Copyright © 2007 Dorling Kindersley
Organisms are adapted to live in particular surroundings with distinctive conditions, such as rainfall and temperature. This is their habitat. The largest habitats include OCEANS, WETLANDS, FORESTS, GRASSLANDS, DESERTS, MOUNTAINS, and POLAR HABITATS.
A habitat is an area occupied by many species. A home is a place within a habitat where a particular animal species can protect itself and its young from the weather and predators. Homes include nests built by birds and wasps, and burrows dug by moles.
A small part of a habitat that has its own conditions of, for example, temperature and light, and its own characteristic species, is called a microhabitat. Microhabitats include the shady area under a tree and the underside of a rock in a stream.
The oceans cover about 70 percent of the Earth’s surface, and form the largest of the world’s habitats. Life is found at all depths, from shallow surface waters to trenches over 7 miles (11 km) down.
All ocean life depends on microscopic plantlike organisms called phytoplankton. Floating near the ocean’s surface, phytoplankton trap sunlight energy to make food. Zooplankton (tiny animals and protists) feed on phytoplankton. They in turn provide food for fish, crabs, squid, and other animals.
Light penetrates the ocean’s surface waters to a depth of only about 650 ft (200 m). Below this, in the twilight zone, it is much dimmer. In the deep zone, it is pitch black, and very cold. Each zone, down to the seabed, has its own community of living things.
Wherever salt or fresh water cannot easily drain, a wetland forms. Wetlands cover over 6 percent of Earth’s surface. They include marshes, swamps, waterlogged forests, peat bogs, and river deltas.
Many wetlands contain a large diversity of species, including birds, mammals, reptiles, insects, amphibians, and plants. Wetlands also serve as nurseries where young fish and other aquatic animals grow and develop.
These are the salt-water wetlands, populated by mangrove trees, found along tropical coastlines. Mangrove swamps teem with life, and they also help to protect the coastline from tropical storm damage.
Habitats dominated by trees and shrubs are called forests. They include tropical rainforests, cool-climate coniferous forests, and temperate broad-leaved forests. These habitats all teem with life.
Evergreen trees continuously grow and provide food in the constant warm and wet climate of tropical rainforests. A huge variety of animals feed and shelter at all levels, from the forest floor to its canopy. Tropical rainforests contain half of all animal and plant species. Yet they cover only about 10 percent of the Earth’s surface.
Wherever it is too dry for forests to grow, or too wet for deserts to form, grasslands appear. The two main types are the tropical African savanna and temperate grasslands, such as the South American pampas.
Grasses can withstand constant grazing because they sprout from the bottom, not from the tips. The more they are eaten, the more they grow. Grasslands support a wide variety of animals that eat grasses, as well as those that prey on grass-eaters.
The African savanna supports many species of plant-eaters, some of which eat different parts of grasses, while others eat different savanna plants. Zebras, for example, eat the coarse, tough tops of grasses, while wildebeest prefer their leafy, middle parts.
This dry and hostile habitat often receives less than 4 in (10 cm) of rain each year. Deserts are very hot by day, but cooler at night. Few animals and plants have adapted to survive these difficult conditions.
Some plants, such as cacti, have deep, wide-spreading roots to reach available water, and small leaves and waterproof skins that limit evaporation. Others spend most of their life cycle as seeds. When rare rains arrive, they sprout, flower, and produce seeds within two weeks. This event is called a desert bloom.
Land that is 1,970 ft (600 m) or more above the sea is a mountain. The higher you climb, the thinner the air, the lower the temperature, and the faster the wind speed. Only the toughest species survive.
Mountains have various zones of vegetation. Deciduous woodlands cover the foothills. These rise up to coniferous forests, which can survive the colder, windier conditions. Above the tree line (where trees can no longer grow) is an alpine meadow of hardy plants. Next is bare rock, capped by a snow field.
Each vegetation zone has its typical species. Woodlands and forests provide habitats for grazers, such as deer and birds. Meadows are home to rodents and rabbits and, in summer, insects and the birds that eat them. Goats and sheep live on the rocky crags, and birds of prey circle above, in search of food.
Cold, icy polar habitats exist at the Earth’s North and South poles. Polar regions have short summers and long, harsh winters. Only animals that have adapted—with thick fur, for example—survive there.
The Arctic surrounds the North Pole and is a frozen ocean. Animals such as polar bears and arctic foxes live on the ice sheet. Antarctica surrounds the South Pole, is a frozen continent, and has few animals. The ocean around it is rich in nutrients and supports fish, seabirds, seals, and whales.