A U.S.-based course in conservation biology wouldn't be complete without a brief introduction to the Endangered Species Act. We won't go into all of its details, but there are a few important features that anyone finishing this course should be aware of.
The Endangered Species Act of 1973 requires the U. S. Fish & Wildife Service (or the National Marine Fisheries Service, for certain marine species) to identify species of wildlife and plants that are endangered or threatened, based on the best available scientific and commercial data.
§3(6) The term ``endangered species'' means any species which is in danger of extinction throughout all or a significant portion of its range other than a species of the Class Insecta determined by the Secretary to constitute a pest whose protection under the provisions of this Act would present an overwhelming and overriding risk to man.
§3(19) The term ``threatened species'' means any species which is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range.
§3(15) The term ``species'' includes any subspecies of fish or wildlife or plants, and any distinct population segment of any species of vertebrate fish or wildlife which interbreeds when mature.
The act recognizes five factors permitting listing:
Wilcove, McMilan, and Winston  point out that animal species proposed for listing between 1985 and 1991 had a median number of 1000 individuals and 2-3 populations when listed. Plants had a median number of 120 individuals and 4 populations. 39 species of plant had 10 or fewer individuals. This suggests that we are trying to save many endangered species when their populations are already drastically reduced.
As of 31 August 2011 (http://ecos.fws.gov/tess_public/pub/Boxscore.do)1
Of the 1380 species listed as threatened or endangered
It seems intuitively obvious that there are some problems here, but before we can say much about how populations of endangered species should be managed, we need to spend some time discussion the problems of life in small populations, which is what we'll be doing for the next three weeks, or so.