HABITAT DESTRUCTION The greats threat to the world's wildlife is the loss of habitat. Rain forests are being destroyed at an alarming rate-in some parts of the world over 12 acres are being cut down every minute-and with them go all the plants and animals that lived there. Scientists have estimated that about 50 species of rain forest plants and animals disappear each day. Thousands more species are in danger of becoming extinct in the next few years. They include the tiger, the Philippine eagle, and many beautiful butterflies. Elsewhere in the world, wetlands are being drained and grasslands are being plowed up for crops or covered with concrete.

In recent years, mangrove forests have been destroyed to make way for shrimp farms, while coral reefs have been "mined" for building materials. Both these activities have a far reaching effect on marine animals because they remove the nursery areas that many species use to breed. Coasts and rivers are effected by dam building: the slit that normally accumulates in a river's delta, helping to protect the shoreline, is trapped behind the dam. This results in the erosion of the shoreline as well as allowing salt water to flow into freshwater habitats. Fast growing cities have created further pressure on the natural habitats that remain.

The pattern that the process of habitat change takes is often as important as its scale. For example, if large areas of habitat are preserved in their original state then the habitat can often function as it did previously, albeit on a reduced scale. But if the same amount of habitat is fragmented divided up into smaller isolated pieces, the effect on animals is much more servere. This is because many species, particularly predators at the top of the food chains, need extensive territories if they are to find enough food to eat. Fragmented habitats are also exposed to more intrusions and disturbance from humans and domestic animals, making it much harder for wild animals to feed and breed.

EXPLOITATION (hunting, collecting, fishing, entertainment and trading) Unlike many of the world's resources, animals can reproduce. This means that - in theory - useful species can be harvested without ever running out. However, many species have been exploited to excess, with the result that a considerable number of them are now in serious danger while others have died out altogether.

The list of casualties of hunting includes the African blue buck, which died out in about 1800, and the North American passenger pigeon - once the most numerous bird in the world - which became extinct in 1914. These animals were killed primarily for food, but animals are now often hunted to meet less pressing needs.

Elephants are in demand for their tusks, and rhinoceroses are killed for their horns. Tigers are hunted for their fur and body pars as the number of surviving animals falls. At sea, fish have become victim to the kind of excessive exploition once reserved for animals on land: plummeting stocks of once-common species, such as the Atlantic cod, are a warning sign that many commercially important fish are experiencing major difficulties.

A range of animals are collected for the pet trade. Apes and tropical parrots are particularly at risk. Some are trapped as adults, but many are taken when they are young. Deprived of parental care, they often fail to survive.

People who traditionally hunted animals for food with bows and arrows did no harm to the wildlife population. The invention of high-powered rifles, harpoons, and other deadly weapons. however, meant that large numbers of animals could be killed for "sport" and for skins, or simply because they competed with domestic livestock. Many animals then became rare, and further hunting of them has put them in even greater danger.

Fishing is also a serious threat to some animals, the fishing nets that are now being used to catch tune and other seawater fishes also kill large numbers of dolphins. The animals get tangled up in the nets and cannot get to the surface to breathe. Conservationists are urging people not to buy fish that are caught by these killer nets in an attempt to prevent the problem.

Captive cetaceans such as Pacific white-sided dolphins, false killer whales, belugas, orcas, Irrawaddy dolphins, bottlenose dolphins and many more are always popular animals at aquariums, but it does not benifit them to keep them in captivity. Although a few of these are bred in captivity, the vast majority are still being taken from the wild. Many of these animals die during the first few hours or days after capture, and the remainder rarely survived more than a few months.

POLLUTION Pollution occurs when chemicals or other agents infiltrate and disrupt natural ecosystems. Sometimes pollution has a natural origin, but in most cases it is the result of human activity. It can effect animals physically - for example, by extangling them in waste or clogging them with oil - but its chemical effects inside the body or in an animal's environment are often more serious, as well as being harder to identify and predict.

The most problematic chemical pollutants are synthetic-organic (carbon-containing) substances, such as solvents, pesticides, and herbicides. Hundreds of thousands of these chemicals now exist, and new kinds are produced every year. Their chemical structure means that they are often absorbed by living tissue, where they cause the most damge. Some of these substances are toxic to all life forms, but other are more selective. They are passed on when predators eat their prey, and so they accumulate in species at the top of the food chains, such as whales, polar bears, and birds of prey. The low breeding success of some predators - beluga whales in the Gulf of St Lawrence, for example - is thought to be a direct result of this kind of pollution.

Animals are also affected by air pollution, which occurs mainly when fules are burned, causing poisonous gases to be released into the atmosphere. This creates localized problems such as acid rain, which has a highly damaging effect on freshwater fish. On a much broader scale, it is also resposible for global warming - an environmental change that could ultimately effect almost every animal species on earth.

THE REMEDIES The most obvious and important thing to prevent animals becoming endangered is to stop destroying wild habitats. Many animals are already protected in nature reserves, where the habitats are safe from destruction, but before creating reserves, biologists need to know how much space the animals need and how far they ravel in their sreach for food. This kind of information is often obtained by putting little radie transmitters on the animals and listening for the signals to find out where the animals go. This has been done with bears, wolves, and big cats.

Many countries have laws that control hunting and prohibit the killing of rare and endagered animals, but it is not easy to enforce these laws, because patrolling vast areas of forest and savanna is impossible.

Elephants and big cats are legally protected, but poachers still kill them for their ivory and skins. The Conservation of International Trade in Endangered Species (CITES) is dedicated to stopping all trade in endangered species and any article that comes from them, such as ivory, tortoiseshell, skins, fur and feathers. If the objects cannot be exported or sold, people will hopefully be less interested in killing the animals.

REINTRODUTIONS Several species that have been threatened with extinction have been saved by being taken into zoos and wildlife parks, and bred there in safety. When the population is big enough, it is sometimes possible to release the animals back into the wild-as long as suitable habitat remains or can be recreated.

The Arabian oryx was the first large animal to be saved from extinction in this way. Other animals that have been successfully reestablished in the wild include the Hawaiian goose, Przewalski's horse, and the gold lion tamarin.