When the word endangered is mentioned, people usually think of particular species, like the panda or whooping crane. However, we would like to encourage you to think about endangerment in a broader context, one that encompasses habitats -- the physical places where species live and interact with one another. Although the development of special breeding programs (also known as captive conservation) may help some species in some cases, it is clearly not an answer to the global problem. Indeed, unless we are able to protect natural areas where endangered species actually live, they have no future.

Species become endangered for a wide variety of reasons. By analyzing and grouping many individual cases, however, we find the same broad causes appearing again and again:

Habitat Destruction

We live in a dynamic world, and habitats are always undergoing changes at all sorts of levels. However, natural changes usually occur at a slow pace, so that impacts on individual species tend to be slight -- at least in the short term. When the pace of change is greatly accelerated, there may be no time for individual species to react to new circumstances, and the effects can be disastrous (see Northern Spotted Owl). In a nutshell, this is the reason that rapid habitat loss is regarded as the chief cause of species endangerment, and there is no force more potent in this regard than human beings. To a greater or lesser extent, every part of the earth has been affected by human activities, especially during this past century. This applies on virtually every scale, from the loss of microbes in soils that once supported tropical forests, to the extinction of fish and other aquatic species in polluted freshwater habitats, to changes in global climate induced by the release of greenhouse gases.

From the perspective of an individual human lifetime, such changes may be hard to detect and their effects on individual species hard to predict. But the lesson is clear enough. For example, despite the lush appearance of many tropical forests, they are highly susceptible to destruction because the soils in which they grow are poor in available nutrients. Centuries may be required to bring back a forest that was cut down or burnt out in the space of a few years. Many of the world's severely threatened animals and plants live in such forests, and it is certain that huge numbers of them will disappear if present rates of forest loss continue.

Introduction of Exotic Species
Species that "belong" to an area are said to be native species. Typically, they have been part of a given biological landscape for a long period, and they are well adapted to the local environment and to the presence of other native species in the same general habitat. Exotic species are interlopers, foreign elements introduced intentionally or accidentally into new settings through human activities. In one context an introduced species may cause no obvious problems and may, over time, be regarded as being just as "natural" as any native species in the same habitat. In another context, exotics may seriously disrupt delicate ecological balances and create a cascade of unintended consequences.

The worst of these unintended consequences arise when introduced species put native species in jeopardy by preying on them, altering their habitats, or outcompeting them in the quest for food resources. Although biological introductions have affected environments the world over, the most destructive effects have occurred on islands, where introduced insects, cats, pigs, rats, mongooses, and other non-native species have caused the grave endangerment or outright extinction of literally hundreds of species during the past 500 years (see Hawaiian Birds).

Overexploitation
This word refers to the utilization of a species at a rate that is likely to cause its extreme endangerment or outright extinction. Among many examples of severe overexploitation, the case of the great whales stands out in special relief. By the middle of the 20th century, unrestricted whaling had brought many species of whales to incredibly low population sizes. In response to public pressure, in 1982 a number of nations (including the USA) agreed to an international moratorium on whaling. As a direct result, some whale species thought to have been on extinction's doorstep 25 years ago have made amazing comebacks (e.g., grey whales in the western Pacific). Others remain at great risk.

Many other species, however, continue to suffer high rates of exploitation because of the trade in animal parts. Currently, the demand for animal parts is centered in several parts of Asia where there is a strong market for traditional medicines made from items like tiger bone and rhino horn (see Black Rhinoceros).

Other Factors
Among other factors threatening particular species are limited distribution, disease, and pollution. Limited distributions are often a consequence of other threats: populations confined to one or a few small areas because of habitat loss, for example, may be disastrously affected by random factors (see Delhi Sands Fly). Introduced diseases can have severe effects on species lacking natural genetic protections against particular pathogens, like the rabies and canine distemper viruses that are currently devastating carnivore populations in East Africa (see African Wild Dog). Domestic animals are usually the reservoirs of these and other diseases affecting wild populations, showing once again that human activities lie at the root of most causes of endangerment. Pollution has seriously jeopardized a number of terrestrial species (see American Peregrine Falcon), although species living in freshwater and marine ecosystems are also suffering (see Coral Reefs).