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BioDiversity PART 1 CHALLENGES TO THE PRESERVATION OF BIODIVERSITY
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BioDiversity Trans-Amazon Highway being cut through the rain forest near Altamaria, Brazil—one example of the detorestation that takes place along with traditional frontier expansion. Photo courtesy of Nigel J.H.Smith.
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BioDiversity CHAPTER 2 THE LOSS OF DIVERSITY Causes and Consequences PAUL R.EHRLICH Professor of Biological Sciences, Stanford University, Stanford, California Discussions of the current extinction crisis all too often focus on the fates of prominent endangered species, and in many cases on deliberate overexploitation by human beings as the cause of the endangerment. Thus black rhinos are disappearing from Africa, because their horns are in demand for the manufacture of ceremonial daggers for Middle Eastern puberty rites; elephants are threatened by the great economic value of ivory; spotted cats are at risk because their hides are in demand by furriers; and whales are rare because, among other things, they can be converted into pet food. Concern about such direct endangerment is valid and has been politically important, because public sympathy seems more easily aroused over the plight of furry, cuddly, or spectacular animals. The time has come, however, to focus public attention on a number of more obscure and (to most people) unpleasant truths, such as the following: The primary cause of the decay of organic diversity is not direct human exploitation or malevolence, but the habitat destruction that inevitably results from the expansion of human populations and human activities. Many of the less cuddly, less spectacular organisms that Homo sapiens is wiping out are more important to the human future than are most of the publicized endangered species. People need plants and insects more than they need leopards and whales (which is not to denigrate the value of the latter two). Other organisms have provided humanity with the very basis of civilization in the form of crops, domestic animals, a wide variety of industrial products, and many important medicines. Nonetheless, the most important anthropocentric reason for preserving diversity is the role that microorganisms, plants, and animals
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BioDiversity play in providing free ecosystem services, without which society in its present form could not persist (Ehrlich and Ehrlich, 1981; Holdren and Ehrlich, 1974). The loss of genetically distinct populations within species is, at the moment, at least as important a problem as the loss of entire species. Once a species is reduced to a remnant, its ability to benefit humanity ordinarily declines greatly, and its total extinction in the relatively near future becomes much more likely. By the time an organism is recognized as endangered, it is often too late to save it. Extrapolation of current trends in the reduction of diversity implies a denouement for civilization within the next 100 years comparable to a nuclear winter. Arresting the loss of diversity will be extremely difficult. The traditional “just set aside a preserve” approach is almost certain to be inadequate because of factors such as runaway human population growth, acid rains, and climate change induced by human beings. A quasi-religious transformation leading to the appreciation of diversity for its own sake, apart from the obvious direct benefits to humanity, may be required to save other organisms and ourselves. Let us examine some of these propositions more closely. While a mere handful of species is now being subjected to purposeful overexploitation, thousands are formally recognized in one way or another as threatened or endangered. The vast majority of these are on the road to extinction, because humanity is destroying habitats: paving them over, plowing them under, logging, overgrazing, flooding, draining, or transporting exotic organisms into them while subjecting them to an assault by a great variety of toxins and changing their climate. As anyone who has raised tropical fishes knows, all organisms require appropriate habitats if they are to survive. Just as people cannot exist in an atmosphere with too little oxygen, so neon tetras (Paracheirodon innesi) cannot survive in water that is 40F (4.4C) or breed in highly alkaline water. Trout, on the other hand, cannot breed in water that is too warm or too acid. And the bacteria that produce the tetanus toxin cannot reproduce in the presence of oxygen. In order to persist, Bay checkerspot butterflies (Euphydryas editha bayensis) must have areas of serpentine grassland (to support the growth of plants that serve as food for their caterpillars and supply nectar to the adults). Whip-poor-wills, red-eyed vireos, Blackburnian warblers, scarlet tanagers, and dozens of other North American birds must have mature tropical forest in which to overwinter (see Terborgh, 1980, for example). Black-footed ferrets (Mustela nigripes) require prairie that still supports the prairie dogs on which the ferrets dine. This utter dependence of organisms on appropriate environments (Ehrlich, 1986) is what makes ecologists so certain that today’s trends of habitat destruction and modification—especially in the high-diversity tropical forest (where at least one-half of all species are believed to dwell)—are an infallible recipe for biological impoverishment. Those politicians and social scientists who have questioned the extent of current extinctions are simply displaying their deep ignorance of ecology;
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BioDiversity habitat modification and destruction and the extinction of populations and species go hand in hand. The extent to which humanity has already wreaked havoc on Earth’s environments is shown indirectly by a recent study of human appropriation of the products of photosynthesis (Vitousek et al., 1986). The food resource of the animals in all major ecosystems is the energy that green plants bind into organic molecules in the process of photosynthesis, minus the energy those plants use for their own life processes—growth, maintenance, and reproduction. In the jargon of ecologists, that quantity is known as the net primary production (NPP). Globally, this amounts to a production of about 225 billion metric tons of organic matter annually, nearly 60% of it on land. Humanity is now using directly (e.g., by eating, feeding to livestock, using lumber and firewood) more than 3% of global NPP, and about 4% of that on land. This is a minimum estimate of human impact on terrestrial systems. Since Homo sapiens is one of (conservatively) 5 million species, this may seem an excessive share of the food resource. But considering that human beings are perhaps a million times the weight of the average animal (since the overwhelming majority of animals are small insects and mites) and need on the order of a million times the energy per individual, this share might not be too unreasonable. Yet human beings can be thought of as co-opting NPP not only by direct use but also by indirect use. Thus if we chalk up to the human account not only the NPP directly consumed, but such other categories as the amount of biomass consumed in fires used to clear land, the parts of crop plants not consumed, the NPP of pastureland (converted from natural habitat) not consumed by livestock, and so on, the human share of terrestrial NPP climbs to a staggering 30%. And if we add to that the NPP foregone when people convert more productive natural systems to less productive ones (such as forest to farm or pasture, grassland to desert, marsh to parking lot), the total potential NPP on land is reduced by 13%, and the human share of the unreduced potential NPP reaches almost 40%. There is no way that the co-option by one species of almost two-fifths of Earth’s annual terrestrial food production could be considered reasonable, in the sense of maintaining the stability of life on this planet. These estimates alone both explain the basic causes and consequences of habitat destruction and alteration, and give reason for great concern about future trends. Most demographers project that Homo sapiens will double its population within the next century or so. This implies a belief that our species can safely commandeer upwards of 80% of terrestrial NPP, a preposterous notion to ecologists who already see the deadly impacts of today’s level of human activities. Optimists who suppose that the human population can double its size again need to contemplate where the basic food resource will be obtained. A standard fool’s answer to that question is that indefinite expansion of the human population will be supported by the immeasurable riches of the sea. Unhappily for that notion, the riches of the sea have been quite carefully measured
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BioDiversity and found wanting. People now use about 2% of the NPP of the sea, and the prospects even for doubling that yield are dim. The basic reason is that efficient harvesting of the sea requires the exploitation of concentrated pools of resources—schools of fishes and larger invertebrates. People cannot efficiently harvest much of the NPP that resides in tiny phytoplankton (the green plants of the sea) or in the zooplankton (animals too small to swim against the currents). Humanity appears to be already utilizing about as much of oceanic NPP as it can on a sustainable basis. This discrepancy in the ability of Homo sapiens to exploit terrestrial and oceanic NPP is reflected in the general lack of an extinction crisis in the seas. Except for such organisms as some whales and fishes that are threatened by direct exploitation, animals that spend their entire lives in the open sea are relatively secure. Aside from some limited environments, such as certain coral reefs, the effects of habitat destruction are relatively small away from shorelines and estuaries. This situation could, of course, change rapidly if marine pollution increases—a distinct possibility. The extirpation of populations and species of organisms exerts its primary impact on society through the impairment of ecosystem services. All plants, animals, and microorganisms exchange gases with their environments and are thus directly or indirectly involved in maintaining the mix of gases in the atmosphere. Changes in that mix (such as increases in carbon dioxide, nitrogen oxides, and methane) can lead to rapid climate change and, in turn, agricultural disaster. As physicist John Holdren put it, a carbon dioxide-induced climatic change could lead to the deaths by famine of as many as a billion people before 2020. Destroying forests deprives humanity not only of timber but also of dependable freshwater supplies and furthermore increases the danger of floods. Destruction of insects can lead to the failure of crops that depend upon insect pollination. Extermination of the enemies of insect pests (a usual result of ad lib pesticide spraying) can terminate the pest control services of an ecosystem and often leads to severe pest outbreaks. The extinction of subterranean organisms can destroy the fertility of the soil. Natural ecosystems maintain a vast genetic library that has already provided people with countless benefits and has the potential for providing many, many more. These examples can be multiplied manyfold—the basic point is that organisms, most of which are obscure to nonbiologists, play roles in ecological systems that are essential to civilization. When a population playing a certain role is wiped out, ecosystem services suffer, even if many other populations of the same organism are still extant. If the population of Engelmann spruce trees (Picea engelmanni) in the watershed above your Colorado home is chopped down, you could be killed in a resulting flood, even though the species of spruce is not endangered. Equally, if that were the last population and it were reduced to just a dozen trees (so that, technically, the species still existed), you would not be spared the flood, and chance events would likely finish off the Engelmann spruce eventually anyway. In most cases, numerous genetically diverse populations are necessary to ensure the persistence of a species in the face of inevitable environmental changes that occur naturally. The existence of many populations spreads the risk so that unfavorable conditions in one or a few habitats do not threaten the entire species. And the presence of abundant genetic variation within a species (virtually assured
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BioDiversity 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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BioDiversity since Homo sapiens is now living largely on its inherited capital and in the future will have to depend increasingly on its income (NPP), one can argue persuasively that the size of the human population and the scale of human activities should be gradually reduced below present levels. Reducing that scale will be an especially difficult task, since it means that the environmental impacts of the rich must be enormously curtailed to permit the poor a chance for reasonable development. Although improvements in the technologies used to support human life and affluence can of course help to ameliorate the extinction crisis, and to a limited extent technologies can substitute for lost ecosystem services, it would be a dangerous miscalculation to look to technology for the answer (see, for example, Ehrlich and Mooney, 1983). In my opinion, only an intensive effort to make those improvements and substitutions, combined with a revolution in attitudes toward other people, population growth, the purpose of human life, and the intrinsic values of organic diversity, is likely to prevent the worst catastrophe ever to befall the human lineage. Curiously, scientific analysis points toward the need for a quasi-religious transformation of contemporary cultures. Whether such a transformation can be achieved in time is problematic, to say the least. We must begin this formidable effort by increasing public awareness of the urgent need for action. People everywhere should understand the importance of the loss of diversity not only in tropical forests, coastal zones, and other climatically defined regions of the world but also in demographically delineated regions such as areas of urbanization. The geological record can tell us much about catastrophic mass extinctions of the past. That, and more intensive studies of the living biota, can provide hints about what we might expect in the future. At the present time, data on the rates and direction of biodiversity loss remain sparse and often uncertain. As a result, estimates of the rate of loss, including the number and variety of species that are disappearing, vary greatly—in some cases, as pointed out by E. O.Wilson in Chapter 1, by as much as an order of magnitude. Moreover, scientists have also differed in their predictions of the eventual impact that will result from the diminishing biodiversity. Some aspects of these challenges are explored in the following five chapters comprising this section and are reflected throughout this volume. REFERENCES Ehrlich, A.H. 1984. Nuclear winter. A forecast of the climatic and biological effects of nuclear-war. Bull. At. Sci. 40(4):S1–S15. Ehrlich, P.R. 1986. The Machinery of Nature. Simon and Schuster, New York. 320 pp. Ehrlich, P.R., and A.H.Ehrlich. 1981. Extinction: The Causes and Consequences of the Disappearance of Species. Random House, New York. 305 pp. Ehrlich, P.R., and H.A.Mooney. 1983. Extinction, substitution, and ecosystem services. BioScience 33(4):248–254. Holdren, J.P., and P.R.Ehrlich. 1974. Human population and the global environment. Am. Sci. 62:282–292. Peters, R.L., and J.D.S.Darling. 1985. The greenhouse effect and nature reserves. BioScience 35(11):707–717.
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BioDiversity Terborgh, J.W. 1980. The conservation status of neotropical migrants: Present and future. Pp. 21–30 in A.Keast and E.S.Morton, eds. Migrant Birds in the Neotropics: Ecology, Behavior, Distribution, and Conservation. A symposium held at the Conservation and Research Center, National Zoological Park, Smithsonian Institution. Smithsonian Institution Press, Washington, D.C. Vitousek, P.M., P.R.Ehrlich, A.H.Ehrlich, and P.M.Matson. 1986. Human appropriation of the products of photosynthesis. BioScience 36(6):368–373.
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BioDiversity CHAPTER 3 TROPICAL FORESTS AND THEIR SPECIES Going, Going…? NORMAN MYERS Consultant in Environment and Development, Oxford, United Kingdom There is strong evidence that we are into the opening stages of an extinction spasm. That is, we are witnessing a mass extinction episode, in the sense of a sudden and pronounced decline worldwide in the abundance and diversity of ecologically disparate groups of organisms. Of course extinction has been a fact of life since the emergence of species almost 4 billion years ago. Of all species that have ever existed, possibly half a billion or more, there now remain only a few million. But the natural background rate of extinction during the past 600 million years, the period of major life, has been on the order of only one species every year or so (Raup and Sepkoski, 1984). Today the rate is surely hundreds of times higher, possibly thousands of times higher (Ehrlich and Ehrlich, 1981; Myers, 1986; Raven, 1987; Soulé, 1986; Western and Pearl, in press; Wilson, 1987). Moreover, whereas past extinctions have occurred by virtue of natural processes, today the virtually exclusive cause is Homo sapiens, who eliminates entire habitats and complete communities of species in super-short order. It is all happening in the twinkling of an evolutionary eye. To help us get a handle on the situation, let us take a lengthy look at tropical forests. These forests cover only 7% of Earth’s land surface, yet they are estimated to contain at least 50% of all species (conceivably a much higher proportion [see Erwin, Chapter 13 of this volume]). Equally important, they are being depleted faster than any other ecological zone. TROPICAL FORESTS There is general agreement that remaining primary forests cover rather less than 9 million square kilometers, out of the 15 million or so that may once have existed
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BioDiversity according to bioclimatic data. There is also general agreement that between 76,000 and 92,000 square kilometers are eliminated outright each year, and that at least a further 100,000 square kilometers are grossly disrupted each year (FAO and UNEP, 1982; Hadley and Lanley, 1983; Melillo et al., 1985; Molofsky et al., 1986; Myers, 1980, 1984). These figures for deforestation rates derive from a data base of the late 1970s; the rates have increased somewhat since then. This means, roughly speaking, that 1% of the biome is being deforested each year and that more than another 1% is being significantly degraded. The main source of information lies with remote-sensing surveys, which constitute a thoroughly objective and systematic mode of inquiry. By 1980 there were remote-sensing data for approximately 65% of the biome, a figure that has risen today to 82%. In all countries where remote-sensing information has been available in only the past few years—notably Indonesia, Burma, India, Nigeria, Cameroon, Guatemala, Honduras, and Peru—we find there is greater deforestation than had been supposed by government agencies in question. Tropical deforestation is by no means an even process. Some areas are being affected harder than others; some will survive longer than others. By the end of the century or shortly thereafter, there could be little left of the biome in primary status with a full complement of species, except for two large remnant blocs, one in the Zaire basin and the other in the western half of Brazilian Amazonia, plus two much smaller blocs, in Papua New Guinea and in the Guyana Shield of northern South America. These relict sectors of the biome may well endure for several decades further, but they are little likely to last beyond the middle of next century, if only because of sheer expansion in the numbers of small-scale cultivators. Rapid population growth among communities of small-scale cultivators occurs mainly through immigration rather than natural increase, i.e., through the phenomenon of the shifted cultivator. As a measure of what ultrarapid growth rates can already impose on tropical forests, consider the situation in Rondonia, a state in the southern sector of Brazilian Amazonia. Between 1975 and 1986, the population grew from 111,000 to well over 1 million, i.e., a 10-times increase in little more than 10 years. In 1975, almost 1,250 square kilometers of forest were cleared. By 1982, this amount had grown to more than 10,000 square kilometers, and by late 1985, to around 17,000 square kilometers (Fearnside, 1986). It is this broad-scale clearing and degradation of forest habitats that is far and away the main cause of species extinctions. Regrettably, we have no way to know the actual current rate of extinction, nor can we even come close with accurate estimates. But we can make substantive assessments by looking at species numbers before deforestation and then applying the analytic techniques of island biogeography. To help us gain an insight into the scope and scale of present extinctions, let us briefly consider three particular areas: the forested tracts of western Ecuador, Atlantic-coast Brazil, and Madagascar. Each of these areas features, or rather featured, exceptional concentrations of species with high levels of endemism. Western Ecuador is reputed to have once contained between 8,000 and 10,000 plant species with an endemism rate somewhere between 40 and 60% (Gentry, 1986). If we suppose, as we reasonably can by drawing on detailed inventories in sample plots, that there are at least 10 to 30 animal species for every one plant
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BioDiversity Forest ecosystems of Caribbean islands have proven to be more resilient than one would assume on the basis of the relationships used in Table 6–2 or the idea of the fragility of island biota (Carlquist, 1974; Soulé, 1983). The Caribbean islands are densely populated (100 to 500 people per square kilometer, or about 10 times more densely populated than surrounding continental tropical lands (Lugo et al., 1981), and their lands have been intensively used and degraded for centuries. All the ills that Carlquist (1974) and Soulé (1983) described for islands (e.g., the introduction of exotic species, intensive predation, and habitat destruction) are present in this region. There are many examples of catastrophic waste of natural resources in the Caribbean islands [see, for example, Ambio 10(6) 1981, which was dedicated to environmental problems of the Caribbean], but there are also examples to give us some hope; these are the ones I am emphasizing. In Puerto Rico, human activity reduced the area of primary forest by 99%, but because of extensive use of coffee shade trees in the coffee region and secondary forests, forest cover was never less than 10 to 15%. This massive forest conversion did not lead to a correspondingly massive species extinction, certainly nowhere near the 50% alluded to by Myers (1983). In an analysis of the bird fauna, Brash (1984) concluded that seven bird species (four of them endemic) became extinct after 500 years of human pressure (this is equivalent to an 11.6% loss of the bird fauna) and that exotic species enlarged the species pool. In the 1980s more birds are present on the island (97 species) than were present in pre-Columbian times (60 species). The resiliency of the bird fauna was attributed to its generalist survival strategy (a characteristic of island fauna) and to the location of secondary forests and coffee plantations on mountaintops along the east-west axis of the island, which acted as refugia. Secondary forests in Puerto Rico have served as refugia for primary forest tree species as well (Wadsworth and Birdsey, 1982; R.O.Woodbury, University of Puerto Rico, personal communication, 1986). After 20 to 30 years of growth, the understory of these ecosystems is supporting species characteristic of mature forests. A random survey of 4,500 trees in secondary forests of two life zones (moist and wet forests) resulted in a tally of 189 tree species (Birdsey and Weaver, 1982). This survey excluded four of the six forested life zones in the island and the species-rich mature publicly owned forests. Yet it is important that 25% of the tree species identified on the island were recorded in this survey of secondary forests. (Puerto Rico has 750 tree species, 203 of which are naturalized; Little et al., 1974.) Dominant species in these secondary forests owe their dominance to human activity, and many of the native species that are typical of mature forests are rare in the forest canopy (142 tree species accounted for 16% of the total basal area of secondary forests) but are now beginning to appear as pole-size individual trees in these forest sites. Secondary forests in high-impact regions obviously require time to fulfill their role as foster ecosystem for endangered species, but in due time, a wide variety of tree species appear to return to forest lands. An extreme example of the importance of species conservation and of human-dominated habitats acting as foster ecosystems for endangered species is that of the Chinese maiden hair tree (Ginkgo biloba). No one has ever seen a wild individual
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BioDiversity of this species. This primitive tree was preserved in courtyards of temples in China and is considered to be the first species saved by humans (Stebbins, 1979). In the United States where extensive human-caused deforestation and subsequent forest recovery have occurred, remnant secondary forest islands account for a large portion of landscape species diversity (Burgess and Sharpe, 1981). As a group, these secondary forest islands constitute a landscape with greater species richness than found in a landscape dominated only by climax forests. Clearly, secondary forests require more scientific attention before their role and value in landscapes affected by human activity can be properly assessed. Catastrophic natural events may also be deleterious to the maintenance of species diversity, particularly to those species already at the edge of extinction. However, these catastrophic events are natural phenomena with predictable rates of recurrence to which the biota as a whole is adapted. Evidence is mounting to show that tropical forest ecosystems have endured catastrophic events for millennia, e.g., periodic fires in the moist forests of the Amazon (Sanford et al., 1985) and in Borneo (Leighton, 1984). In the Caribbean, hurricanes appear to be important in the maintenance of species diversity. Long-term studies in areas of the Luquillo Experimental Forest Biosphere Reserve have shown that there are progressive reductions in tree species between hurricane events (Crow, 1980; Weaver, 1986). The effects of periodic hurricanes maintain a diverse mix of successional and climax species on a given site. Without hurricanes, successional species would be more restricted. Sanford et al. (1985) suggest that fire performs the same function in Amazonian moist forests; Sepkoski and Raup (1986) expanded this idea to the effects of global perturbations on the history of life on the planet. Studies of regeneration strategies for mature forests have indicated that disturbance is usually associated with the early phases of seedling germination and establishment in most forest types, including tropical forests (Pickett and White, 1985). This has led Pickett and White to propose the concept of “patch dynamics” as a focus of scientific inquiry aimed at understanding ecosystem dynamics. The relevance of this to the maintenance of species diversity is that environmental change and disturbance may be required to maintain a species-rich tropical landscape. Because humans have facilitated immigration and created new environments, exotic (nonnative) species have successfully become established in the Caribbean islands. This has resulted in a general increase in total species inventories of birds and trees. Some of these exotic species are pests and thus are called biological pollutants (CEQ, 1980). However, many exotic species have become so well integrated into the natural landscape that most islanders consider them native. Although conservationists and biologists have an aversion to exotic species such as predatory mammals and pests (with good reason!), this may not be totally justified if the full inventory of exotic fauna and flora and certain ecological arguments are taken into consideration. For example, the growth of exotic plant species is usually an indication of disturbed environments, and under these conditions, exotic species compete successfully (Vermeij, 1986). They accumulate and process carbon and nutrients more efficiently than do the native organisms they replace. In so doing, many exotic species improve soil and site quality and either pave the way for the
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BioDiversity succession of native species or form stable communities themselves. There is no biological criterion on which to judge a priori the smaller or greater value of one species against that of another, and if exotic species are occupying environments that are unavailable to native species, it would probably be too costly or impossible to pursue their local extinction. The paradox of exotic species invasions of islands with high levels of endemism is discussed by Vitousek in Chapter 20. He correctly points out that if the invasion of exotic species is at the expense of the extinction of local endemics, the total species richness of the biosphere decreases and the Earth’s biota is homogenized since most of the invading exotics are cosmopolitan. NEED FOR BETTER LAND AND RESOURCE MANAGEMENT In summary, strong evidence can be assembled to document the resiliency of the functional attributes of some types of tropical ecosystems (including their ability to maintain species richness) when they are subjected to intensive human use. Initial human intervention results in the loss of a few, highly vulnerable species. Massive forest destruction is probably required to remove more widely distributed species. Because massive species extinctions may be possible if human destruction of forests continues unabated, the evidence for ecosystem resiliency is not to be construed as an excuse for continued abuse of tropical environments. Rather, ecosystem resiliency is an additional tool available to managers if they choose to manage tropical resources prudently. We cannot tell the needy of the tropical world that they must cease and desist in their struggle for survival to prevent a catastrophe whose dimensions, consequences, or mitigating conditions we cannot define with any certainty. It may turn out that the public call for conserving natural diversity is also an expression of frustration over the poor use of the natural resources of the tropics and our apparent inability to do something about it. Scientists have the responsibility of focusing the debate. Its fundamental essence, I believe, is the need for better land and resource management. Experience in the Luquillo Experimental Forest Biosphere Reserve in Puerto Rico has demonstrated that species richness can be partially restored to lands previously used heavily for agriculture, that growing timber need not eliminate all natural species richness on site, and that tropical lands respond to sensible care through management. I know of no technical reason why sensible land management in tropical areas cannot lead to the success that is usually associated with temperate zones. The obstacles to progress are social and rooted in poor training and education programs, lack of facilities and infrastructure, weak institutions, misguided foreign aid programs, lack of commitment to forestry research and to enforcement of regulations, and the absence of a land conservation ethic. A strategy for forest and species conservation in tropical regions should focus on the restoration of forest production on former forest lands where food production is not sustainable. This, and sensible use of secondary forests and tree plantations, will reduce pressure on
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BioDiversity forest lands with mature forests or with unique ecological characteristics and set us on a course to meet the needs of the needy while protecting species diversity. ACKNOWLEDGMENTS In this article, I benefited from the comments of S.Brown, P.Kangas, E.Medina, O.Solbrig, R.Waide, C.Asbury, J.Lodge, W.Lawrence, and colleagues at the Institute. I thank all of them. This work was done in cooperation with the University of Puerto Rico. REFERENCES Ambio 10(6) 1981. An entire issue devoted to environmental problems of the Caribbean. Birdsey, R.A., and P.L.Weaver. 1982. The Forest Resources of Puerto Rico. USDA Forest Service Southern Forest Experiment Station. Resource Bulletin SO-85. New Orleans, U.S. Department of Agriculture. 59 pp. Brash, A.R. 1984. Avifaunal Reflections of Historical Landscape Ecology in Puerto Rico. Tropical Resources Institute. Yale University, New Haven, Conn. 24 pp. Brown, S., and A.E.Lugo. 1982. The storage and production of organic matter in tropical forests and their role in the global carbon cycle. Biotropica 14:161–187. Burgess, R.L., and D.M.Sharpe, eds. 1981. Forest Island Dynamics in Man-Dominated Landscapes. Ecological Studies 41. Springer-Verlag, New York. 310 pp. Carlquist, S.J. 1974. Island Biology. Columbia University Press, New York. 660 pp. CEQ (Council on Environmental Quality). 1980. Environmental Quality-1980. The Eleventh Annual Report of the CEQ. U.S. Government Printing Office, Washington, D.C. 497 pp. Crow, T. 1980. A rain forest chronicle: A 30-yr record of change in structure and composition at El Verde, Puerto Rico. Biotropica 12:42–55. Ehrlich, P., and A.Ehrlich. 1981. Extinction. The Causes of the Disappearance of Species. Random House, New York. 305 pp. Ewel, J.J. 1977. Differences between wet and dry successional tropical ecosystems. Geo-Eco-Trop 1:103–177. Ewel, J.J. 1983. Succession. Pp. 217–223 in F.B.Golley, ed. Tropical Rain Forest Ecosystems, Structure and Function. Elsevier, Amsterdam. Food and Agriculture Organization. 1981. Los Recursos Forestales de la America Tropical. Informe Tecnico 1; Forest resources of tropical Asia, Technical Report 2; Forest Resources of Tropical Africa, parts 1 and 2, Technical Report 3. UN32/6.1301–78–04, Food and Agriculture Organization, Rome. 4 volumes. Gentry, A.H. 1979. Extinction and conservation of plant species in tropical America: A phytogeographical perspective. Pp. 110–126 in I.Hedberg, ed. Systematic Botany, Plant Utilization, and Biosphere Conservation. Proceedings of a symposium held in Uppsala in commemoration of the 500th anniversary of the university. Almquist and Wiksell International, Stockholm. Gentry, A.H. 1982. Patterns of neotropical plant-species diversity. Evol. Biol. 15:1–85. Holdridge, L.R. 1967. Life Zone Ecology. Tropical Science Center, San Jose, Costa Rica. 206 pp. Holdridge, L.R., W.C.Grenke, W.H.Hatheway, T.Liang, and J.A.Tosi. 1971. Forest Environments in Tropical Life Zones, a Pilot Study. Pergamon, New York. 747 pp. Lanly, J.P. 1982. Tropical Forest Resources. FAO Forestry Paper 30. Food and Agriculture Organization, Rome. 106 pp. Leighton, M. 1984. Effects of drought and fire on primary rain forest in eastern Borneo. P. 48 in B.C.Klein-Helmuth and J.L.Hufnagel, compilers. Abstracts of Papers. AAAS Meeting, New York. 24–29 May, 1984. American Association for the Advancement of Science, Washington, D.C. Little, E.L., R.O.Woodbury, and F.H.Wadsworth. 1974. Trees of Puerto Rico and the Virgin Islands. USDA Forest Service, Agricultural Handbook 449, Vol. 2. Washington, D.C. 1,024 pp. Lovejoy, T.E. 1980. A projection of species extinctions. Pp. 328–331, Vol. 2 in G.O.Barney (study director). The Global 2000 Report to the President. Entering the Twenty-First Century. Council on Environmental Quality, U.S. Government Printing Office, Washington, D.C.
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BioDiversity Lovejoy, T.E. 1981. Prepared statement. Pp. 175–180 in Tropical Deforestation, An Overview, the Role of International Organizations, the Role of Multinational Corporations. Hearings before the Subcommittee on International Organizations of the Committee on Foreign Affairs. House of Representatives, 96th Congress, second session, May 7, June 19, and September 19, 1980. U.S. Government Printing Office, Washington, D.C. Lugo, A.E., and S.Brown. 1981. Tropical lands: Popular misconceptions. Mazingira 5(2):10–19. Lugo, A.E., R.Schmidt, and S.Brown. 1981. Tropical forests in the Caribbean. Ambio 10:318–324. Myers, N. 1979. The Sinking Ark. A New Look at the Problem of Disappearing Species. Pergamon, New York. 307 pp. Myers, N. 1980. Conversion of Tropical Moist Forests. National Academy of Sciences, Washington, D.C. 205 pp. Myers, N. 1982. Forest refuges and conservation in Africa with some appraisal of survival prospects for tropical moist forests throughout the biome. Pp. 658–672 in G.T.Prance, ed. Biological Diversification in the Tropics. Columbia University Press, New York. Myers, N. 1983. Conservation of rain forests for scientific research, for wildlife conservation, and for recreation and tourism. Pp. 325–334 in F.B.Golley, ed. Tropical Rain Forest Ecosystems, Structure and Function. Elsevier, Amsterdam. NRC (National Research Council). 1980. Research Priorities in Tropical Biology. National Academy of Sciences, Washington, D.C. 116 pp. Norton, B.J., ed. 1986. The Preservation of Species. Princeton University Press, Princeton, N.J. 305 pp. Oldfield, M.I. 1984. The Value of Conserving Genetic Resources. U.S. Department of the Interior, National Park Service, Washington, D.C. 360 pp. Pickett, S.T.A., and P.S.White, eds. 1985. The Ecology of Natural Disturbance and Patch Dynamics. Academic Press, Orlando, Fla. 472 pp. Raven, P.H. 1977. Perspectives in tropical botany: Concluding remarks. Ann. Mo. Bot. Gard. 64(4):746–748. Sanford, R.L., Jr., J.Saldarriaga, K.E.Clark, C.Uhl, and R.Herrera. 1985. Amazon rain-forest fires. Science 227:53–55. Sepkoski, J.J., Jr., and D.M.Raup. 1986. Periodicity in marine extinction events. Pp. 3–36 in D. K.Elliott, ed. Dynamics of Extinction. John Wiley and Sons, New York. Simberloff, D. 1983. Are We on the Verge of Mass Extinction in Tropical Rain Forests? Unpublished monograph, July 1983. Simberloff, D. 1986. Are we on the verge of a mass extinction in tropical rain forests? Pp. 165–180 in D.K.Elliott, ed. Dynamics of Extinction. John Wiley and Sons, New York. Soulé, M.E. 1983. What do we really know about extinctions? Pp. 111–124 in C.M.Schonewald-Cox, S.M.Chambers, B.MacBryde, and W.L.Thomas, eds. Genetics and Conservation. Benjamin/ Cummings, London. Stebbins, G.L. 1979. Strategies for preservation of rare plants and animals. Great Basin Naturalist Memoirs 3:87–93. Tosi, J. 1980. Life zones, land use, and forest vegetation in the tropical and subtropical regions. Pp. 44–64 in S.Brown, A.E.Lugo, and B.Liegel, eds. The Role of Tropical Forests on the World Carbon Cycle. A Symposium held at the Institute of Tropical Forestry in Rio Piedras, Puerto Rico, on March 19, 1980. CONF-800350, U.S. Department of Energy Carbon Dioxide Program. National Technical Information Service, Springfield, Va. Tosi, J., and R.F.Voertman. 1964. Some environmental factors in the economic development of the tropics. Econ. Geogr. 40:189–205. Vermeij, G.J. 1986. The biology of human-caused extinction. Pp. 28–49 in B.G.Norton, ed. The Preservation of Species. Princeton University Press, Princeton, N.J. Wadsworth, F.H., and R.A.Birdsey. 1982. Un nuevo enfogue de los bosques de Puerto Rico. Pp. 12–27 in Noveno Simposio de Recursos Naturales. Puerto Rico Department of Natural Resources, San Juan, Puerto Rico. Weaver, P.L. 1986. Hurricane damage and recovery in the montane forests of the Luquillo Mountains of Puerto Rico. Caribb. J. Sci. 22:53–70. WRI and IIED (World Resources Institute and International Institute for Environment and Development). 1986. World Resources 1986. Basic Books, New York. 353 pp.
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BioDiversity CHAPTER 7 CHALLENGES TO BIOLOGICAL DIVERSITY IN URBAN AREAS DENNIS D.MURPHY Research Programs Director, Center for Conservation Biology, Stanford University, Stanford, California Jaws, claws, an explosion of spray, and a grizzly emerges from the shallows, a salmon in its grasp. Mixed herds of elk, deer, and pronghorn antelope graze rolling, grassy slopes. A cougar surveys from broken chaparral and woodland above. A scene from the shores of Yellowstone Lake? Perhaps. But it is also a scene from the shores of San Francisco Bay just 150 years ago. Now only deer and cougar remain, but well away from those shores in mountainous habitats above the sprawling metropolitan Bay Area. It seems that only the relatively recent European settlement of the West has spared those species at all. In wooded patches surrounding Milwaukee, the woodland bison, moose, wolverine, black bear, elk, and lynx have been long extinct. Now just a very few forest specialists, such as the raccoon, chipmunk, and white-footed mouse, survive in the region, and those species are gone from all but the very largest woodland patches (Matthiae and Stearns, 1981). In patches of eastern deciduous forest near Washington, D.C., migrant bird species restricted as breeders to forest interiors also survive in only the largest natural habitat remnants. A number of warbler species there show signs of imminent regional extinction (Whitcomb et al., 1981). These are merely obvious examples of an accelerating decline in the global diversity of living things. The term biological diversity has been used to describe “the variety of life forms, the ecological roles they perform, and the genetic diversity they contain” (Wilcox, 1984, p. 640). While scientists argue about the relative enormity of tropical deforestation and its impact on biological diversity, the loss of populations, species, and entire ecological communities in human population
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BioDiversity centers and their surrounding landscapes is well documented and inarguably immense. In urban areas of the eastern United States, only species with the most general habitat and resource requirements have remained in urban corridors. Moreover, the prospect of further erosion of biological diversity looms. In Great Britain, where the sustained assault on the environment is measured in millennia rather than in centuries, and where most vertebrate species are distant memories, a cascade of invertebrate extinctions is now being observed. For example, 80% of the resident butterfly species have declined in number in at least a major part of their British ranges during the past decade (Thomas, 1984). A number of those survive only on reserves and under rigorous management regimes. An estimated 18% of all European butterfly species are considered to be vulnerable to or imminently faced with extinction (Heath, 1981). Unfortunately, losses of animal and plant species are restricted neither to temperate zone urban areas nor to the developed world. Urban impacts on biological diversity reach their most devastating in the Third World. Less than 2% of the Atlantic forests of coastal Brazil within the urban reach of Sao Paulo remain, and it has been estimated that thousands of species from this region of high endemism have been driven to extinction, most never having been described by taxonomists. Although the full extent of this urban environmental degradation is virtually impossible to convey, its underlying causes are comparatively simple to identify. With few exceptions, losses of naturally occurring biological diversity are incidental to human activities. Thus, urban areas are effectively synonymous with ecosystem disruption and the erosion of biological diversity. Natural habitats are replaced directly by houses, condominiums, hotels, and malls, as well as by streets, highways, and utilities that support them. Historically, urban areas were the first regions subjected to local overkill of wildlife for food, fur, and feathers, and through misdirected predator control programs. They were also the first to experience logging and weed eradication programs. The biological diversity of urban areas has also been among the most severely affected by the introduction of animal species, which prey on native animal populations, compete for limited resources, and act as vectors for novel diseases and parasites to which native organisms can be particularly susceptible. Great effects on biological diversity in urban areas also can result from less direct sources, including many of the air- and water-borne pollutants that imperil human health. Toxic by-products of industrial production, such as polychlorinated biphenyls (PCBs), sulfur dioxide, and oxidants as well as pesticides directed at noxious species, have been found to disrupt natural ecosystems (Ehrlich and Ehrlich, 1981). Airborne pollutants are especially insidious, since they expand the reach of urban blight far beyond city limits. More subtle impacts on biological diversity result from overdrafting local aquifers, dropping water tables, and ground subsidence. These processes are often compounded by changes in natural patterns of groundwater percolation caused by the destruction of wetlands and diversion of runoff. This wide array of obvious and subtle factors contribute to the disruption of ecosystem function, the decoupling of interactions among species, and the disappearance of populations of organisms from urban locales. Why should that concern us? Because losses of just a few populations can result in a great destabilization
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BioDiversity of natural ecological communities and, as a consequence, in a decrement in the ability of those communities to provide a wide array of services. Thus, many reasons for protecting diversity in urban areas are often highly utilitarian. Benefits include amelioration of climate, because foliage in cities contribute to the reduction of ambient temperatures. Large trees and shrubs reduce wind velocity and reduce evaporation of soil moisture. Plants are also useful in architecture, erosion control, watershed protection, wastewater management, noise abatement, and air pollution control (Grey and Deneke, 1986). Nevertheless, the aesthetic reasons for preserving biological diversity are often those that most obviously affect the populace of urban areas. The great parks and natural areas of the world’s major cities, such as Central Park and the Gateway National Recreation Area in New York City and Golden Gate Park and the Golden Gate National Recreation Area in San Francisco, are regarded as prized jewels, providing opportunities for recreation and relaxation as well as habitat for a wide variety of species. The arguments for protecting biological diversity in urban areas seem straightforward, but the implementation of conservation programs in urban areas is among the most difficult problems faced by environmentalists. Some areas are so disturbed that functioning, naturally occurring ecosystems are no longer identifiable, whereas other urban habitats remain effectively undisturbed. Open spaces in inner cities often support only species that are particularly well adapted to human impact. Such areas are nearly always small and extremely isolated, and their maintenance and enhancement demand extensive and continuous hands-on management. The conservation goals in such areas must usually aim at maximizing biological diversity to the extent possible, rather than preserving all remaining resident species. Inner city park developers have traditionally introduced plantings of exotic species. Such settings fulfill many of the aesthetic and utilitarian roles that natural habitats offer, but their establishment and maintenance costs tend to be high, since few of the self-regenerating functions of natural ecosystems are available. Yet, although human-induced intervention such as the replacement of ecosystem components can increase the number of species locally over at least the short run, these processes nearly always upset the ecological balance of communities; hence it ultimately exerts a negative impact on naturally occurring biological diversity. Where larger, intact ecosystems exist within cities, they are often restricted to corridors alongside steep stream canyons, such as Rock Creek Park in Washington, D.C., and Fairmont Park in Philadelphia. But the most extensive expanses of natural habitat in urban areas are those surrounding city limits. In those relatively undisturbed areas, prescriptions for the preservation of biological diversity are quite different from those for maximizing diversity in more disturbed areas. Corridors and surrounding habitats are among the most valuable urban natural areas, providing for extensive biological diversity and reducing the isolation of the largest surviving ecosystems, which may be far from urban centers. The single greatest threat to the biological diversity of relatively intact natural communities in and around urban areas is the destruction of natural habitats and their conversion to other uses. The paving over of natural habitats as urban activities sprawl outward destroys and fragments remnant functioning ecosystems. The re-
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BioDiversity distribution of water through channelization and impoundment of flowing waters, and the draining of some wetlands and the flooding of others, destroys undeveloped habitat areas. Activities as seemingly benign as the planting of exotic trees and shrubs in parks and along byways or the conversion of open space to golf courses disrupt the distribution of natural components of biological diversity. These activities combine to decrease, habitat area and disturb the equilibrium between extinction and immigration among remaining natural habitats, with the frequent result that some species are permanently lost. Decreases in local biological diversity resulting from losses of habitat area and insularization of habitat remnants are compounded by the more subtle effects of fragmentation. Losses of single, specific microhabitats within an otherwise undisturbed habitat can cause the local extinction of certain species. Disruption of even narrow corridors of natural habitat between large habitat patches can lead to losses of species. The removal of understory foliage in manicured park areas and suburban housing developments can result in the loss of numerous species, most conspicuously species of birds. Vast differences in temperature, humidity, light availability, and wind exposure exist between forest edges and interiors and affect habitat suitability for some species. In addition, losses of certain species due to any one or more causes can affect closely associated species sometimes leading ultimately to secondary extinction events (Wilcox and Murphy, 1985). In light of these basic ecological facts, conservation of the full range of urban biological diversity necessitates the protection of the largest possible expanses of natural habitat. Yet, that simple prescription is usually impossible to fill in urban areas, where the forces acting to decrease the size of remaining natural habitats are greatest. These conflicting pressures interact to determine urban conservation policy and to force biologists to justify the sizes of biological preserves. Economic and political considerations in urban areas make preservation particularly difficult. Land costs are high because of high demand, and the vast majority of urban space is private property. The few publicly owned open spaces are subject to intensive, varied uses, many of which are incompatible with preserving biological diversity. Local political institutions usually favor development over preservation, and many agencies concerned with land and resource management, such as the U.S. Forest Service and Bureau of Land Management, have no presence in urban areas. Many conservation organizations with largely urban memberships virtually limit their concern to nonurban environments, and those involved with local issues rarely have the resources available for protracted fights over development. The Endangered Species Act with its mandate outlawing the “take” of any endangered species is the best tool for protecting biological diversity in urban areas of this country. Although the goal of the Act is protection of individual species of concern, its “purposes…are to provide a means whereby the ecosystems upon which endangered species depend may be conserved” (USC, 1983, p. 1, §1531). Its strength resides in its ability to protect species regardless of land ownership. Efforts to conserve the full extent of biological diversity by using the Endangered Species Act must target species that are most susceptible to habitat loss. The protection of extinction-prone species can be the key to facilitating the conservation
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BioDiversity of biological diversity in urban areas. Species especially prone to extinction include those high on trophic pyramids, widespread species with low vagility (i.e., with poor dispersal ability), endemic and migratory species, and species with colonial nesting habits (Terbough, 1974). Many such species inhabit urban areas during all or major portions of their lives and can act as umbrellas of sorts, often conferring protection to great numbers of species in the same habitats. The greatest erosion of extinction-prone species has usually occurred in habitat remnants that survive in those urban areas with the longest histories of settlement. Hence prescriptions for conserving remaining biological diversity differ substantially among urban areas. For example, forest patches support many more bird species than do grassland patches of similar size. All else being equal, therefore, protection of the total remaining biological diversity of oak woodlands surrounding San Francisco will demand more and larger preserves than protection of similar habitats to achieve a similar goal near less biologically diverse Washington D.C. In addition, the sizes of preserves necessary to protect biological diversity within an urban area will vary because the diversity itself varies greatly among different natural communities. Oak woodland preserves near San Francisco are likely to require more area to protect their complement of biological diversity than will native grassland preserves in the same geographic area. In the urban United States, three groups must interact to assist the Endangered Species Act in protecting biological diversity. Field biologists must aid in the identification and survey of potential umbrella species. Conservation organizations must use that information and citizen petitions to get appropriate umbrella species protected via the endangered list. In response, the Office of Endangered Species will have to reassess listing priorities. The San Francisco Bay Area exemplifies the challenge of preserving urban biological diversity. Without the grizzly bear, tule elk, and even the Xerces blue butterfly, San Francisco might be viewed as biologically impoverished in a sense, but the urban Bay Area remains an exceptionally rich natural region in the biologically richest state in the union. The ecological communities within a 25-kilometer radius of Berkeley include redwood, Douglas fir, and digger pine forests as well as coastal sage and inland chaparral, annual grasslands, dunes, riparian corridors, freshwater lakes, bay marshlands, and even pelagic marine communities and offshore seabird rookeries, an extraordinary array of ecological communities supporting immense biological diversity. The conservation challenge is great, especially in the shadow of a population growing at more than 3% per year; moreover, that shadow is not cast evenly. Less than 15% of San Francisco Bay marshlands remain, but much inland chaparral remains untouched. Can this urban biological diversity be protected? In this country, the answer is a qualified yes. In many other countries the outlook is not that sanguine. In Austria, prohibitions against the collection of wildlife and plants are strictly enforced, while the conversion of natural habitats to cultivation is effectively subsidized by the government. In the Federal Republic of Germany, as the Black Forest dies from acidification, powerful lobbies thwart the implementation of speed limits on the autobahns; consequently high levels of pollution continue to prevail. Overpopu-
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BioDiversity lation, chronic poverty, and fuel shortages in the Third World create unrelenting pressures to exploit all available local resources. These pressures certainly will become more overwhelming in the future. Our urban centers can be viewed as bellwethers of our global environmental fate. Our success at meeting the challenges of protecting biological diversity in urban areas is a good measure of our commitment to protect functioning ecosystems worldwide. If we cannot act as responsible stewards in our own backyards, the long-term prospects for biological diversity in the rest of this planet are grim indeed. REFERENCES Ehrlich, P.R., and A.H.Ehrlich. 1981. Extinction. The Causes and Consequences of the Disappearance of Species. Random House, New York. 305 pp. Grey, G.W., and F.J.Deneke. 1986. Urban Forestry. 2nd edition. John Wiley & Sons, New York. 299 pp. Heath, J. 1981. Threatened Rhopalocera (Butterflies) in Europe. Council of Europe, Strasbourg, France. 157 pp. Matthiae, P.E., and F.Stearns. 1981. Mammals in forest islands in southeastern Wisconsin. Pp. 55–66 in R.L.Burgess and D.M.Sharpe, eds. Forest Island Dynamics in Man-dominated Landscapes. Springer-Verlag, New York. Terborgh, J. 1974. Preservation of natural diversity: The problem of extinction prone species. BioScience 24:715–722. Thomas, J.A. 1984. The conservation of butterflies in temperate countries: Past efforts and lessons for the future. Pp. 333–353 in R.I.Vane-Wright and P.R.Ackery, eds. The Biology of Butterflies. Academic Press, London. USC (United States Code). 1984. Title 16, Conservation; Section 1531 et seq. Endangered Species Act of 1973. United States Code, 1984 Lawyers Edition. Lawyers Co-operative, Rochester, N.Y. Whitcomb, R.F., C.S.Robbins, J.F.Lynch, B.L.Whitcomb, M.K.Klimkiewicz, and D.Bystrak. 1981. Effects of forest fragmentation on avifauna of the eastern deciduous forest. Pp. 125–205 in R. L.Burgess and D.M.Sharpe, eds. Forest Island Dynamics in Man-dominated Landscapes. Springer-Verlag, New York. Wilcox, B.A. 1984. In situ conservation of genetic resources: Determinants of minimum area requirements. Pp. 639–647 in J.A.McNeeley and K.R.Miller, eds. National Parks, Conservation, and Development: The Role of Protected Areas in Sustaining Society. Proceedings of the World Congress on National Parks, Bali, Indonesia, 11–22 October 1982. Smithsonian Institution Press, Washington, D.C. Wilcox, B.A., and D.D.Murphy. 1985. Conservation strategy: The effects of fragmentation on extinction. Am. Nat. 125(6):879–887.
Representative terms from entire chapter: