Extinction is a normal process in the course of evolution. Species have slowly evolved and disappeared throughout geologic time as the result of climate 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 human 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 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.
The 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 Pacific rim are some of the richest marine habitats known-they are the home or breeding ground of many species of fish and other wildlife, such as sea otters. When the sea otter population off the western coast of Canada and the United States was hunted almost to extinction in the 19th and early 20th centuries, invertebrates such as the sea urchin were left without a major predator. The population of sea urchins increased dramatically and rapidly consumed the kelp and other seaweeds, turning the rich ecosystem into a barren undersea terrain. Conservation efforts throughout the latter half of the 20th century, such as the U.S. Marine Mammal Protection Act (1972), allowed for the protection and reintroduction of the sea otter to these ecosystems and the kelp forests once again thrived.
The 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 modern medicines are derived from plants or animals. A small plant from Madagascar, the rosy periwinkle, produces substances that are effective in fighting two deadly cancers, Hodgkin’s disease and leukemia. Yet the forest habitat of the rosy periwinkle is rapidly disappearing to supply firewood and farmland for the impoverished people of Madagascar, and most of the endemic species there-that is, species that live nowhere else-are endangered.
In the United States the Endangered Species Act of 1973, legislation enacted to protect endangered species and the habitats on which they depend, established two degrees of endangerment. Endangered species, such as the California condor, are at immediate risk of extinction and probably cannot survive without direct human intervention. Threatened species, such as the gray wolf, are abundant in parts of their range but are declining in total numbers and are at risk of extinction in the foreseeable future. The World Conservation Union (IUCN), a nongovernmental organization compiling global information on endangered species, has established similar categories of endangered species, referring to the categories as critically endangered, endangered, and vulnerable.
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