Endangered Species, plant and animal species that are in danger of extinction, the dying off of all individuals of a species. Over 34,000 plant species and 5,200 animal species around the globe are threatened with extinction, and many thousands more become extinct each year before biologists can identify them. The primary causes of species extinction or endangerment are habitat destruction, commercial exploitation (such as plant collecting, hunting, and trade in animal parts), damage caused by nonnative plants and animals introduced into an area, and pollution. Of these causes, direct habitat destruction threatens the most species.
A steady rate of extinction is a normal process in the course of evolution, and is called the background rate of extinction. Species have slowly evolved and disappeared throughout geologic time because of climatic changes and the inability to adapt to survive competition and predation. Since the 1600s, however, the rate of extinction has accelerated rapidly because of human population growth and resource consumption. Today, most of the world’s habitats are changing faster than most species can adapt to such changes through evolution, or natural selection. The current global extinction rate is estimated at about 20,000 species per year, exponentially greater than the background extinction rate. Many biologists believe that we are in the middle of the greatest mass extinction episode since the disappearance of the dinosaurs 65 million years ago.
survival of ecosystems (plant and animal communities and their physical
surroundings) such as forests, coral reefs, or wetlands depends on their
biodiversity, or variety of plants, animals, and habitats, as well as the many
interactions among these species. The removal or disappearance of one or
several species may irreversibly damage the ecosystem and lead to its decline.
For example, the undersea kelp forest ecosystems of the northern
irreversible loss of biodiversity has a serious impact on the ability of
remaining species, including humans, to survive. Humans depend on species
diversity and healthy ecosystems to provide food, clean air and water, and
fertile soil for agriculture. In addition, we benefit greatly from the many
medicines and other products that biodiversity provides. As
many as 40 percent of our modern pharmaceutical medicines are derived from
plants or animals. A small plant from
Species become extinct or endangered for a number of reasons, but the primary cause is the destruction of habitat by human activities (see Environment). As species evolve, most adapt to a specific habitat or environment that best meets their survival needs. Without this habitat the species may not survive. Pollution, drainage of wetlands, conversion of shrub lands to grazing lands, cutting and clearing of forests, urbanization, coral-reef destruction, and road and dam construction have destroyed or seriously damaged and fragmented available habitats. Habitat fragmentation, the isolation and division of habitats into smaller areas, has caused plant and animal species in the remaining islands of habitat to lose contact with other populations of their own kind. This reduces their genetic diversity and makes them less adaptable to environmental or climatic change. These small populations are highly vulnerable to extinction. For some species, the fragmented habitats become too small to support a sustainable population.
1600s, worldwide commercial exploitation of animals for food and other products
has caused many species to become extinct or endangered. The whaling industry,
in which whales are slaughtered for oil and meat, for example, has led many
whale species to the brink of extinction. The African black rhinoceros, killed
for its horn, which is prized as a medicine and aphrodisiac, is also critically
endangered. The great auk and the passenger pigeon became extinct in the 19th
and early 20th centuries because of overhunting, and
species introduced to a new ecosystem have caused serious native species
declines. The nonnative, or exotic, species may be introduced either
accidentally or intentionally, and either out-compete the native species or
prey on them. Native plants and animals may have no defenses against these
invaders, which can exterminate or greatly reduce the populations of many
native species. The damaging effects are generally irreversible, and there is
often no way to keep the damage from spreading. The accidental introduction of a blight, for example, virtually eliminated the American
chestnut tree from North American hardwood forests. The zebra mussel, first
Pollution is another important cause of extinction. Toxic chemicals—especially chlorinated hydrocarbons, such as dichlorodiphenyltrichloroethane (DDT) and polychlorinated biphenyls (PCBs)—have become concentrated in food webs, the interconnected food chains that circulate energy through an ecosystem. These toxic chemicals strongly affect species near the top of the food chain. Both DDT and PCBs 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 species of fish in many habitats. Oil spills destroy birds, fish, and mammals, and may contaminate the ocean floor for many years after the event. Acid rain, the toxic result of extreme air pollution, has been known to kill organisms in freshwater lakes and destroy large tracts of forested land.
III PRESERVATION EFFORTS
In an effort to protect global biodiversity and encourage the study, restoration, and sound management of endangered species, the IUCN and the World Conservation Monitoring Centre (WCMC) maintain a global list of endangered and vulnerable animal species called the Red List. A framework for international conservation efforts, the Red List database assesses the status of, and threats to, animal species worldwide. To add to this and other biodiversity databases, nongovernmental organizations such as Conservation International and World Wildlife Fund conduct periodic rapid assessments (focused, intensive evaluations) of biodiversity in various hotspots—regions like Madagascar that are both rich in endemic species and environmentally threatened.
This information is used in the administration of international agreements such as the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), signed by 125 nations in 1973 and put into effect in 1975. The purpose of CITES is to restrict exploitation of endangered plants and wildlife by regulating and restricting their trade. Despite legal protection, however, the future of many species, such as the African black rhinoceros, is in doubt because of poor law enforcement and the activities of poachers and dealers who supply the $6 billion per year trade in endangered animals and animal parts. Similarly, the International Whaling Commission is charged with protecting global whale populations, but lacks the authority to enforce its rulings.
private and governmental efforts in the
Efforts to save endangered species also include captive breeding of severely endangered species later released in the wild to restore or add to a breeding population (a population of individuals capable of reproducing). Due to breeding in captivity (such as in zoos and specialized animal clinics), the number of known California condors grew from 27 in 1987 to 157 by 2000, including 52 living in the wild in southern California; the number of whooping cranes rose from 21 worldwide in 1941 to at least 180 in the wild and 130 in captivity in 1999; and the American peregrine falcon has been reintroduced into the wild in eastern North America where it had become extinct as a breeding population. In 1999 scientists successfully transplanted an embryo from an African wildcat into the womb of a domestic cat. The domestic cat gave birth to an African wildcat, giving scientists hope that this procedure may one day help forestall extinction for endangered species that reproduce poorly on their own.
Captive breeding and embryo transplant programs are considered a last resort because such breeding may reduce the genetic diversity of the species and its ability to survive in the wild. For example, Pacific salmon raised in hatcheries may be less capable of surviving in adverse conditions than wild salmon, and if they interbreed with the wild fish, they may dilute the natural survival ability of wild salmon. Extremely difficult to successfully reintroduce to the wild, captive-bred animals are also more likely to carry disease, which they may transmit to the wild population. These programs are extremely costly, sometimes reaching $500,000 per year per species. Moreover, captive breeding and embryo transplant programs are a short-term repair that may divert attention from finding solutions to the original causes of the species decline, such as habitat destruction or toxic pollution. For some species, however, such as the California condor and the peregrine falcon, captive breeding has made the difference between survival and extinction.
IV CONSERVATION BIOLOGY
The scientific field of conservation biology evolved to study the complex problems surrounding habitat destruction and species protection. The objectives of conservation biologists are to understand how humans affect biodiversity and to provide potential solutions that benefit both humans and nonhuman species.
One of the approaches that conservation biologists study and implement is the development of habitat reserves and wildlife corridors. All species require a minimum amount of habitat for survival. Wildife habitat reserves are established to meet these requirements for as many species as possible. Some national parks, wilderness areas, and other protected habitats are suitable for the survival of a wide range of species. Unfortunately, the minimum area required by large carnivores such as grizzly bears or cougars may be larger than the area protected in a reserve. In order to effectively enlarge the area available to the species, protected wildlife corridors that interlace or connect separate reserves may be developed. These areas are narrower than the actual reserve but ideally allow for unimpeded travel between reserves. Conservation biologists recommend that both reserves and corridors be surrounded by a wide belt, or buffer zone, of similar landscape to maintain the effectiveness of the reserve. Conservation biologists also study and promote habitat restoration and extensive public education, and work with ecologists and taxonomists (scientists who study the scientific classification of organisms) to identify and describe the millions of imperiled species not yet known to science.
The current global extinction crisis is one of the greatest challenges posed by the rapid growth and expansion of human populations. The protection of endangered species and habitats should be a top priority for international organizations, governmental agencies, industry, and individuals if there is hope for preserving the earth’s valuable biodiversity for future generations.
Reed F. Noss
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