In definition, extinction is the disappearance of an entire species. Over the last 50 years, about half of the world's tropical rain forests have been burned to make pasture and farmland or have been cut for timbers. Many thousands of square miles more will be destroyed this year. The people responsible, often poor farmers, view the forests lands as a resource to be developed, much as American view North American forests as a century ago.
The problem is that as the rain forests disappear, so do their inhabitants. No one knows how many species are being lost. To find out, scientists carefully cataloged all of the residents of one small segments of forest and then extrapolate their data. That is, scientists use what they know to predict what they do not know.
The resulting estimates vary widely, but it is clear that Earth is losing many species. Some 10 percent of well-known species teeter on the brink of extinctions. Worst-case estimates are that we will lose up to one-fifth of the world's species of plants and animals-about 1 million species-during the next fifty years. An extinction of this size has not occurred in at least 65 million years, since the end of the age of the dinosaurs.
The tragedy of extinction is that as species disappear, so do our chances to learn about them and their possible benefits. This situation is comparable to burning library before reading the books-we lose forever the knowledge that we have might gained. For example, two potent anticancer drugs have been isolated from the rosy periwinkle, a flower that occurs naturally only on Madagascar, an island being devastated by deforestation.
Any the time the rate of individuals within a species is greater than the birth rate, extinction is a possibility. Generally, extinction occurs when the environment changes. Temperature change; sea levels rise and fall. Grasslands become deserts; clear lakes become polluted. The variation of organisms within a species will survive the changing environments,
the species will become extinct.
The fossil record shows that throughout geological time, millions of species have evolved, survived for a while, then failed to adapt successfully, and finally became extinct. It is a surprisingly common process. In fact, from the number of fossils of extinct organisms found, it is apparent that a majority of the species that ever lived on Earth is now extinct.
Extinctions of species can also be caused by the introduction of nonnative species. Some species move from one ecosystems to another on their own. Animals may migrate to new areas, and seed can be varied by wind or water to different places. Even humans can influence the spread of nonnative species to other ecosystems, sometimes accidentally and sometimes on purpose.
Starlings, for instance, were purposely brought to the United States. In the 1800s, a few dozen European starlings were released in New York City. The birds rapidly multiplied , and today there are millions of them across the United States.
Both starlings and native North American bluebirds nest in holes in tree trunks and fence posts. As a result, the two species compete for shelter. The bluebirds nearly lost the battle. Concerned citizens launched a multi state effort to build and distribute bird boxes. These boxes, specially designed with small entrances, provided nesting nesting places for the blue birds and kept larger starlings out.
In conclusion, in its new environment, the nonnative species may have no natural enemies to keep its numbers in check. As a result, its members can quickly multiply and take over an ecosystem. The new competing species may wipe out an existing native species and cause changes in the ecosystem.
Source:http://www.helium.com/items/898067-definition-extinction-disappearance-entire
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