Animal Distribution

I

Introduction

Animal Distribution, patterns that characterize where animals are found around the globe. When scientists study the distribution of animals, they investigate why reindeer, for instance, are found only in certain parts of the arctic tundra, or why malaria-bearing mosquitoes proliferate in damp subtropical areas. Scientists study animal distribution to understand the spread of animal-borne diseases, to acquire knowledge about the preservation of rare species that may have special needs, and to become informed about the changing geography of the world, and our place in its history and its future. To understand these issues, scientists need to identify the specific climates, foods, and geographic features different animals require, and what areas best provide them. The study of animal distribution is called zoogeography.

Animals vary widely in their tolerance of environmental conditions. Some can survive in a variety of habitats, 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 environments are the subject matter of ecology. The factors affecting animal distribution range from global geological events to local weather conditions.

II

Global Conditions

Present distribution patterns of plants and animals are largely a result of shifting landmasses (see Paleoclimatology). About 225 million years ago, all the continents were connected into one supercontinent called Pangaea. Over the course of about 50 million years, Pangaea split into two huge chunks of land that drifted farther and farther apart. About 65 million years ago, these large landmasses split into the continents we recognize today, and began inching toward their present positions (see Plate Tectonics). When the continents separated 65 million years ago, populations of animals that could not fly or swim were unable to move to other continents. They evolved, isolated from their relatives on other continents, in response to their own unique environments.

The regions that were to become South America, Africa, and Australia, for example, were once joined in a chunk of land that separated from Pangaea, called Gondwanaland. Flightless birds evolved in Gondwanaland, and over the centuries, became widely dispersed over the landmass. When Gondwanaland broke up, a flock of flightless birds, along with plants and other animals, was carried away on each floating continent. Separated by the ocean and unable to fly, the isolated birds could not breed with their family members on the other continents. As a result, four distinct species of flightless birds evolved, each adapted to its own region: the cassowary and emu in Australia, the rhea in South America, and the ostrich in Africa. Fossil evidence and comparisons of deoxyribonucleic acid (DNA), the distinct genetic material found in all cells, confirm that these flightless birds are related; all are descendents of the original Gondwanaland flightless bird family.

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As continents drifted over the course of millions of years, their distances from the Earth’s poles and equator changed, altering the amount of sunlight they received. This produced drastic changes in long-term meteorological conditions such as temperature, rain, and wind, collectively known as climate, which in turn produced changes in the types of organisms that could live in one region. Dramatic temperature cycles of global warming and cooling have also punctuated the Earth’s history, resulting in extensive changes in plant and animal distribution. About seven million years ago, for example, a period of extreme cooling caused glaciers to spread over much of North America, Europe, and Asia, changing the northernmost regions from warm, temperate habitats into vast sheets of ice (see Ice Ages). The animals that lived in these areas migrated south to warmer climates, and animals that thrived in freezing temperatures took over their territory. When the Earth slowly warmed again, the ice sheets melted, and the migrations reversed. The Earth has experienced at least four major cycles of cooling and warming; the most recent Ice Age ended about 10,000 years ago. Evidence of these changes is seen in the fossils of animals adapted for extreme cold, such as mastodons, that are found in temperate, or mild, regions of Europe and North America. Fossils of tropical animals such as jaguars found beneath arctic ice are also testimony to major climate fluctuations of the past.

In some instances, factors other than the movement of continents cause animals to move from one landmass to another. For example, iguanas have been found to travel from one Caribbean island to another by riding on trees that had been uprooted and hurled into the ocean by hurricanes. Some scientists believe that other animals have also traveled on floating logs; in particular, rodents are thought to have rafted to Australia from other landmasses between five million to six million years ago.

III

Biomes

The Earth in its current form encompasses many different climatic regions, called biomes. Each biome is home to a characteristic community of plants and animals. Similar organisms are found in a rain forest biome, for example, whether the rain forest is in Africa or South America, though particular species of plants and animals may differ. Likewise, the plants and animals of the Gobi Desert in Asia resemble those of the Sahara in Africa and the Mojave Desert in the United States. In addition to the biomes, which are terrestrial, or located on land, unique communities of plants and animals are found in saltwater and freshwater environments.

Rainfall and temperature play the major roles in determining the types of organisms that live in a biome. The geographic features of an area also shape the distribution of organisms. Rivers and lakes harbor unique plants such as willow, cottonwood, and cattails that attract specific birds, insects and animals; the cold temperatures at mountaintops result in species that are similar to, or have similar adaptations to, those found at the poles. Ptarmigans, for example, are cold-adapted birds found in the arctic and in the alpine regions of mountains at the equator.

A

Terrestrial Communities

The plants that populate each biome attract specific insects and other animals that feast on them. The plant-eating organisms, or herbivores, are inviting prey for carnivores, animals that devour other animals. Omnivores are animals that dine on both plants and animals. The links between plants, herbivores, carnivores, and omnivores in a particular environment is called a food web. Each biome has a unique food web. The cold, mossy tundra biome of the poles, for example, with its patchwork of lakes and bogs, attracts vast clouds of mosquitoes and flies. These insects draw many species of birds to feed during the brief, cool summers. In turn, small mammals like the arctic fox eat the eggs and young of ground nesting birds. Caribou graze on the grasses, moss, and lichens; and lemmings and arctic hares provide food for the bears and other large predators.

South of the tundra is the taiga biome, or boreal forest, with more warmth and enough rainfall to encourage conifer forests of pine, spruce, fir, and hemlock. The dense growth of the conifers provides shelter and a good breeding ground for small animals—lynx, wolverine, porcupine, snowshoe hare, and small rodents are abundant. Wolves prey on the lynx and other mammals; black bear and grizzly bear munch on berries, nuts, and buds; and mule deer and moose browse on shrubs. A host of plant-eating insects and the birds that devour them are also abundant in the taiga.

Still further from the poles is the temperate forest biome, an area with varied rainfall and seasonal temperature differences that encourage much more plant and animal diversity than tundra and taiga. Temperate zones support forests of maples, ash, aspen, and other trees that lose their leaves in the autumn. The fallen leaves provide food for a variety of fungi, bacteria, and other decomposers that create a rich carpet of spongy humus, or organic matter, on the forest floor. The humus provides nutrients and soaks up moisture for the plants. Enough light reaches the forest floor to encourage growth of shrubs and trailing plants. Rabbits, raccoons, squirrels, opossums, and a variety of birds make their home in these forests. Human encroachment has made wolves scarce, the predator that once controlled the abundant deer population. The bear, fox, and mountain lion that prey on the smaller animals of the forest are also declining as forests give way to pavement and lawns.

More diverse still are the rain forest biomes that characterize the tropics. These include some of the most varied and productive plant and animal communities on Earth. The warmth and abundant moisture of the rain forest supports close-growing trees; their branches touch overhead and form a thick canopy, blocking light from reaching the rain forest floor. Lianas, or vines, twine up and around the trees, striving to reach light, and epiphytes, plants that live on trunks or branches, position themselves to catch every ray that leaks through the dense canopy. The abundance and diversity of plant life supports a great variety of animal life. A 13-sq-km (5-sq-mi) rain forest preserve in Costa Rica, for example, is home to 450 species of trees, more than 1,000 other plant species, 400 species of birds, 58 species of bats, and 130 species of amphibians and reptiles.

Much of the Earth is covered with grasslands, biomes that include the prairies or plains of the United States, the savannas of Africa, and the steppes of Russia. Uncertain and low rainfall does not permit trees to grow in these areas, and grasses that can withstand unpredictable moisture have become the dominant vegetation. Unlike most plants, grasses grow from the young shoots at the base, and growth is stimulated when animals chew off the upper leaves. Thus, grasslands can support great numbers of grazing animals because when they feed, they encourage the vigorous growth of grass. As a result of human activity, cattle have replaced most of the huge bison herds that once browsed the grasslands of North America. Smaller herbivores such as jackrabbits and prairie dogs are numerous in grasslands; they provide a meal for coyotes, badgers, rattlesnakes, hawks, and owls. The savannas support dwindling herds of elephant, gazelle, wildebeest, zebra, and antelope, all of which are threatened by human invasion of their habitat.

Shrub lands are biomes with long, dry summers and short, rainy, warm winters. Called chaparral in California and Baja California, maquis in the Mediterranean regions, the matorral in Chile, and phrygana in the Balkans, these regions are characterized by gnarled, dwarfed, broad-leaved evergreen trees and shrubs, often with waxy leaves that conserve water. Scrub oak and dense shrubs such as mesquite, manzanita, and chamiso provide habitat and food for insects, birds, and rabbits. Lizards, snakes, and bobcats are typical inhabitants of the chaparral.

The desert biome, with its very low rainfall and very high temperatures, surprisingly enough harbors a diversity of plants and animals second only to the rain forest. Cactus, ocotillo, the boojum tree, and other plants that store water in their stems or leaves thrive in deserts. Rodents such as the kangaroo rat feed on the fruits and seeds of desert plants while lizards and snakes prey on rodents, bird eggs, and insects. Mammals, including the dingo of Australia and the coyote of North America, eat rodents, insects, and the dead flesh, or carrion, of other animals. Vultures, strictly carrion eaters, soar over deserts in search of a feast. Many desert animals survive the heat by hunting at night and hiding in burrows during the day. Some animals such as kangaroo rats seldom need a drink of water because their efficient kidneys enable them to get all the water they require from their food.

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