Animal

B

Speciation and Extinction

Like other living things, animals evolve by adapting to and exploiting their surroundings. In the billion-year history of animal life, this process has created vast numbers of new species, each capable of using resources in a slightly different way. Some of these species are alive today, but these are a minority; an even greater number are extinct, having lost the struggle for survival.

Speciation, the birth of new species, usually occurs when a group of living things becomes isolated from others of their kind (see Species and Speciation). Once this has occurred, the members of the group follow their own evolutionary path and adapt in ways that make them increasingly distinct. After a long period—typically thousands of years—their unique features mean that they can no longer breed with their former relatives. At this point, a new species comes into being.

In animals, this isolation can come about in several different ways. The simplest form, geographical isolation, occurs when members of an original species become separated by a physical barrier. One example of such a barrier is the open sea, which isolates animals that have been accidentally stranded on remote islands. As the new arrivals adapt to their adopted home, they become more and more distinct from their mainland relatives. Sometimes the result is a burst of adaptive radiation, which produces a number of different species. In the Hawaiian Islands, for example, 22 species of honeycreepers have evolved from a single pioneering species of finch-like bird.

Another type of isolation is thought to occur where there is no physical separation. In this case, differences in behavior, such as mate selection, may sometimes help to split a single species into distinct groups. If the differences persist for a long enough time, new species are created.

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The fate of a new species depends very much on the environment in which it evolved. If the environment is stable and no new competitors appear on the scene, an animal species may change very little in hundreds of thousands of years. But if the environment changes rapidly and competitors arrive from outside, the struggle for survival is much more intense. In these conditions, either a species changes, or it eventually becomes extinct.

During the history of animal life, on at least five occasions, sudden environmental change has triggered simultaneous extinction on a massive scale. One of these mass extinctions occurred at the end of the Cretaceous Period, about 65 million years ago, killing all dinosaurs and perhaps two-thirds of marine species. An even greater mass extinction took place at the end of the Permian Period, about 200 million years ago. Many biologists believe that we are at present living in a sixth period of mass extinction, this time triggered by human beings.

X

Animals in the Balance of Nature

Compared to plants, animals make up only a small part of the total mass of living matter on earth. Despite this, they play an important part in shaping and maintaining natural environments.

Many habitats are directly influenced by the way animals live. Grasslands, for example, exist partly because grasses and grazing animals have evolved a close partnership, which prevents other plants from taking hold. Tropical forests also owe their existence to animals, because most of their trees rely on animals to distribute their pollen and seeds. Soil is partly the result of animal activity, because earthworms and other invertebrates help to break down dead remains and recycle the nutrients that they contain. Without its animal life, the soil would soon become compacted and infertile.

By preying on each other, animals also help to keep their own numbers in check. This prevents abrupt population peaks and crashes and helps to give living systems a built-in stability. On a global scale, animals also influence some of the nutrient cycles on which almost all life depends. They distribute essential mineral elements in their waste, and they help to replenish the atmosphere’s carbon dioxide when they breathe. This carbon dioxide is then used by plants as they grow.

A

Animals and People

Until relatively recently in human history, people existed as nomadic hunter-gatherers. They used animals primarily as a source of food and also for raw materials that could be used for making tools and clothes. By today’s standards, hunter-gatherers were equipped with rudimentary weapons, but they still had a major impact on the numbers of some species. Many scientists believe, for example, that humans were involved in a cluster of extinctions that occurred about 12,000 years ago in North America. In less than a millennium, two-thirds of the continent’s large mammal species disappeared.

This simple relationship between people and animals changed with domestication, which also began about 12,000 years ago. Instead of being actively hunted, domesticated animals were slowly brought under human control. Some were kept for food or for clothing, others for muscle power, and some simply for companionship.

The first animal to be domesticated was almost certainly the dog, which was bred from wolves. It was followed by species such as the cat, horse, camel, llama, and aurochs (a species of wild cattle), and also by the Asian jungle fowl, which is the ancestor of today’s chickens. Through selective breeding, each of these animals has been turned into forms that are particularly suitable for human use. Today, many domesticated animals, including chickens, vastly outnumber their wild counterparts. In some cases, such as the horse, the original wild species has died out altogether.

Over the centuries, many domesticated animals have been introduced into different parts of the world only to escape and establish themselves in the wild. Together with stowaway pests such as rats, these feral animals have often had a highly damaging effect on native wildlife. Cats, for example, have inflicted great damage on Australia’s smaller marsupials, and feral pigs and goats continue to be serious problems for the native wildlife of the Galápagos Islands.

Despite the growth of domestication, humans continue to hunt some wild animals. Some forms of hunting are carried out mainly for sport, but others provide food or animal products. Until recently, one of the most significant of these forms of hunting was whaling, which reduced many whale stocks to the brink of extinction. Today, highly efficient sea fishing threatens some species of fish with the same fate (see Fisheries).

Since the beginning of agriculture, the human population has increased by more than two thousand times. To provide the land needed for growing food and housing people, large areas of Earth’s landscapes have been completely transformed. Forests have been cut down, wetlands drained, and deserts irrigated, reducing these natural habitats to a fraction of their former extent.

Some species of animals have managed to adapt to these changes. A few, such as the brown rat, raccoon, and house sparrow, have benefited by exploiting the new opportunities that have opened up and have successfully taken up life on farms, or in towns and cities. But most animals have specialized ways of life that make them dependent on a particular kind of habitat. With the destruction of their habitats, their number inevitably declines.

During the last century or so, animals have also had to face additional threats from human activities. Foremost among these are environmental pollution and the increasing demand for resources, such as timber and fresh water. For some animals, the combination of these changes has proved so damaging that their numbers are now below the level needed to guarantee survival.

Across the world, efforts are currently under way to address this urgent problem (see Endangered Species). In the most extreme cases, gravely threatened animals can be helped by taking them into captivity and then releasing them once breeding programs have increased their number. One species that was restored in this way is the Hawaiian mountain goose or nēnē. In the 1950s, its population had been reduced to about 25 birds. Captive breeding has since helped the population increase, although the nēnē remains on the endangered list.

While captive breeding is a useful emergency measure, it cannot assure the long-term survival of a species. Today animal protection focuses primarily on the preservation of entire habitats, an approach that maintains the necessary links between the different species the habitats support. With the continued growth in the world’s human population, habitat preservation will require a sustained reduction in our use of the world’s resources to minimize our impact on the natural world.

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