The tremendous variety of species held in wilderness areas, particularly the tropics, is our bank and lifeline, our agricultural and medical insurance policy. Three-quarters of the world's food supply comes from twelve plant species, but those species are dependent on thousands of others: pollinators (insects, bats, birds), soil microbes, nitrogen-fixing bacteria, and fungi. The tropical rain forests contain a pool of genetic diversity for important food crops, a source for vital new strains that can be hybridized to fight pests and diseases. Botanists are combing the planet for wild ancestors of soybeans, tomatoes, hard wheat, and grapes, believed to contain valuable genes for drought tolerance and other characteristics, but much diversity has already been lost. Genetic engineering alone cannot replace what hundreds of millions of years of evolution have given us. Wild replacements for pineapples, pomegranates, olives, coffee, and other crops lie in biodiversity-rich areas that must be saved.

In terms of medicine, our most important modern pharmaceuticals, including quinine, morphine, aspirin, penicillin, and many other antibiotics, are derived from microbes, plants, and animals found in tropical and marine environments. The first comprehensive scientific treatise on our reliance on other species, Sustaining Life: How Human Health Depends on Biodiversity, published in 2008, confirmed the importance of genetic variety, describing groups of threatened organisms crucial to agriculture and human medicine. Predictably, our close relatives, primates, make up a key group. Contributing to work on smallpox, polio, and vaccine development, primates allow research on potential treatments for hepatitis C and B, Ebola and Marburg viruses, and HIV/AIDS.

The list of threatened plants and animals we rely on is weird and varied, including amphibians, bears, gymnosperms (the family of plants that includes pine trees), cone snails, sharks, and horseshoe crabs. Cone snails, a large genus of endangered marine mollusks, inject their prey with paralyzing toxins that are prized in medical research for their use in developing pain medications for cancer and AIDS patients who are unresponsive to opiates. The blood of the horseshoe crab, which carries antimicrobial peptides that kill bacteria, is being tested in treatments for HIV, leukemia, prostate cancer, breast cancer, and rheumatoid arthritis; it also yields cells crucial in developing tests to detect bacteria in medical devices, and its eyes have allowed Nobel Prize–winning researchers to unravel the complexities of human vision.

Cone snails and horseshoe crabs are exactly the kinds of species that people tend to dismiss, seeing no utility in them, no connection to human need. This was the attitude expressed in 1990 by Manuel Lujan Jr., secretary of the interior during the George H. W. Bush administration, who asked in exasperation, "Do we have to save every subspecies?" It was the attitude expressed in 2008 by presidential candidate John McCain, who repeatedly declared his opposition to the funding of research on grizzly bear DNA. He got a cheap laugh whenever he said, "I don't know if that was a paternity issue or a criminal issue." Medical researchers were not laughing: bears, too, are essential to human medicine. Bear bile yields ursodeoxycholic acid, now used in treating complications during pregnancy, gallstones, and severe liver disease. Denning bears enter a period of lethargy during the winter and recycle body wastes in a process unique in mammals; this process is studied for insights in treating osteoporosis, renal disease, diabetes, and obesity.

If species are crucial to medicine, ecosystems are indispensable to the health of the planet. Ecosystems provide the most basic provisioning services — food, firewood, and medicines — along with the so-called regulating services of a fully functional environment, which include cleaning the air, purifying water, controlling floods