Seen from space, the most striking feature of the earth is its sheer physical variety. Not only is there dry land and open sea, but there are also mountains, plains, rivers, coastal shelves, and deep oceanic trenches.
The earth also varies in its climate: in some parts of the world, weeks or months pass by under almost cloudless skies, while in others, the ground is scoured by icy winds or soaked by intense tropical storms. Differences like these create a complex jigsaw of varied habitats, enabling the earth to support a rich diversity of animal life.
Some species are highly adaptable, and can survive in a wide range of conditions, but the vast majority are found in one kind of habitat and nowhere else.
WHAT ARE HABITATS?
In its narrowest sense, a habitat is the environment in which something lives. For some animals, a habitat might be as restricted as a temporary pool in a desert or as small as a piece of decaying wood. In a broader sense a habitat can mean a characteristic grouping of living things, together with the setting in which they are found, in ecology, a habitat defined in this way is known as a biome.
Habitats contain both living and non living matter in some — for example, true desert – living things are thinly scattered, so the non-living part of the environment is dominant. In others, such as forest and coral reefs, living things are so abundant that they fill all the available space and create habitats for each other. In these habitats, huge numbers of species exist side by side, forming extraordinarily complex webs of life.
FACTORS THAT SHAPE THE HABITATS
Geology plays a part in shaping habitats, but by far the most important factor is climate. As a result, differences in climate – which sometimes occur over remarkably small distances — can have a huge effect on plant and animal life. A classic example of this occurs where mountain ranges intercept rain-bearing winds. On the windward side of the mountains, heavy rainfall often creates lush forests teeming with all kinds of animal life. But in the“rainshadow’ to the lee of the mountains, low rainfall can produce desert or scrub, where only drought-tolerant animals can survive.
Temperature is another climatic factor that has an important effect both on land and in the sea. For example, in the far north, coniferous forest eventually peters out in the face of biting winter frosts. This northern Tree-line which runs like a ragged ring around the Arctic, marks the outer range of cross-bills, wood wasps. and many other animals that depend on conifers for survival.
On coasts and at sea, temperature changes are usually more gradual than they are inland. However, warmth or lack of it — still determines where some habitats are found. For example, reef-building corals do not thrive at temperatures of less than about 20°C (68° F). So most reefs are found in the tropics. However, on the west coast of Africa and the Americas, reefs are rare because, although the climate is warm, cold currents pass close to the shore. Mangrove swamps present a similar pattern: in the southern hemisphere, they reach as far as South Australia; in the northern hemisphere, they extend only just out of the tropics.
From the earliest days of scientific exploration, naturalists noticed great variations in biodiversity, or species richness, in the far north and south, species totals are low compared with the numbers found near the equator. Arctic tundra, for example, is inhabited by just a few hundred species of insects, while in tropical forests, the total is probably at least a million.
A similar picture — albeit on a smaller scale — is true for mammals and birds. However, high-latitude habitats make up for their lack of biodiversity by having some phenomenally large species populations. The seas around Antarctica, for example, possibly harbor tens of millions of crab eater seals — one of the most numerous wild large mammals on earth.
The reasons for such variation in biodiversity are still not fully understood. Although climate almost certainly plays a part. However, in an age in which many animal species are endangered, biodiversity — and ways of maintaining it — has become an important topic. Tropical forests and coral reefs are especially rich in species‘ which is why so much attention is currently focused on preserving them and their animal life.
A glance at the map on this page shows that various types of habitat are spread across large expanses of the world. However, within a few exceptions, most of their animals are not. Instead, each species has a characteristic distribution which comes about partly through its evolutionary history, and partly as a result of its way of life. In many cases, an animals lifestyle shapes its distribution in unexpectedly subtle ways.
For example, in the Americas, the brown pelican is found all along the western coast, apart from the far north and south: in the east, it does not reach south of the Caribbean. The reason for this is that, unlike its relatives, the brown pelican feeds by diving for fish, and needs clear water to spot its prey. The Caribbean is clear, but further south lies the Amazon River, which pours vast amounts of mud-laden water into the sea. For the pelican, this muddy water is a barrier that cannot be crossed.
Many animals distributions are linked to those to particular plants. Extreme examples include the yucca moth, which depends on yucca plants: the fig wasp, which develops inside figs: and countless bees that depend on particular flowers. Not all plant-dependent animals are insects, however. The robber crab – the largest and heaviest terrestrial crustacean — feeds predominantly on rotting coconuts. Which it scavenges along the shore. As a result, it is found only where coconuts grow. Mammals can be just as particular. The giant panda – one of the most famous examples — depends on about 2 dozen species of bamboo. which are found only at mid-altitudes in the mountains of central China.
In nature, habitats change all the time. Forest and grassland catch fire, rivers burst their banks, and storms batter coral reefs and coasts. Such unpredictable occurrences are facts of life, and animals — along with other living things – have evolved ways of surviving them.
Habitats can also change in much more profound ways, over much longer periods of time. Here, the driving force is usually climate change, a natural process that is triggered by a host of factors, including continental drift. On several occasions in the distant past — most recently about 12,000 years ago — the polar ice caps expanded, destroying existing habitats and evicting their animals. On each occasion, when the ice eventually melted, plants moved back into the empty landscape, and animals followed suit. The worlds climates are interrelated, which means that changes in one area can have long-term effects all over the globe.
For example, during the last ice age, the climate in the tropics became drier, and the Amazon rain-forest shrank to form scattered “refugia” — islands of forest surrounded by grassland. Even today, these areas of forest still contain a wider variety of birds than the relatively new forest that has grown back. Ice ages also affect sea levels, by locking up water as ice. When sea levels fall, land habitats expand; when they rise, the land is drowned again and plants and animals are forced to retreat.
Since the last ice age ended, natural changes have not been the only ones that have affected the world’s habitats. On a local and global scale, human activity has had an increasing impact and,as a result the pattern we now see is partly man-made. This is especially true of forests, which have been cut back to make space for agriculture, but it is also true of some grasslands, wetlands, and even deserts. In some remote regions — particularly in the far north — the original pattern still remains, but in populated regions, it has been transformed, creating a world where wild animals can have difficulty finding a home.