if its populations are living in different geographic areas) increases its potential for successfully evolving in response to long-term environmental changes. Today, this genetic diversity within species is declining precipitously over much of Earth’s land surface—an unheralded loss of one of humanity’s most vital resources. That resource is largely irreplaceable. Along with fossil fuels, rich soils, ancient groundwater, and mineral deposits, genetic diversity is part of the inheritance of capital that Homo sapiens is rapidly squandering.

What then will happen if the current decimation of organic diversity continues? Crop yields will be more difficult to maintain in the face of climatic change, soil erosion, loss of dependable water supplies, decline of pollinators, and ever more serious assaults by pests. Conversion of productive land to wasteland will accelerate; deserts will continue their seemingly inexorable expansion. Air pollution will increase, and local climates will become harsher. Humanity will have to forego many of the direct economic benefits it might have withdrawn from Earth’s once well-stocked genetic library. It might, for example, miss out on a cure for cancer; but that will make little difference. As ecosystem services falter, mortality from respiratory and epidemic disease, natural disasters, and especially famine will lower life expectancies to the point where cancer (largely a disease of the elderly) will be unimportant. Humanity will bring upon itself consequences depressingly similar to those expected from a nuclear winter (Ehrlich, 1984). Barring a nuclear conflict, it appears that civilization will disappear some time before the end of the next century—not with a bang but a whimper.

Preventing such a denouement will prove extremely difficult at the very least; it may well prove to be impossible. Earth’s habitats are being nickeled and dimed to death, and human beings have great difficulty perceiving and reacting to changes that occur on a scale of decades. Our nervous systems evolved to respond to short-term crises—the potential loss of a mate to a rival, the sudden appearance of a bear in the mouth of the cave. For most of human evolutionary history there was no reason for natural selection to tune us to recognize easily more gradual trends, since there was little or nothing one could do about them. The human lineage evolved in response to changes in the ecosystems in which our ancestors lived, but individuals could not react adaptively to those changes, which usually took place slowly. The depletion of organic diversity and the potential destruction of civilization may, ironically, be an inevitable result of our evolutionary heritage.

If humanity is to avoid becoming once again a species consisting of scattered groups practicing subsistence agriculture, dramatic steps will be necessary. They can only be briefly outlined here. Simply setting aside preserves in the remaining relatively undisturbed ecosystems will no longer suffice. In most parts of the planet such areas are too scarce, and rapid climatic changes may make those preserves impossible to maintain (Peters and Darling, 1985). Areas already greatly modified by human activities must be made more hospitable for other organisms; for example, the spewing of toxins into the environment (leading to intractable problems like acid deposition) must be abated.

Above all, the growth of the human population must be halted, since it is obvious that if the scale of human activities continues to increase for even a few more decades, the extinction of much of Earth’s biota cannot be avoided. Indeed,



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