National Geographic: Most people know what it means to be a threatened species--its something thats rare and may become extinct. What isnt often explained is how we know something is threatened and who decides whether a species is threatened or not.
Read almost any article about species in peril and there will be some reference to the level of threat faced by that species. Words like Vulnerable, Endangered, and Critically Endangered are usually mentioned without any real explanation of where these categories come...