Under the law, only certain animals classify as a population. As defined by NMFS, a population or group of populations may be protected as a species under ESA if it is reproductively isolated from other populations and if it represents an important component in the evolutionary legacy (Waples 1991)
¶ … Improving the Endangered Species Act Since its inception in 1973, the Endangered Species Act (ESA) had 109 species listed as endangered. Today there are at last count 1,500 endangered species that the ESA is designed to protect and ensure their long-term survival (Robbins, 2010)
In the Yellowstone National Park for instance, the Rocky Mountain Wolves were hunted into extinction by 1926. At the overall level of Northern America, 42 per cent of the grey wolf population was eradicated by the twentieth century (Constible, Sandro, Lee Jr
Eight years in the making, the plan was finally implemented in the winter of 1994-1995, when Canadian gray wolves were captured and released into two areas: Yellowstone National Park and central Idaho. The program is believed to be highly successful, resulting in a population of about 170 adult wolves in each area, and mating between wolf packs indicates that the gray wolf is closer to becoming one integrated population" (Li, 2000)