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Protection of Global Biodiversity
By Lakshman D. Guruswamy, Jeffrey A. McNeely
Duke University Press Copyright © 1998 Duke University Press
All rights reserved.
Biological Extinction: Its Scope and Meaning for Us
by Peter H. Raven and Jeffrey A. McNeely
The earth is home to an estimated 10 million species, of which about 1.5 million have been formally described. Due to population growth and increasing rates of consumption, however, the natural wealth of our planet is being lost at an estimated rate of 5 percent per decade. This is a tragic loss of the biological wealth of our planet, for ethical, aesthetic, and economic reasons, as well as for reasons of ecological functioning. A major international initiative is required to make the business community, decisionmakers, and the general public aware of this unparalleled loss of the productive capacity of our planet.
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We are confronting an episode of species extinction greater than anything that the world has experienced for the past 65 million years. To understand species extinction in the broadest possible terms, consider this fundamental fact: the earth, our planetary home, is finite. Since the earth, and everything on or in it, is limited, the economic formulas developed over the past few hundred years to keep track of the values involved in human transactions cannot make it any larger, nor give us any more of the productive systems and commodities on which we depend. It does not matter what conversions may be possible: no matter how clever we may be, the earth remains the same. Contrary to the wishful thinking embodied in some cornucopian scenarios (see, for example, Simon and Kahn 1984), the earth and its systems can either be used in such a way as to provide a sustainable context for our operations, or we shall destroy them. We are currently losing the biological diversity on which we depend at a rate that will greatly limit our future options.
Our species first appeared perhaps 500,000 years ago, at the last instant of the 4.5-billion-year history of the planet earth. As our hunter-gatherer ancestors moved over the face of the earth, they began to exterminate many of the large mammals and birds that they hunted for food—on numerous islands, in the New World 8,000 to 12,000 years ago, and elsewhere (Martin and Klein 1984). After the development of agriculture at scattered localities 11,000 to 6,000 years ago, a population estimated at 5 million people just before the era of recorded history began to increase rapidly. At the same time, with the extensive land clearing and grazing that characterized early agriculture, species became extinct at rapidly increasing rates. For example, in Hawaii, the activities of the Polynesians led to the documented extinction of half the original 100 or so land birds in the 1,200 years before Europeans arrived. Over the subsequent two centuries, 18 additional species have been eliminated, leaving 18 more at such low numbers that they are in immediate danger of extinction and only 9 with populations that appear large enough to be viable over time (Olson 1989, 50–53).
What is occurring now, during the second half of the twentieth century? The human population is in the process of growing from 1.8 billion people at the turn of the century (WRI 1995), a third of them living in industrialized countries, to more than 6 billion by the year 2000, with only a fifth in industrialized countries (United Nations 1992). The population of developing countries during this same period is increasing from about 66 percent of the world's population to roughly 80 percent, or from around 1.2 billion people to perhaps 5 billion (WRI 1995). About 40 percent of those people are living in absolute poverty, dependent on firewood, with no reliable access to clean freshwater, and half of them receive less than the minimum recommended daily amount of food. Many of the women and children who live in these societies are essentially enslaved by these conditions (Sadik 1992). Collectively, the people of developing countries control about 15 percent of the world's cash economy, use some 20 percent of the industrial energy and less of most other materials that contribute to their standard of living, and include among their numbers only about 6 percent of the world's scientists and engineers (United Nations 1992; World Bank 1992).
Over the past 40 years, while the human population increased at unprecedented rates, we have wasted about a fifth of the world's topsoil (Pimental 1993); lost around an eighth of the world's cultivated lands to desertification and salinization; increased greenhouse gases in the atmosphere by over a third, thus setting the world on a course that is leading inexorably to warmer climates (Houghton, Jenkins, and Pephramus 1990); destroyed more than 5 percent of the stratospheric ozone layer, thus increasing the incidence of malignant skin cancer in the latitude of the United States by an estimated 20 percent (EPA 1987; Jones and Wigley 1989); and cut or converted to simplified biological deserts about a third of the forests that existed in 1950 (Brown, Flavin, and Kane 1992). Grain production per capita began to decline in 1986, bringing Thomas Malthus's 200-year-old prediction closer to reality (Pinstrup-Andersen 1993). These grim facts demonstrate that we are not managing the world sustainably now, even as we consider how it might be made sustainable when more people consume more resources in the future.
Since the human populations of developing countries contain such high proportions of young people, even the devotion to family planning that has been exhibited by most of these countries for more than 20 years will not result in population stability for another two or three generations. The most optimistic scenarios for a stable world population range from 8.5 billion to 14 billion people, somewhere toward the end of the next century. If we fail to maintain and strengthen family planning programs worldwide, the situation could be even more serious; if fertility were to remain at the 1990 level of 4.3 children per woman until 2025, the world population would be 109.4 billion by the year 2100, but if fertility were to have dropped in 1990 to the replacement level of 2.1 children per woman, the population would be 8.1 billion in the year 2100 (United Nations 1992).
Perhaps the simplest way to summarize the conditions we face now, and distinguish them from those prevailing at any earlier time, is to point out that human beings, one of an estimated 10 million species on earth (WCMC 1992), are currently estimated to be consuming, wasting, or diverting 40 percent of the total net photosynthetic production on land (Vitousek et al. 1986). Since our population is likely to double or triple over the next century, many of the other species on earth will simply have "no place to hide." There is no rational scenario by which most of them could find a corner somewhere, leading to the inevitable conclusion that species are becoming extinct at levels unprecedented for the past 65 million years. We turn next to a consideration of those rates.
Future Extinction Rates
How do we project rates of species loss reasonably into the future? First, the 1.4 million species that have been named and classified probably represent no more than 15 percent of the world's total species, or about 10 million species, approximately 85 percent of them terrestrial; some scientists consider far higher numbers to be credible (Erwin 1982; May 1990). Probably at least 7 million of the species exist in the tropics and subtropics, with 1.5 to 2 million in the better-known temperate regions. These estimates refer to species of eukaryotic organisms only (organisms with cells that have a contained nucleus); current methods are unable to estimate the numbers of distinct kinds of bacteria or viruses. Insects have an especially interesting history. Labanderia and Sepkoski (1993) have found that an extensive fossil record demonstrates that the diversity of insects at the family level exceeds that of preserved vertebrate tetrapods through 91 percent of their evolutionary history. The great diversity of insects was achieved not by high origination rates but, rather, by low extinction rates comparable to the low rates of slowly evolving marine invertebrate groups. The great radiation of modern insects began 245 million years ago and was not accelerated by the expansion of angiosperms—flowering plants—during the Cretaceous period. The basic trophic machinery of insects was in place nearly 100 million years before angiosperms appeared in the fossil record, so the loss of these ancient life-forms seems especially tragic.
The geological record suggests that despite recent depredations, we now share the earth with the largest number of species that have ever existed at one time, with the communities and ecosystems of the tropics containing the richest and most complex number of species (WCMC 1992).
Judging from the fossil record, the average life span of a species is approximately 4 million years, and of a mammal is about 2 million years (Raup 1986). If the total number of species in the world is around 10 million, then the background rate of extinction can be calculated at about four species per year. This appears to be a reasonable order-of-magnitude estimate, even though extinction rates (and the longevity of species) vary widely between groups. At such a rate, some 40,000 species of organisms would have become extinct during the first 10,000 years following the appearance of agriculture. For the approximately 250,000 species of plants, it would amount to the disappearance of one species every 10 years; for the 4,500 mammals, one every 275 years; and for the 9,000 bird species, one every 275 years (assuming an avian species life span of 4 million years).
In actuality, at least 115 species of birds and 58 species of mammals have become extinct during the past 400 years (WCMC 1992), during which time the number of human beings has increased 11-fold to its current level of 5.5 billion people. This documents a rate of extinction for these groups more than 50 times the base level. Since 1930, when the global human population reached 2 billion people, at least 19 species of birds and 14 species of mammals have become extinct, with more extinctions probable but not yet finally documented. The rates for the past 64 years, therefore, can be documented at roughly 100 times the base level for these groups, and are probably considerably higher—despite intensive efforts to conserve species of birds and mammals.
In temperate regions, for groups that are well known—such as plants, butterflies, and vertebrate animals—it is generally the case that about 10 percent of the species in a given area are currently regarded by specialists as threatened or endangered (IUCN 1993). The figures are much higher for freshwater organisms than for terrestrial ones; thus, more than 70 percent of the 297 native freshwater mussels in the United States are considered threatened, endangered, or of special concern. Although some industrialized countries, such as the United States, are in a position to put substantial resources into the preservation of biodiversity, other nations are not so fortunate. Therefore, the losses in temperate countries such as Chile and South Africa, with large numbers of species found nowhere else, are likely to be great.
Predictions of extinction rates in the future have been based largely on the demonstrated relationship between species number in a given group of organisms and habitat area. The relationship between species number and area is expressed by the formula S = CA2, where S is the number of species, A is the area of the place where the species live, and C and z are constants. For the purpose of calculating rates of species extinction, C can be ignored, and z is what counts. Generally, the value of z varies between 0.15 and 0.35, with the exact value depending on the kind of organism being considered and the habitats in which those organisms are found. As Wilson (1985) has pointed out, when species are able to disperse easily from one place to another, z is low; when they do not have this ability, z is high. Thus, birds have a low z value, while orchids a high one.
The rule of thumb, which corresponds to the commonly observed z value of 0.3, is that when an area is reduced to one-tenth of its original size, the number of species eventually drops to half (MacArthur and Wilson 1967). The word "eventually" is used because some species may disappear immediately when the forest is cleared, while others decline slowly and constitute what Janzen (1987) called "the living dead." Also, for as many as half of the original species to persist in a habitat that has been reduced to a tenth of its original size, an undivided block of the original habitat in optimal condition must remain. Even now, about two-thirds of the surviving tropical rain forests are highly fragmented. Large blocks of individual forests rarely persist, so that the rate of survival at equilibrium is likely to be much lower, with as few as 30 percent, 20 percent, or even 10 percent of the original species surviving. When the last forest patches are cleared in a given area, the rate of survival suddenly may drop to near zero (Wilson 1988, 3–18). It is worth turning here to the misconceptions that have been presented in this context about extinction rates in Puerto Rico and the eastern United States.
Taking a perspective based on forestry, Lugo (1988a) has asserted that MacArthur and Wilson ignored the concept of equilibrium in their development of the theory of island biogeography, when in fact, their theory is based explicitly on survival and equilibrium. Lugo and his colleagues properly emphasize the role of plantations in maintaining some species, which could then repopulate reestablished forests if the pressures are relaxed. They have also asserted that there has been little extinction in Puerto Rico, despite a great reduction in its forest cover around the turn of the century.
These claims, which in any case, cannot be applied to a world in which the population will be doubling or tripling over the course of the next century, ignore the fact that the Greater Antilles were inhabited by dense populations of native people at the time that Christopher Columbus reached them, and that these islands had already been altered significantly 500 years ago—as documented clearly in the early chronicles of the area (Denevan 1992). By comparison with Hawaii, where the fossil record of birds is better known, one might reasonably expect more than a third of the biota to have gone extinct earlier as a result of human activities. The assertions also ignore the fact that the dozens of Puerto Rican plant species represented by one or a few individuals must be members of a class from which not every species survived—many have clearly been lost. The fossils of birds, mammals, and other vertebrates taken from the caves of Puerto Rico amply document widespread extinction since people first reached the island, so that the encouraging discovery that some species do indeed survive in plantations (Lugo 1988b)—a very good thing for the world of the future—cannot logically be taken to dispute the extremely general relationship between species number at equilibrium and area, once the facts are properly analyzed. What is of key importance, however, is the general point that Lugo has presented so well over the years—that human beings are, in fact, managing the entire biosphere and must make serious choices about how to do it well, or suffer the consequences. The survival of species beyond the twenty-first century will not depend, for the most part, on the preservation of whatever approximations of pristine wilderness remain, but rather, on our collective ability to manage and restore to sustainability the various kinds of degraded lands that our activities have created.
Excerpted from Protection of Global Biodiversity by Lakshman D. Guruswamy, Jeffrey A. McNeely. Copyright © 1998 Duke University Press. Excerpted by permission of Duke University Press.
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