dignity. The decisive factors will be social, political, and economic; they will not be limited simply to a willingness to conserve.
Putting these relationships in another context, and assuming that two-thirds of the world’s 4 to 5 million species are located in the tropics and subtropics, nearly half the world’s species of plants, animals, and microorganisms will be destroyed or severely threatened over the next quarter century—well within the expected life span of most people living today. If half these organisms become extinct during the next several decades—surely a conservative estimate—the world will experience a major episode of extinction. This episode could amount to the loss of perhaps 10% of the world’s species by the end of the century and to more than a 25% loss within the next couple of decades. These estimates are compatible with the predictions of extinction rates for primates and other relatively well-studied groups of organisms and with the closely coupled nature of the biological relationships involved. To find a comparable rate of extinction, one needs to go back more than 65 million years to the end of the Cretaceous period, when the dinosaurs disappeared along with a major loss of other life on Earth. In fact, the rate of extinction that will be characteristic of most of the remaining lifetimes of those now living is estimated to be at least 1,000 times the normal rate. Since there are now many more species than there were 65 million years ago, the absolute loss in number of species will be much greater.
For a more concrete example, consider the flowering plants. We obtain 85% of our food directly or indirectly from just 20 kinds of plants, and about two-thirds from just three: maize (corn), wheat, and rice. The 20 species were brought into cultivation thousands of years ago, largely because they were easy to grow; they were not selected because of their ability to contribute to the needs of a modern industrial civilization. Despite that, they are precious. Widespread starvation in the tropics and subtropics, however, reminds us that temperate-zone agriculture is not suitable everywhere, and suggests that an enhanced ability to cultivate some of the other 250,000 species of flowering plants might offer rewards by providing food crops that can be cultivated successfully in areas where the cultivation of present food plants is now difficult or impossible.
In evaluating our future opportunities to use the lesser known plants, however, consider the significance of the extinction projections reviewed above. Some 25,000 species of plants—about 5 species a day—are expected to disappear between now and the end of the century, and then perhaps 10 species a day will become extinct over the following couple of decades. Clearly, many of the 50,000 species of plants expected to vanish forever during our lives hold exceptional promise for producing food, fodder, wood, medicine—all the factors that increase the quality and stability of human existence on Earth. Given our record numbers, and the extreme pressure with which we are assaulting the global ecosystem, it seems absolutely mandatory that we redouble our efforts to survey, classify, preserve, and understand these plants, as well as members of other groups of organisms, while they still exist.
The consequences of the destruction of tropical and subtropical forests are grave; basically, our collective actions are denying to our children and grandchildren the ability to play the game of survival with the tools that we have at our disposal