Biodiversity—the vast numbers of species of plants and animals interacting on Earth—provides people with food, building materials, fibers, and medicines. Insects pollinate crops, wild plant genes reinvigorate domestic ones, forests provide breathable air, microorganisms create soil, and aquatic microorganisms cleanse and purify water. Without the millions of known and unknown species operating interdependently, life on Earth would be impossible for human beings. Yet life-forms are dying off at an unprecedented rate. Biologists estimate that there were 1 million fewer species in 2000 than in 1900.

The U.S. Endangered Species Act of 1973 defines an endangered species as one that is on the brink of extinction. A threatened species is defined as one with a good chance of becoming endangered in the foreseeable future. What pushes a healthy animal species into the threatened category? Almost all of the answers involve human beings. Either through misuse, thoughtlessness, or overconsumption...