Some species adapt to natural changes in their environment such as climate change and increased competition from other species. The two to four million plants and animals that exist today have adjusted to meet these challenges, out of the estimated 500 million species that have existed since life began on Earth. Because of increased human activities around the world that destroy other habitats, most extinction (death of a species in the wild) has occurred over the past 200 years. With the accelerated destruction and pollution of the world's tropical rain forests, in particular, at least 500,000 insect, plant, and animal species are expected to disappear over the last two decades of the twentieth century.

Before a species becomes extinct, it is classified by the International Union for the Conservation of Nature and Natural Resources (IUCN) as either endangered, critically endangered, threatened, or rare. An endangered species faces immediate danger...