The vocabulary of endangerment has some special terms that will be used throughout this expedition. (You can find definitions of these and other important words in the Glossary.) Most important are the terms which describe the risk status of individual species. Risk status is simply a determination, based on a careful scientific analysis, of the likelihood that a particular species will disappear in the foreseeable future.

This guidebook uses the terms Threatened, Endangered, Vulnerable, and Extinct as originally defined by the IUCN (International Union for the Conservation of Nature, part of the World Conservation Union) and ESA (United StatesEndangered Species Act). Additional terms are also used by conservation biologists.

The IUCN is part of a worldwide group of governmental and nongovernmental conservation organizations, and publishes the authoritative Red List series of threatened plants and animals. The Endangered Species Act of 1973 , the premiere piece of Federal conservation legislation in this country, is designed to protect both endangered and threatened species and the habitats upon which they depend. Although the IUCN and ESA may each define the threat status of individual species somewhat differently, they are in broad agreement on which species need the most protection. And there is uniform agreement on one point: if nothing is or can be done to alter a species' fate, that species will become extinct.