Themes > Science > Zoological Sciences > Endangered and Rare Species > Endangered Species > Endangered Species
Endangered Species, plant and
animal species that are in immediate danger of extinction. The following
degrees of endangerment have been defined. Critically endangered species,
such as the California condor, are those that probably cannot survive
without direct human intervention.
Threatened species are abundant but are declining in total numbers. Rare
species exist in relatively low numbers over their ranges but are not
necessarily in immediate danger of extinction. Extinction is actually a
normal process in the course of evolution. Throughout geological time,
many more species have become extinct than exist today. These species
slowly disappeared because of climatic changes and the inability to adapt
to such conditions as competition and predation.
Since the 1600s, however, the process of extinction has accelerated
rapidly through the impact of both human population growth and
technological advances on natural ecosystems. Today the majority of the
world's environments are changing faster than the ability of most species
to adapt to such changes through natural selection.
Causes
Species become extinct or endangered for a number of reasons, but the primary cause is the destruction of habitat. Drainage of wetlands, conversion of shrub lands to grazing lands, cutting and clearing of forests (especially in the Tropics, where the rain forests will be gone by AD 2000 if destruction continues at its present rate), urbanization and suburbanization, and highway and dam construction have seriously reduced available habitats. As the various habitats become fragmented into "islands", the remaining animal populations crowd into smaller areas, causing further habitat destruction. Species in these small islands lose contact with other populations of their own kind, thereby reducing their genetic variation and making them less adaptable to environmental change. These small populations are highly vulnerable to extinction; for some species, the fragmented habitats become too small to support a viable population.Since the 1600s, commercial exploitation of animals for food and other products has caused many species to become extinct or endangered. The slaughter of great whales for oil and meat, for example, has brought them to the brink of extinction; the African rhinoceros, killed for its horn, is also critically endangered. The great auk became extinct in the 19th century because of overhunting, and the Carolina parakeet perished as a species because of a combination of overhunting and habitat destruction.
Introduced diseases, parasites, and predators against which native flora and fauna have no defenses have also exterminated or greatly reduced some species. The accidental introduction of a blight, for example, eliminated the chestnut tree from North American hardwood forests. Predator and pest control also have adverse effects. Excessive control of prairie dogs, for example, has nearly eliminated one of their natural predators, the black-footed ferret.
Pollution is another important cause of extinctions. Toxic chemicals-especially chlorinated hydrocarbons such as dichloro-diphenyl-trichloroethane (DDT) and polychlorinated biphenyls (PCBs)-have become concentrated in food webs, affecting most strongly those species at the end of the chain. Both DDT and the PCBs, for example, interfere with the calcium metabolism of birds, causing soft-shelled eggs and malformed young. PCBs also impair reproduction in some carnivorous animals. Water pollution and increased water temperatures have wiped out endemic races of fish in several habitats.
Preservation Efforts
Some private and governmental efforts have
been directed at saving declining species. One immediate approach is to
protect a species by legislation. Laws were enacted in the United States
in the early 1900s, for example, to protect wildlife from commercial trade
and overhunting.
In 1973 the Endangered Species Act provided mechanisms for the
conservation of ecosystems on which endangered species depend; it also
discouraged the exploitation of endangered species in other countries by
banning the importation and trade of any product made from such species.
The United States also has various agreements with other nations-for
example, with Canada and Mexico for the legal protection of migratory
birds.
International efforts centre on the Convention on International Trade in
Endangered Species of Wild Fauna and Flora, ratified by 51 nations. Its
purpose is to restrict exploitation of wildlife and plants by regulating
and restricting trade in species. The effectiveness of such laws in
various countries, however, depends on enforcement and support by people
and the courts. Because of a lack of law enforcement, the willingness of
some segments of society to trade in endangered species, and the
activities of poachers and dealers who supply the trade, the future of
many species is in doubt in spite of legal protection.
Efforts to save endangered species also include the propagation of
breeding stock for release in the wild, either to restore a breeding
population (as in the case of the peregrine falcon) or to augment a
natural population (as in the case of the whooping crane). Due to breeding
in captivity, the number of known California condors had risen from 27 in
1987 to 52 by 1992. Another approach involves the determination of
critical habitats that must be preserved for endangered species. These
habitats may be protected by the establishment of reserves; the value of
these may be limited, however, because of the island effect. The
objections of special interest groups also make land preservation for the
protection of endangered species difficult.
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