The Diversity of Life
Edward O. Wilson, the Harvard scientist who is one of the world’s foremost authorities on the lives of ants and on the problems of ecology, begins his book on a dark night in the middle of the Brazilian rain forest:
The forest at night is an experience in sensory deprivation most of the time, black and silent as the midnight zone of a cave. Life is out there in expected abundance. The jungle teems, but in a manner mostly beyond the reach of the human senses. Ninety-nine percent of the animals find their way by chemical trails laid over the surface, puffs of odor released into the air or water, and scents diffused out of little hidden glands and into the air downwind. Animals are masters of this chemical channel, where we are idiots. But we are geniuses of the audiovisual channel, equaled in this modality only by a few odd groups (whales, monkeys, birds). So we wait for the dawn, while they wait for the fall of darkness; and because sight and sound are the evolutionary prerequisites of intelligence, we alone have come to reflect on such matters as Amazon nights and sensory modalities.
But is this as it should be? Wilson asks. Only one species is able to reflect on nature, but is it right that this unique species should commandeer the earth’s resources to the detriment of millions of others? There is a moral dilemma here, one that is felt acutely by a biologist whose passion is the diversity of life on earth.
What is the biodiversity of which Professor Wilson writes with such conviction and authority? What is the scope of it? How did it originate? Where is it found? How are humans diminishing it? And why should it matter that humans are diminishing it? Professor Wilson offers answers to all these questions in engaging and nontechnical prose. His account, accompanied by a scattering of line drawings, is carefully organized, systematic, and instructive, and could serve as a college text. But it is much more than a text in its prodigious erudition and its original and fascinating insights into the Lilliputian world of insects, mites, and microbes.
Biodiversity, in the simplest terms, is the sum total of life, comprehending all species of organisms, large and small, terrestrial and aquatic. Notice that the units are species, mutually interbreeding populations which are reproductively isolated from other sets of interbreeding populations. Some scientists would expand the definition of biodiversity to include the genetic information contained in all organisms, on the grounds that the genetic makeup of every individual of a sexually reproducing population is unique. But, with few exceptions, the genes carried by individuals are not unique. Most genes are widely dispersed in populations. Individuals serve only as their transitory bearers. Genetic continuity is maintained in populations, and the distinctions between the populations of a species tend to blur over time as individuals disperse and habitats shift in space. Hence it is most reasonable, as well as practical, to consider species and their component populations as the units of biodiversity, just as they are the units of evolution.
At another level, biodiversity can be divided into two great categories, one encompassing natural species and populations, and the other encompassing the products of man’s efforts to elaborate on natural diversity through artificial selection. Man’s additions to the evolutionary process have provided the foundations for modern agriculture, yielding for example domestic cattle and hybrid corn. Less in the forefront of modern agriculture, but priceless for the wealth of genetic diversity they contain, are thousands of varieties of wheat, rice, potatoes, and other crop plants that are propagated by traditional societies. Such artificially selected varieties make up a highly utilitarian element of biodiversity, one of many stressed by Wilson in his plea for “precaution” in our stewardship of the world’s genetic resources. Nevertheless, and rightly so, Wilson’s primary concern is for natural biodiversity, both for its own sake and because nature is the source of all selected varieties. If nature is preserved, utilitarian strains can in principle be recreated by artificial breeding; the reverse is simply not possible.
The immensity of natural biodiversity is beyond one’s intuitive grasp. Ten thousand species of microbes may teem in a pinch of European soil; 12,000 species of beetles may dwell in the treetops of a single hectare of Panamanian rain forest. This is not the biodiversity one sees on television nature programs, but it is an integral part of life on earth. It is so unexplored that biologists can’t say with certainty whether the total number of species on our planet is 10 million, 30 million, or, not impossibly, even 100 million. Does it matter?
To Wilson, it matters as much as life itself. It should matter to the rest of us too, for most of the hidden, unglamorous components of biodiversity are intimately engaged in the biological recycling processes that purge the air, soil, and water of toxins and wastes. The small organisms are immensely important, but their services are ones we take for granted. We would quickly acquire a keen appreciation of their unesteemed roles if they were suddenly to go on strike.
Most contemporary biodiversity is to be found in the tropics. Depending on how many insects there are—estimates run from 2 to 30 million—at least 50 percent, and perhaps 90 percent of all living species occur in tropical forests. Tropical lakes and rivers nurture many more fish than their temperate counterparts, the Amazon alone containing more than 2,000 species. And tropical seas and reefs are vastly richer in fish, molluscs, corals, and almost any other type of marine life than the cool waters of higher latitudes. The disproportionate concentration of the earth’s biological wealth in the tropical belt enormously complicates the challenge of conserving it, because the natural resources of many tropical countries are under intense and mounting pressure as impoverished human populations struggle to improve their lot.
Wilson ranks the task of conserving biodiversity as the greatest challenge of our time. In his estimation, the masterworks of nature deserve at least the same solicitous attention we lavish on the masterworks of man. During World War II, both sides secured their art treasures in deep underground vaults. No human beings were so well sheltered. Artworks are unique and priceless, and represent deep cultural values. It would be a crass person indeed who questioned these values. But the truth is that our cultural origins are far more deeply and pervasively embedded in nature than in art. Yet why are we not equally concerned about nature? Perhaps it is because nature has no appointed custodian. If nature is to survive, we must all become its custodians. If we fail,we must bear responsibility for what Professor Wilson has called the one misdeed of this generation which future generations will never forgive.
How to preserve biodiversity is the central theme of the newly emergent discipline of conservation biology. Species of organisms live in natural assemblages that biologists refer to as ecological communities. The species that compose these communities cannot readily be bought, sold, traded, separated, or transported. They are best preserved in their natural habitats. This is because ecological communities are structured by webs of interdependency; and links between producer and consumer, predator and prey, host and parasite, provide the checks and balances that confer longterm stability. Disrupt these links, and the interlocking mechanism begins to fail. Simplification of the community through loss of diversity is the predictable result.
Ecologists are just now beginning to comprehend the systems of checks and balances that maintain biodiversity. No species is independent of the others with which it lives. Although the functions of some species in any community may potentially be fulfilled by others, we don’t know enough about most communities to make such predictions in confidence. Therefore when Manuel Lujan, the current secretary of the interior, asks, “Do we have to save them all?” the only safe answer is “Yes.”
The same caution, Wilson writes, should lead us to regard with special concern a roster of “keystone” species, the most equal among equals in the panoply of organisms. By definition, keystone species are critical in the functioning of a community. If such a species is removed or incidentally lost, the community will decay to a less diverse state. The principle of keystone species was discovered by the University of Washington biologist Robert Paine, who found that a rich intertidal community of limpets, chitons, snails, barnacles, brachipods, bivalves, and other invertebrates would be quickly monopolized by mussels if the top predator, a starfish, were removed from the system. In the absence of the keystone predator, a highly diverse community collapsed into one of monotonous simplicity.
Ecologists are now finding that the same principle applies to terrestrial communities as well. An example of increasing familiarity to suburban Americans is the exploding whitetailed deer population, an explosion attributable to the absence of wolves and mountain lions, their natural predators. Overbrowsing by legions of deer threatens the survival of numerous endangered plant species, and is drastically altering the patterns of tree regeneration in forests throughout the US. It is unfortunate that the keystone species in this case, wolves and mountain lions, are ones that are persecuted by human beings and that require vast expanses of wilderness in which to maintain themselves.
The persistence of keystone predators and other wide-ranging species is severely jeopardized when human beings enter the picture. As settlers begin to colonize an erstwhile wilderness, the natural habitat is almost invariably converted in a patchwork. Little clearings first appear. Later, these are expanded, while more are created nearby. Soon the landscape is a checkerboard of fields, pastures, and woodlots. Any time you fly over the eastern US, you will see such patterns. Ecologists refer to this process as habitat fragmentation. Formerly continuous populations of plants and animals are broken up into small isolated subunits. Usually fragmentation is accompanied by various sorts of disturbance, including increased pressure on predators, aggravating an already precarious situation. The largest species, such as bear, wolves, and elk, cannot maintain themselves in small habitat patches, and disappear almost immediately. The remnant populations of many other species are too small to survive for long, and these gradually fade out in piecemeal progression.
Other species, such as white-tailed deer, thrive in the mosaic of forests and fields and, in the absence of predators, increase until their populations are held in check by starvation. Other animals also increase in response to fragmentation, including raccoons and opossums. But raccoons and opossums relish the eggs and nestlings of songbirds; so where they have increased, bird populations have gone into decline. Fragmentation thus disrupts nature’s system of checks and balances. The consequence is an ecological chain reaction that unleashes a cascade of local extinctions. Local extinctions then spread and merge, just like the expanding patchwork of pioneer fields in a virgin forest, until global extinctions are the result. For this reason, conservation biologists regard fragmentation as perhaps the greatest threat to biodiversity short of complete loss of the natural habitat.