This determination is a complex process that has no set of fixed criteria that can be applied consistently to all species. The known number of living members in a species is not the sole factor. A species with a million members known to be alive but living in only one small area could be considered endangered, whereas another species having a smaller number of members, but spread out in a broad area, would not be considered so threatened. Reproduction data—the frequency of reproduction, the average number of offspring born, the survival rate, etc.—enter into such determinations. In the United States, the director of the U.S. Fish and Wildlife Service (within the Department of the Interior) determines which species are to be considered endangered, based on research and field data from specialists, biologists, botanists, and naturalists.

According to the Endangered Species Act of 1973, a species can be listed if it is threatened by any of the following:

1. The present or threatened destruction, modification, or curtailment of its habitat or range;
2. Utilization for commercial, sporting, scientific, or educational purposes at levels that detrimentally affect it;
3. Disease or predation;
4. Absence of regulatory mechanisms adequate to prevent the decline of a species or degradation of its habitat;
5. Other natural or man-made factors affecting its continued existence.

If the species is so threatened, the director then determines the “critical habitat,” which is the species’ inhabitation areas that contain the essential physical or biological features necessary for the species’ preservation. The critical habitat can include non-habitation areas, which are deemed necessary for the protection of the species.