmer ones at breeding time. Some species of salmon migrate only twice. Young salmon leave the freshwater streams where they were spawned and travel to the sea, where they grow to maturity. They then migrate from the sea to lay eggs in the waters where they were spawned.
The environments of many animals include cover and shelter. Tall grasses give cover for many land animals, including antelopes, lions, snakes, and birds. Fish take cover among the stalks of water plants, and even the huge hippopotamus can hide under water-lily pads by submerging up to its nostrils. In addition, an animal may take shelter from predators by climbing a tree, or by dodging into a hole in the ground or a hollow log. Gorillas and chimpanzees take shelter in trees at night, building temporary platforms of boughs and leaves in which to sleep. Bears and bats are among animals that are attracted to caves for sleeping and raising their young.
Some animals build special places called nests in which they produce their young and care for them. Birds build their nests in a variety of places, including rocky ledges, trees, shrubs, or clumps of grass. Some fishes hollow out nests in the mud or gravel of pond bottoms or stream beds. Termites build large and elaborate "hills" of earth in which to breed, take shelter, and hoard food.
The natural environment of an organism is called its habitat. The term is applied to the three large life zones: (1) salt water (divided further into depth zones), (2) fresh water (running water and standing water), and (3) land (divided into climatic or vegetation zones). The term is also used more specifically. For example, the habitat of certain land-dwelling beetles is the tropical forest zone, and more specifically the underside of rotting logs.
No particular animal is found in all the places in which it could survive. It is found only in a certain range, or area of distribution. Distribution is limited by geographic and climatic barriers such as water (for land animals), land (for water animals), high mountains, and temperature extremes, and by biological barriers such as the absence of food or the presence of predators. Population increase, climatic changes, and destruction of shelter are among other influences on distribution, forcing animals to expand their range or shift from one range to another. Ranges are sometimes extended when changing conditions have made new areas suitable.
When the ranges of the various undomesticated animals, especially mammals, are marked on a world map, several distinct regions, each with a unique composition of wildlife, are evident. These regions are called zoogeographic regions. (Similar maps showing the ranges of undomesticated plant species can be drawn; these regions are called phytogeographic regions. When the ranges of both undomesticated animals and plants are marked together on a map, these regions are called biogeographic regions.) Although there are many animals, such as bats and mice, that have a worldwide distribution, and many animals that overlap between two or more regions, the combination of animals found in each region is distinct. In addition, each region has certain animals that are not found in other regions.
There is no complete agreement among biologists on how many of these regions there are or what they should be called, but most accept the following six:
The Palearctic region includes most of Europe, central and northern Asia, Africa north of the Sahara, and most of China. Animals unique to this region include the mole rat and hedgehog.
The Nearctic takes in the United States, Canada, Greenland, Iceland, and the northern section of Mexico. Animals found exclusively in the Nearctic include the Rocky Mountain goat, musk ox, and American bison.
The Ethiopian region covers most of Africa and the Arabian peninsula. Here the zebra, African elephant, and gorilla are unique.
The Oriental region consists of the tropical areas of Asia, including Borneo, the Philippines, and most of India. Animals native only to this region are the gibbon, tree shrew, and tarsier.
The Neotropical region includes southern Mexico, Central America, and South America. Exclusive to this region are the sloth, New World monkeys, and the giant ant-eater.
The Australian region covers Australia, New Guinea, and a few of the smaller islands of the Malay Archipelago. Here the wombat, kangaroo, and koala are unique.Many different animals share the same habitat.