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Do We Still Need Nature?
The Importance of Biological Diversity

Our reliance on the Earth's non-renewable resources of oil and other fuel and non-fuel minerals is well understood by most people. Yet, when caught in the tide of technological advances that seem to dominate our everyday lives, we can easily forget the extent to which the modern, industrial world still depends on the biological world: on both the ecological systems that we have already learned to manage, such as farms and orchards, and on those we have not.

A fundamental property of ecological systems is a certain mixture, or diversity of living things: we cannot expect to find deer or ducks in the wild in the absence of the interconnected web of other plants and animals on which their lives depend. Biological diversity, or biodiversity, is a term that is now commonly used to describe the variety of living things and their relationships to each other and interactions with the environment.

The notion of biodiversity encompasses several different levels of biological organization, from the very specific to the most general. Perhaps the most basic is the variety of information contained in the genes of specific organisms, be they petunias or people. Different combinations of genes within organisms, or the existence of different variants of the same basic gene are the fundamental "stuff" of evolution. At the next level is the variety of different species that exist on the Earth: a concept that includes the relationship of different groups of species to each other. Biodiversity also describes the varied composition of ecosystems, and the variety of different sorts of ecosystems that are found in regions of study that biologists call landscapes.

It has been clear for some time that at all of these levels of organization the rich biodiversity that has always characterized the natural world is today declining. The extinctions or threatened extinctions of many species are but the most visible and well-known manifestation of a deeper and more far-reaching trend. What has been less obvious to many people are the potential consequences of these changes.

Our Dependence on Biodiversity

Our lives depend on biodiversity in ways that are not often appreciated. A case in point is agriculture. Society has learned a tremendous amount about techniques to maximize crop yields, both in temperate climates such as the grain belt of the U.S. and Canada, and in subtropical and tropical environments, where the "green revolution" that gained initial momentum in the 1960s vastly increased yields of rice and other crops. In both cases, the advances relied in part on biodiversity, and specifically on the availability of diverse strains of cereal grains capable of responding positively to heavier applications of fertilizer. The need continues, for we are still learning how to sustain tropical agriculture and to minimize adverse environmental impacts of fertilizers and pesticides while maintaining high yields, and how to sustain the highly-managed agro-ecosystems on which we more and more depend.

Much of today's world is also dependent on wild resources, of which the best known examples are probably marine fisheries. The industrial nations of the world support large and technologically- advanced fleets whose sole purpose is to harvest wild fish for human consumption, either directly or indirectly as fishmeal for fertilizers, cattle feed, and aquaculture. Averaged globally, people derive about 16 percent of their total animal protein from marine fisheries. Many developing nations also support a combination of open-ocean fishing industries and intensive coastal and local fisheries, upon which coastal populations depend both for food and for their economic livelihood. About a sixth of the world's population, much of it in the developing world, derives more than a third of their total protein from marine fisheries.

Our long-standing dependence on the natural world for wood is another example that is still much in evidence around the world. Only a small fraction of the timber that is cut in the U.S., for instance, is harvested from plantations: most is taken from natural forests that are not intensively managed. Worldwide, an even greater fraction comes from trees grown in the wild: by far the most important source is unmanaged or lightly managed forest stands. The use of wood for fuel, while of little consequence in technologically advanced countries like our own, is an abiding staple in many developing nations, and the twin demands for shelter and fuel have led to extensive deforestation in many parts of the world, such as Madagascar and Indonesia.

Four out of every five of the top 150 prescription drugs used in the U.S. have had their origins in natural compounds. An example is aspirin--a derivative of salicylic acid which was first taken from the bark of willow trees. Today aspirin and many other drugs are synthesized more efficiently than they can be extracted from the wild, but they were first discovered in naturally occurring compounds, which then formed the basis for subsequent improvement. The process of discovery still continues. For example, taxol, a promising anti-cancer drug, was first extracted from a tree found in the wild: the Pacific yew. The chemical substance from which taxol came has since been discovered in close relatives of that species, thus reducing pressures for harvesting what is already a small population.

Other economic gains derive from our interaction with the natural world, of which the best known example may be the economic value of tourism. Much, although obviously not all vacation travel comes under the rubric of "eco-tourism," driven by a desire to see and experience the natural world. The total economic activity generated by tourists of this kind has been recently estimated by the United Nations at nearly $230 billion each year. Even on regional and local scales, the revenue generated by tourism can be substantial, and a major component of local and regional economies (Table 1).

Each of the activities cited above provides resources and economic gains for citizens in all societies. Yet each is at risk due to the continued erosion of the resource on which they are based, which is biodiversity. In what follows we review what is known of the forces that are reducing biodiversity and some of the possible consequences of this loss, and suggest areas in which additional research and policy analyses are most needed.

The Winds of Change

The recent Global Biodiversity Assessment of the United Nations Environment Program (UNEP) has identified four major causes of the present decrease in biodiversity, and a fifth which may yet prove to be important.

Land Use

(See Figure 1)

Changes in how the land is used are probably the principal contributor to the current decline in biodiversity. About 1 to 2 percent of the land surface of the Earth is now devoted to urban use, but other changes in land cover and land use far exceed the direct impact of the small fraction that is paved or developed for homes and factories and other buildings. Homo sapiens has already converted about a quarter of all the land surface to agricultural uses. By some estimates we now appropriate directly or indirectly about 40 percent of what biologists call the primary production of the Earth's biota (the products of photosynthesis on which all other life depends), and the percentage that comes under our control in this way is increasing.

The pressures on terrestrial resources and land depend very much on population growth and the demands of early stages of economic development. Moreover, land acquisition, especially for agriculture and forestry, focuses initially on those areas with the most fertile soils and equable climates, which are often the areas of greatest biological diversity.

Deforestation in the humid tropics is probably the best-known current example of rapid land-use change. During the decade of the 1970s, vast areas of tropical forest in South America, Africa, and Southeast Asia were cleared and converted to agriculture and other uses. In the middle-to-late 1980s, the rates of deforestation in South America slowed dramatically, largely due to economic and tax policy changes in Brazil, but the pace of cutting in Africa and Southeast Asia, though poorly quantified, remains high. Globally, the rate of loss of tropical forests for the 1980s has been estimated at about 1 percent per year, but there is still considerable uncertainty. The rates of extinction of local species that accompany these rapid changes in land cover may soon be far in excess of what is found today, reaching as high as 10,000 times the natural background rate.

In the industrialized nations of the Northern Hemisphere the most rapid and widespread conversion of forest to other uses took place over the last several hundred years. In this time, much of the northeastern U.S., for example, was deforested at least once, in connection with the rise of agriculture and timber industries. But as regional and national economies changed, many previously cleared areas were left to return to their natural vegetation. As a result, forests have reappeared in parts of the Northeast, and indeed the country as a whole has probably gained forested land over the last several decades.

The current trend of most concern with respect to tree-cover in the U.S. is a shift to smaller parcel sizes. What once were continuously forested landscapes are now a quilt of small patches of trees, criss- crossed with roads, subdivisions, agricultural tracts, and a variety of different land-uses and land-covers: a scene that is familiar to anyone who has looked out an airplane window. The average size of tree-covered parcels is smaller than was the case twenty, fifty, or a hundred years ago, resulting in a landscape that is highly fragmented and partitioned.

The difference in terms of the natural world is great, and several studies now point with concern to the biological impacts of the shift to less continuous landscapes. The known consequences of these changes are reduced numbers of both plants and animals and a greater possibility of the outright loss of some of them--when in effect, they are painted into a corner with nowhere left for them to go. The interweaving of favorable and unfavorable habitats also curtails the ability of organisms to disperse, and makes recolonization of distant areas more difficult.

An analogous pattern of fragmentation can be found in parts of South America where deforestation was previously extremely rapid. Although the amount of new cutting appears to have fallen from that of previous decades, it seems to be increasing again in the rain forest of the Amazon, and the deforested, newly colonized regions now have their own distinctive appearance. Patchworks of active fields, orchards, abandoned fields, second growth forest, and primary forest are the norm. But the scene is ever changing through an interplay of active use by initial colonizers, abandonment, partial recovery through natural processes, and as then often happens, subsequent re-use. Analyses of potential impacts on biodiversity that are based on simple measures of deforested area can provide little more than very general conclusions.

Deforestation is not the only land-use change of interest or concern. Another with broad implications for biodiversity is the intensification of agriculture and grazing on those lands that have been traditionally devoted to these purposes. Of particular importance for biodiversity are the secondary impacts of intensive agriculture. Heavy applications of fertilizers and pesticides have the potential of creating additional environmental problems as well as affecting the abundance and viability of the other plants and animals and micro-organisms in the same or adjoining areas.

The adverse effects of non-point-source pollution due to the run-off of pesticides and herbicides from intensively-used fields are well- known. In addition, because of the understandable tendency to put the best land into production first, the expansion of agriculture into less fertile areas typically requires heavier applications of chemicals, more extensive site preparation, and other forms of more intensive management. The typical result is increased chemical run-off to the landscape, and with ensuing degradation, additional pressure for expansion, and so on. It is such a cycle that has led to widespread desertification in some parts of the world, primarily through overgrazing that can be compounded by naturally occurring droughts.

Over-exploitation

Many of the best documented cases of individual species being driven to extinction or near-extinction by humans are those of over- exploitation.

The passenger pigeon--a species that resembled the smaller, mourning dove--was in the early 1800s the most abundant bird in North America, and so plentiful that migrating flocks of a billion or more individuals would darken the skies of parts of the eastern U.S. for days at a time. By the end of the last century it had been hunted to the brink of extinction, and in September of 1914, in a Cincinnati zoo, the passenger pigeon disappeared forever with the death of the last remaining bird. The American bison, or buffalo, of the Great Plains was also nearly hunted out of existence in the same century, and its larger, woods-dwelling relative was driven to extinction.

As many as a quarter of all the bird species in the world may have similarly vanished in the course of the last 1000 years with the expansion of human populations through the islands of the South Pacific. The spread of early people through the New World, about 10,000 years ago, was probably responsible for the extinction of many of the large mammals that were originally here: now-extinct mammoths, sloths, and cave bears are known to have been hunted by those who first walked through North and South America. The same impact was felt by large mammals in Australia, New Zealand, and Madagascar. The current and rapid loss of tropical hardwoods in many regions due to high commercial demand, low rates of successful replacement, and the long periods of growth necessary to produce new, marketable resources has raised concern about over- exploitation of some species, such as rosewood, although none of the trees are known to have been driven to extinction.

Over-exploitation is also a major factor in reducing the natural biodiversity of marine fisheries through major reductions in populations, although again, no extinctions have been documented. During the last two decades, the world has seen the collapse of a number of marine fisheries. Some of these have recovered, but others, such as the cod and haddock fisheries in the North Atlantic, have not. Even for those that recover, the consequences of the original over-exploitation on population dynamics and genetic diversity are now only poorly understood. What is often apparent is a systematic decrease in the size, and hence age, of the individuals that are harvested. The selective loss of larger fish has significant impacts on those that remain. If fertility is strongly related to body size, as is the case for many fish species, over-exploitation not only reduces the abundance of a species, but it may also make recovery more difficult in systematically removing the most fecund individuals. The ensuing consequences for overall ecosystem functioning and biodiversity are as yet not well understood.

Whole ecosystems can also be affected by over-exploitation. For example, a reduction in organic carbon and nutrients, including phosphorus or nitrogen, as may occur in intensively farmed areas, decreases the fertility of soils. When losses are severe, the resulting depletion can lead to either more intensified use by adding more fertilizers and then herbicides and pesticides to control weeds and pests (in the cycle noted above), or to abandonment. If abandoned, the land will probably not recover its original component of plant and animal species because of the depleted nutrients. Through this chain of happenings, an over-exploitation of the soil for agricultural gain can have long term, negative impacts on the biodiversity of the region.

Alien introductions

Introductions and invasions of alien species of plants and animals is a long-recognized problem, as detailed in an earlier issue of CONSEQUENCES. We have only limited ability to predict quantitatively the results of any particular intruder, including its capability of establishing a permanent, reproducing population. What is certain is that some areas are by nature more susceptible. Continental forests are reasonably resistant to newly introduced tree species, except in cases where they have been disturbed by heavy cutting or partial clearing. Native meadows and prairies, when disturbed, have also proven particularly susceptible to intruders, as is the case for the many grasslands around the world that have been converted to pasture or cultivated land. For example, many of the now common grasses in the intermontane western U.S. and southwestern Canada are transported Eurasian weeds. These species were able to invade and become established because the original perennial tussock grasses were unable to support the intensification of grazing from large-scale cattle ranching.

Freshwater lakes and streams have little immunity to invading species. Alien plants or animals seem able to establish reproducing populations relatively easily, and the new species often have significant impacts on biotic composition, and on a variety of ecosystem processes. Two examples of the kind of changes that can result from even well-meant introductions are the purposeful introduction of game fish to many lakes and streams throughout the world that replaced native varieties, and the ecological havoc that followed the introduction of the Nile perch in Lake Victoria in 1960 to benefit commercial fishing. In less than thirty years, the appetite of the Nile perch, whose food is smaller fish, led to the extinction of about thirty species of fish that were native to the lake.

In terrestrial ecosystems, the largest changes occur when the intruder brings quite different traits from those of native species. The best documented example is that of the introduction of the exotic tree, Myrica faga, into Hawaii, which has resulted in large changes in ecosystem dynamics. The significant difference, in this case, was the ability of the introduced tree--a legume like peas and beans and clover--to convert atmospheric nitrogen to ammonia, a characteristic not previously present in those ecosystems. This ability of the introduced tree increases the nitrogen content of soils, and thus alters the raw materials on which many other plant species depend.

Introduced species with characteristics that are not qualitatively different from those of native species, can through force of numbers have large and long-lasting effects on them. About 100 European starlings were released in New York City in 1890-91 by a collector bent on establishing all the birds mentioned in the writings of William Shakespeare. The result, evident throughout the country today, is a diminished number of many native American songbirds, through competition for nest-sites, in which the aggressive and now very abundant starling has been extremely successful.

Pollution and toxification

The widespread increase of various pollutants and poisonous or toxic substances in the environment has had obvious local impacts on biodiversity in acutely affected areas. In western industrialized countries the more severe instances of air and water pollution are for the most part now being addressed through regulation and clean-up. In many developing nations, however, the financial resources needed to correct acute pollution problems are not available, and air- and water-borne pollution continue to pose great environmental problems, including local reductions in the diversity of living species.

In many parts of the industrialized world, long-term chronic pollution of air, water, and soil pose problems that are difficult to resolve and rectify. The degree to which chronic, low-level pollution constitutes a risk to biodiversity is less clear than for acute exposures, but several facts are worth noting. One is that the transport of many pollutants has been surprisingly wide and rapid. Organic compounds of chlorine such as DDT, for example, can literally be found all over the world due to atmospheric transport, even though many of these substances have long been banned in western industrialized countries, and they often remain and accumulate in parts of both terrestrial and aquatic food chains.

Another concern is that long-term pollution, at even low levels, can affect whole ecosystems, with resultant impacts on biodiversity that bring about additional changes in how ecosystems operate. The chain of events through which central European forests have responded to acid deposition is a ready example. There, the chronic deposition of airborne acidic substances from industrial effluents affects both chemical processes and essential microorganisms in the soil, lowering the vitality of trees and ground-cover and making groundwater, streams, and lakes more acidic.

A third concern is that pollutants in soils and ground water, once introduced, remain there for a long time, due both to the chemical stability of many of the compounds involved, and the extremely slow rates at which ground water is circulated or exchanged. A fourth arises through the fact that many insecticides work by a process of mimicry--copying the behavior of vital hormones in the species they are designed to attack. But hormones that regulate the physiological functions of non-target species, especially the reproductive functions, can also be mimicked by the same insecticides. This raises the concern that the ever-increasing load of these compounds in natural systems may impact wildlife in general, and even humans. The recent discovery of prevalent reproductive anomalies in some forms of wildlife, such as some frogs and toads, is believed by some scientists to be due to hormonally active substances introduced into the environment.

Climate change

Natural variations in climate are among the more important causes of past changes in biodiversity, both locally and for the Earth as a whole. The prospect of human-induced changes in the climate adds new concerns, however, particularly if the rate of change should prove as rapid as many now foresee.

Current projections of general circulation models imply rates of change of the global climate system that exceed those of almost any natural variation in the geologic past. These rapid changes in surface temperature and other weather parameters could lead to severe mismatches between regional conditions and the physiological requirements of many plant species. Were the average surface temperature to rise by several degrees C, that warming would probably be followed by potentially large re-organizations of some ecological communities. For example, some model simulations suggest that with such a change, much of deciduous forest in the eastern U.S. could be replaced by savanna-like vegetation, composed of mixtures of open park land and grasses. Similarly, some forest ecosystem models have suggested that rapid and extensive diebacks of certain trees could occur over much of their current range. Were this to happen, there would undoubtedly be an associated wave of changes in the fauna as well.

By itself, a systematic change in predominant vegetation, which has happened before in the geologic past, would not necessarily imply an increase in the rate of species extinctions. At the same time, with global warming some species will find less optimal habitat than before, particularly in higher latitudes. Plants and animals that now inhabit montane and alpine habitats--and which are there through an evolved dependence on cooler temperatures and higher altitudes- -may with warming of these regions have nowhere else to go. The combination of potentially rapid climate change with the increased fragmentation of land cover is especially troubling: while it would be conceivable for some organisms to disperse to climatically more favorable habitats, the shift could in practice carry them across many unfavorable regions.

The recent consensus findings of the Intergovernmental Panel on Climate Change of a now detectable signature of anthropogenic influences on the physical climate system adds new impetus to examine the probable impacts of continued global warming on living things, and biodiversity in particular.

The Loss of Species

Species extinctions have received the lion's share of the attention in debates regarding biodiversity and the need to sustain it. The loss of individual species in ecosystems, such as frogs in wetlands or ferns in a forest, can certainly affect the ways in which those systems work together to cycle essential nutrients and water and process energy. Since we have only limited ability to predict how ecosystems will respond in terms of replacement or built-in redundancy to the possible loss of a specific species, there is some reason to be concerned when any are threatened by extinction.

At the same time, the same degree of concern should apply to reductions in the populations of species, even if they are not reduced to disappearance altogether. The role that classes of organisms play in ecosystems depends not only on what they do in terms of material cycling and energy flow, but on how many are there to do it. Reductions in abundance of an essential species can clearly affect overall system functioning, and therefore the degree to which ecosystem services will continue to be provided.

Some, known as keystone species, play a role in ecosystems that seems out of proportion to their number, such that even small changes in their abundance may have great impacts on the ecosystems in which they live. A common example is the sea- otter, a marine mammal that lives along the coasts of the northern Pacific Ocean. They dive and prey on sea-urchins that principally feed, in turn, on large seaweed called kelp. When sea-otters are present, the number of urchins is kept sufficiently low that stands of kelp--which are of commercial value as a source of potash and iodine--can become established and thrive. When otters vanish from the scene, the resulting growth in urchin populations effectively prevents the plant's successful regeneration, and eventually leads to the loss of kelp forests.

In time, all classes of living things--like the dinosaurs, or, we must presume, our own species--must face extinction. But the disappearance of any of them is a critical endpoint, marking the end of 3.5 billion years of evolutionary development. In Nature it represents a permanent depletion of biodiversity and a loss of genetic information on which evolution is based. In terms of people and nations, it counts as a loss of potential economic value in terms of services or products. Each species is a reservoir of unique genetic information that cannot be reproduced once it is gone. In this broader sense, any extinction, however trivial it may seem, represents a permanent loss to the biosphere as a whole.

What we need to know for informed policy decisions are the ecosystem services that a threatened species provides; the degree to which it offers opportunities for direct economic benefit; how expected benefits weigh against costs of preservation; and on a more general level, how present or expected rates of extinction compare to what might be expected through natural changes. The telling questions are whether and by how much the present rate of species loss differs from the rate that Nature would exact, were we not here, and whether the species that are lost play important keystone roles. The challenge is that this sort of information is only rarely available. Nor do we have, as yet, a predictive theory of keystone species.

Rates of loss

The UNEP Global Biodiversity Assessment has recently reviewed the methods that have been used in the literature to calculate natural, or background extinction rates and have compared them against current trends. The results, which are intentionally conservative, are sobering. Best estimates are that current extinction rates for well- documented groups of primarily, but not exclusively, vertebrates and vascular (in general, seed-bearing and fern-like) plants are at least 50 to 100 times larger than the expected natural background. There is no good reason to expect these rates to differ very much for plant or animal groups that are less well-studied.

On the basis of recent estimates of land-use change, largely in the tropics, there is a reasonable expectation that extinction rates in the very near future could rise, worldwide, to as much as 10,000 times the natural level. Extinctions of this number and extent would approach, and possibly surpass, the major mass extinctions of the geologic past, as when dinosaurs and other life forms disappeared, about 65 million years ago.

The total number of species that inhabit the planet is unknown. The UNEP Global Biodiversity Assessment uses an estimate of about 13 million, but the range varies from 8 to 50 million or more. Only about 2 million species have been described scientifically, and they are distributed very unevenly among different taxonomic groups (Table 2). While important in its own right, the number need not be precisely known to be concerned about the rates at which the better documented species are now disappearing. In today's world, most extinctions will occur before the species have even been named and described, much less known ecologically.

The Services that Biodiversity Provides

Assessments of the economic benefits of biological diversity have been based primarily on our ability to generate revenue from biodiversity, through activities that produce measurable results in current markets, such as pharmaceuticals or tourism. But there are additional benefits from biodiversity that are not so easily included in commercial market analyses, and that come under the name of ecosystem services. These are the end results of natural biological processes that either improve the overall quality of the environment, or provide some benefit to the human users of the landscape--such as improvement of water quality and reduction of flooding. The concept of ecosystem services is unabashedly tilted toward human uses.

The study of ecosystem services is relatively new, but what is known points consistently in one direction: maintaining diversity on a variety of levels of ecological and biological organization--within forests, or among the trees that are there, or even within the genes of a single variety---is critical if services are to be maintained on a sustainable basis.

Ecosystem services can be provided in a variety of forms. One example is the purification of water that generally occurs by flowing through forested ecosystems and wetlands, which is an extremely important function from the standpoint of human populations that live downstream. The presence of living vegetation provides an efficient sink for many atmospheric pollutants as well. The regulation of stream flow by vegetation in the upper reaches of watersheds has long been recognized as an important ecosystem service, and watershed managers manipulate both the amount and type of vegetation in watersheds to help control sedimentation, floods, and sometimes stream flow.

The services that ecosystems provide often depend on the underlying physical structure of the habitat, such as the conditions for feeding and breeding that may be needed for the continued survival of an important animal species. What is often required is a diversity of habitats over an entire landscape. Ecosystem services may also depend on the presence of a particular species, as is the case in highly co-evolved plant-pollinator systems, or in highly managed agroecosystems that rely on specific pollinators, such as honeybees.

Biodiversity also plays an important role in maintaining ecosystem services over long periods of time, through the ups and downs of natural variations. Ecosystems that have lost either genetic or species diversity are less resistant to the effects of environmental perturbations, such as droughts, and are slower to recover when disturbed. Diversity is a form of ecosystem health insurance: those ecosystems that include several species that serve the same or similar functions tend to be more resistant to environmental stress and recover faster from perturbations.

The Economic Value of Ecosystem Services

The economic value of ecosystem services is difficult to calculate, and this raises several important problems when we look at biodiversity in the context of public policy. How can we measure the economic value of ecosystem services such as water purification, or resistance to environmental disturbances? Since the maintenance of biodiversity involves choices and ultimately, costs, how can markets reflect and distribute these values appropriately?

The task may be somewhat easier in the case of new products and materials that are derived from the natural world. Prospecting for new pharmaceuticals is the most publicized, but not the only example. New food crops are also a possibility, although to date there have been very few such introductions that have achieved more than regional importance, either dietarily or economically. More intriguing, perhaps, is the use of genetic engineering to extract biochemical processes from the natural world. Research of this kind has found application in biological clean-up, or bioremediation of toxic waste and oil spills. An even more promising and somewhat more controversial opportunity is found in harnessing processes at the most fundamental levels of biological structure.

The pool of resources hidden in the genetic resources of living things is potentially huge. An example is the polymerase chain reaction (PCR) that is used in genetic research and in commercial applications to manipulate DNA. The ready availability of substances that speed up the rate at which the cells replicate--the catalysts that in living matter are proteins known as enzymes--has literally made genetic engineering practical on industrial scales.

The enzymes used to catalyze PCR were first isolated from bacteria that can survive only in high temperatures, and the source from which they were taken was natural hot springs in Yellowstone National Park. In this case, to say that an entire new industry depended on the diversity of organisms and habitats in the National Park system is no exaggeration. Substantial prospecting is now underway in these and other extreme environments to find enzymes that will catalyze other, industrially-useful reactions.

What Lies Ahead?

The population of the Earth will likely double by the year 2050, resulting in a world of at least 10 billion people, the largest number of whom, by far, will live in tropical and subtropical Asia, Africa, and South America. These are as well the regions in greatest need of economic development, and the twin pressures of population growth and economic expansion can only increase the demands on biological resources. We can anticipate an ever-increasing competition among different uses of the available land, and the maintenance of biodiversity may not rank high in the face of other, more obvious demands.

Many of the existing policies of our own country that have been enacted to preserve biodiversity have been focused on threatened species, or to preserve striking or unique ecosystems, such as Yellowstone National Park. The Endangered Species Act, the Convention on International Trade in Endangered Species, and our system of National Parks will continue to help in preserving biodiversity. But there are other areas of public policy that are as useful and important. In fact, it may well be that lands and waters that are necessarily exploited for their natural resources will hold the key for practical strategies to maintain biodiversity, for parks and preserves, alone, are inadequate for the task.

In truth, much that happens to preserve or decrease biodiversity arises through secondary effects of policies that are enacted for other reasons. Fisheries policies that aim to maintain fishery harvests; forestry policies that seek to maximize the economic yield of marketable timber; agricultural policies that maintain subsidies for keeping land in production that might be used for other beneficial purposes; and policies for the management of public lands that encourage overgrazing by maintaining artificially low grazing fees all have important negative effects on biodiversity, although not by design.

Other existing policies have impacts that work in the other direction. But unless the impacts on biodiversity of private acts or public policies are understood, and until there exists a broader consensus regarding the relative value of biodiversity, there is little hope, in this or any country, of holding the line at the levels that are needed for almost any use or service. We all need to be more aware of the direct benefits, indirect benefits, services, and future potential that biodiversity offers for both private gain and public benefit. We need greater awareness and coordination of policies that affect biodiversity, and national goals that go deeper than the protection of endangered species and the preservation of public parks.

From an economic perspective, much more work needs to be done to put a fair and meaningful valuation on biodiversity. The service aspects of biodiversity must be understood, and market mechanisms put in place to include these very real factors in both policy and business decisions.

From a scientific perspective, we need to learn more, and more quickly, about the role that biodiversity plays in the working of ecosystems. Gaps in our present knowledge of these connections now limit our assessments of the risks imposed when biodiversity declines, and preclude more complete economic evaluations.

In all of this, calls will be heard to defer action until we have in hand a more complete and reliable inventory of the present extent and variety of life on Earth, in terms of the number of species of plants and animals. Although counting must go on, it is now clear that waiting to learn the full extent of biodiversity before acting to stem so precipitous a decline is not a prudent choice, for both ecological and economic reasons.

Last, but certainly not least, are the issues of stewardship and ethics. In the long run, we must be concerned about maintaining the capability of the biological world to adapt, through adjustment and evolution, to changes in the physical environment. In addition, many would agree that as a society we bear the ethical obligation to protect the habitability of the planet, and to act as responsible stewards of its biological riches for the present and future welfare of the human species. To do that requires an appreciation of the value of biodiversity--both what it provides for the natural world and the ways that we can use it--and a commitment to preserve it so that our children and their children will continue to realize the benefits of a biologically rich Earth. Surely such a challenge demands the attention of scholars and policy-makers alike.

Reviewers

Dr. Thomas Lovejoy is an ecologist and widely-known expert on biological diversity at the Smithsonian Institution in Washington, D.C., where he serves as Counselor to the Secretary for Biodiversity and Environmental Affairs and as Director of the Smithsonian's Institute of Conservation Biology.

Dr. Gordon Orians, an ecologist, is Professor Emeritus of Zoology at the University of Washington in Seattle, a Past-President of the Ecological Society of America, and a member of the National Academy of Sciences. For many years he has contributed to the science-policy interface through the Science Advisory Board to the EPA, the National Research Council, the World Wildlife Fund, and the Smithsonian Institution.

Scientific reviewers provide technical advice to the authors and Editor, who bear ultimate responsibility for the accuracy and balance of any opinions that are expressed.

For Further Reading

Biodiversity, edited by E. O. Wilson. Contributions from a National Forum on BioDiversity. National Academy Press, Washington, D.C., 1988, 521 pp.

Global Biodiversity Assessment, Policymakers' Summary, United Nations Environment Programme, 1996.

The National Biological Service homepage at http://www.its.nbs.gov/ [now the USGS's Biological Resources Division]

The Diversity of Life by E. O. Wilson, Harvard Univ. Press, Cambridge, Mass., 1992, 424pp.

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