Lecture

= What are endangered, threatened and extinct species? = Most people have a general idea of what an **endangered**, **threatened**, or **extinct** species is, but biologists have rather precise definitions for each term. An endangered species is a type of animal or plant that is in immediate danger of extinction. The species usually has a small **population** and needs protection in order to survive. The mountain gorilla, the Indian python, the lady slipper orchid, and thousands of other plant and animal species throughout the world are endangered. Biologists use the word threatened to describe species that face serious problems, but whose populations are not in immediate danger of becoming extinct. Some examples of threatened species are the African elephant, the northern spotted owl, and the eastern indigo snake. Extinct species no longer exist or live anywhere in the world. The dodo, the passenger pigeon, and the dinosaurs are examples of extinct species.

= **An Increasing Rate of Extinction** = Although extinction itself is not an old phenomenon, the current rate of extinction is something new. Biologists say that **at least three animal and plant species become extinct every day**, a rate much higher than anything in the past 65 million years.

= **Why Species Become Endangered** = Species become endangered for a wide variety of reasons. However, when individual cases are grouped and studied, the same broad causes appear again and again: Another people-related problem that harms wildlife is the introduction of **exotic species** //-// foreign species that are deliberately or accidentally introduced into new habitats by human activities. Sometimes an introduced species causes no obvious harm, but in other cases the introduced species causes serious problems. The worst of these problems is when introduced species begin to prey on native species and cause them harm.
 * Rapid habitat destruction** is the main reason that species become endangered. Natural changes usually occur at a slow rate, so the effects on individual species are usually slight, at least over the short term. When the rate of change is greatly speeded up, there may be no time for individual species to adapt to new conditions. The results can be disastrous. This increase in the rate of habitat destruction is directly linked to the rise in human population. As more people use more space--for homes, farms, shopping centers, and so on--there is less living space for species that cannot adapt to changing conditions. People also affect plant and animal habitats when they take wood, oil, and other products from the land.
 * Overexploitation** is another reason species become endangered. One example of this is the case of the great whales, many of which were reduced to extremely low population sizes in the mid-20th century because of unrestricted whaling. In 1982, a number of countries agree to put a ban on commercial whaling. As a result, some whale species that used to be endangered have made great comebacks. Many other species, however, are still at risk. Some other animal species experience high rates of exploitation because of the trade in animal parts**.** Currently, this trade 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. Other people-related problems that put plant and animal species at risk include **poaching**, **pollution,** and **overcollecting**.