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Nature and Human Society: The Quest for a Sustainable World (1997)

Chapter: 5 Threats to Sustainability

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Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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PART 5—
THREATS TO SUSTAINABILITY

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Nature Displaced:
Human Population Trends and Projections and their Meanings.

Richard P. Cincotta
Robert Engelman
Population Action International, 1120 19th Street, NW Ste. 550, Washington, DC 20036-3678

Unlike the great species extinctions of Earth's past, the one occurring today is less an episode than a process, whose full results will not be known for hundreds of years. Between the linked human-induced phenomena of global climate change and biodiversity loss, the planet could be passing into the equivalent of an entirely new geological epoch in just a few human generations. Or it could be that biodiversity loss will amount to little more than a manageable depletion, incurring regrettable scientific and economic losses but leaving the basic services provided by most major ecosystems largely intact.

The size and distribution of human population over the near and distant future will surely be a dominant factor in determining whether the loss of biodiversity that the world faces turns into merely a source of wistful regret for future generations, a planetary catastrophe, or something in between. Population growth enlarges the scale and extent of the human enterprise and hence inflates the likelihood that human activities will push native nonhuman populations and biotic communities past critical thresholds of tolerance and renewal.

Demands for housing (Mason 1996), food energy and arable land (Bongaarts 1994; Engelman and LeRoy 1995; Smil 1994), freshwater (Engelman and LeRoy 1993; Falkenmark and Widstrand 1992), and industrially fixed nitrogen (Howarth and others 1996; Smil 1991; Vitousek and others 1997) appear more sensitive to the growth of human population than to the growth of per capita income or even to recent changes in technological efficiency. Habitat conversion, historically the greatest threat to biodiversity, has been driven by these very demands—by housing needs, pressures to expand and intensify agriculture, and the quest to harness

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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additional freshwater supplies. Climate change, the demise of commercial fish populations and coastal reefs, widespread soil degradation, and the re-emergence of infectious disease also reflect the strong influence of population dynamics and take a growing toll on biodiversity. These global changes threaten ecosystem function and raise the risk of future extinction. It thus makes sense to consider the prospects for human population growth.

In this article, we consider those prospects by examining the United Nations (UN) population projections—both how and what they project. The methods and meaning of UN projections are poorly understood by scientists outside the field of demography. And the recent misuses of the projections in the press have confounded the public.

Despite widespread perceptions to the contrary, there is nothing inevitable about most future human population growth. Our species now numbers 6 billion and is growing at a pace of just over 80 million per year. More than 95% of this growth is occurring in countries of the developing world. Most demographers expect human population at least to approach 8 billion in the next half-century. Beyond that expectation, however, no one can be certain that world population will ever rise to greater levels. There is equal uncertainty that population will stop growing at any particular time in the not too distant future.

We can be certain, however, that today women in most developing countries desire fewer children than their mothers or even their older sisters sought or had (Westoff 1991). Over the last 30 years, that trend, when supplemented with access to modern contraception and the information needed to use it safely and effectively, clearly has resulted in lower rates of childbearing in countries with traditionally high fertility (Robey and others 1994). In the future, changes could occur even more rapidly. Decisions made today will have an enormous influence on the demographic future. These decisions are likely to be among the most important that we can make to conserve as much as possible of the planet's remaining biodiversity.

Humanity's Place in Nature

Few scientists outside the field of ecology are aware of how ecologically unprecedented is the scale of human numbers—not just present numbers, but also those of the last several millennia. No other mammal of comparable body weight has ever attained anywhere near such abundance. By manipulating the qualities and quantities of other species through agriculture, Homo sapiens broke through the energy and nutrient constraints that limited it as a hunter-gatherer.

Statistical models relating the adult body weight of mammals to their observed abundance (Peters 1983, p 166–7) predict that the equilibrium density of mammalian species in their home ranges will vary according to the following relationships: DC = 15 W - 1.16 for carnivores and DH = 103 W - 0.93 for herbivores (grazers and browsers), where D is animal density expressed in individuals per square kilometer, and W is the adult body weight in kilograms. For a carnivorous mammal or herbivore the size of Homo sapiens (roughly 65 kg), these relationships predict 0.12 individual/km2, and 2.1 individuals/km2, respectively. The natural

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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availability of preagriculture human diets, however, fell between carnivorous and herbivorous diets. In fact, we are still largely grain-, fruit-, and tuber-eating with a predilection for meat. A liberal estimate of the average density that our species would likely have attained without agriculture is around 1 individual/km2—similar to the density at which hunter-gatherers and nomadic pastoralists lived until relatively recently.

If preagriculture humans at that density were to exploit every square kilometer of Earth's habitable terrestrial surface, about 130 million square kilometers (Hannah and others 1994), the world would support roughly 130 million people. According to one estimate, world population surpassed that number during the early years of the Roman Empire (Biraben 1979 reprinted in Livi-Bacci 1992; Cohen 1995). The United States alone surpassed it just before World War II (US Bureau of the Census 1995).

Demographics Then and Now

We know with reasonable certainty that Homo sapiens has expanded in numbers from at most a few tens of millions in prehistory to nearly 6 billion at the close of the 20th century. Most of these billions arrived in the 20th century, as the march of technology (especially in sanitation, immunization, and agriculture) allowed, for the first time, the vast majority of babies born to survive to become parents themselves. Some of the most rapid population growth during the 19th century occurred in the United States, where annual increases, roughly 2.5–3%, were as high then as in sub-Saharan Africa today. The consistently high 19th-century growth rates are a major reason that the United States is today the third most populous country in the world.

The result of the victory over infant and child death is evident in every region and major city. The planet sustains nearly half its humans in urban areas. Roughly three of every five people live in Asia. Each of the other major world regions is home to several hundred million people, but the populations of the continents are growing at markedly different paces: Europe, with about 730 million people, at a mere 0.2% per year (UN 1996a); North America (mostly the United States and Canada), with 300 million, at about 1.0% annually; Asia, with 3.5 billion, at about 1.5% per year; and the Latin American and Caribbean region, with about 485 million, at about 1.7% per year. Standing apart from the rest of the world demographically is Africa, with 708 million, where population growth has continued for decades at nearly 3% per year, falling slightly now to 2.7%. The average of all uneven rates of growth worldwide is equivalent to that of Asia, or about 1.5% per year.

Despite the ever-larger population base, world population growth is gradually slowing. The annual rate peaked at 2.1% in the late 1960s and has drifted down since. When a growth rate decreases, however, growth itself continues until the rate reaches zero. And substantial growth continues decades after fertility descends to replacement levels (slightly more than two children per woman) or even dips below. That effect, known as population momentum, is due to the long lag time between birth and reproductive maturity that characterizes our species. The

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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lag allows past growth to continue to augment the absolute size of the reproductive segment of the population (women roughly 15–49 years old), thus supporting high numbers of births despite a decrease in fertility to replacement level.

As world population increases, more modest rates of growth can add larger annual increments to the population base. That has occurred although the highest rates of global population growth, estimated to have occurred around 1970, saw only about 72 million people added to world population each year. Current lower rates of growth are adding more than 80 million people per year. The global annual growth increment itself has declined since 1988 and could continue to decline—although by how much and for how long is unknown. A previous temporary decline during the middle 1970s, reflecting devastating effects of famine and political upheaval on the age structure of China's huge population (NRC 1984), illustrates how uncertain demographic projections can be.

During the 1970s and 1980s, human fertility in industrialized countries, which was already near replacement levels, declined once more. Nearly all the European countries fell below the roughly two-children-per-couple average that, in the absence of immigration, is necessary to replace each generation with the one that follows. The meaning of that trend for future population is potentially enormous.

Throughout the developing world, couples desire smaller families and later childbirths and they increasingly have the means to achieve the family size they seek. Several good examples can be gleaned from East Asia and Southeast Asia. During the middle 1960s, South Korea, Taiwan, Singapore, Thailand, and the former Hong Kong Territory began effective programs to lower infant mortality, establish easy access to family-planning services (ADB 1997; Tsui 1996), and increase primary-school enrollments and educational attainment (ADB 1997; Birdsall and Sabot 1993; Birdsall and others 1996; UNDP 1996; World Bank 1993). Thirty years later, average fertility in each of these Asian states is below two children per woman (the US average).

Other developing countries—including Mexico (3.1 children per woman), Brazil (2.4), Indonesia (2.9), Tunisia (3.3), and Sri Lanka (2.2)—are also experiencing downward trends in fertility (UN 1996b). A recent analysis of regional patterns of demographic change (Bongaarts and Watkins 1996) suggests that the first country in each developing region to begin its transition to lower fertility was endowed with relatively high indicators of social and economic progress, as measured by the UN's Human Development Index (UNDP 1996). In each case, however, fertility decline spread to nearby countries—probably via transfers of expertise, experience, and information at the government and local levels—despite the neighbors' lower scores for economic and social development.

In most developed countries, where there is access to affordable, effective contraception and safe abortion, women are more likely to have the number of children they want than are women in developing countries. Where these circumstances prevail and where childbearing and rearing are expensive or constrain economic mobility, total fertility consistently remains below the replacement level of slightly more than two children per woman (Potts 1996).

The other great trend shaping world population, aside from changes in fertility, is rapid mortality decline—or rapid increase in average life expectancy. Life

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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expectancy began its climb in middle-18th-century Europe (figure 1). By the late 20th century, people in all corners of the world had longer life expectancies. The dominant influences are at both ends of the age spectrum: smaller proportions of children are dying in the first few years of life, and larger proportions of adults are surviving to old age.

Demographers assume that the mortality decline will continue, placing some further upward pressure on the pace of population growth. Falling mortality, however, could moderate worldwide as additional improvements in health care and nutrition become more difficult to achieve. In eastern Europe, mortality has actually risen in recent years; and in sub-Saharan Africa, the AIDS pandemic is reversing recent progress in infant mortality (US Bureau of the Census 1994). Both trends and the growing specter of emerging infectious diseases (Olshansky and others 1997) raise questions about the strength of the UN's assumption of continued mortality decline well into the 21st century.

The Project of Projecting

In projecting an image of the future, the challenge for demographers is to understand the complex and uneven trends in fertility, mortality, and migration and to consider to what extent they are likely to continue and—perhaps most critical—at what levels they might end. Given the hodgepodge of modern demographic trends, all that can be said with certainty about future trends and end points is that we cannot be certain. The UN Population Division, which produces the most widely cited tables of international population information, has addressed such uncertainty by computing every 2 years a three-piece set of population projections. The most recent series, published in 1996, projects populations for each of the UN's 185 member countries to 2050 (see UN 1996b).

image

Figure 1
Life expectancy in three developed countries (1750–2000) and three developing
countries (1950–2000). European life expectancies for years before 1950 from tables
compiled by Livi-Bacci (1992), citing various authors who have analyzed
historical records. Data from 1950 and beyond from current UN tables (UN 1996b).

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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But scientists and journalists should take note: the UN projections are not statistically predictive. They are not estimates calculated from models of underlying behavioral relationships, nor are they the extrapolated curves with which biologists are most familiar. For that reason, the projections tend to be poorly understood and commonly misused.

The three pieces making up the UN's set of projections are its low, medium, and high variants generated for each country. Each variant differs from the other two in just a couple of key assumptions—its fertility end point and the path of fertility to that end point (see country examples in figure 2). When plugged into a model that generates births and eliminates the dead from each age group (and adds immigrants and subtracts emigrants where necessary), each variant traces a different population trajectory through the future.

To generate the medium variant's fertility curve, UN demographers use assessments of each country's situation and progress to make an educated guess of when each country will achieve replacement-level fertility. In each case, for projection purposes, this date is assumed to fall before midcentury. A fertility trajectory is then created that allows national fertility to fall—or, where needed, to rise—smoothly to its replacement-level end point. Once fertility arrives at this point, it is assumed to stay there indefinitely. By completing this exercise for fertility of each country (using standard mortality assumptions) and migration, adding all national populations for each year computed, the UN arrives at a continuous medium variant trajectory for world population.

The 1996 UN medium variant projects a global population of about 9.4 billion people around the middle of the 21st century, compared with the known 2.5 billion in 1950 and the 5.9 billion in 1998. If extended beyond 2050, as the UN does in its long-range projections (UN 1992; also see Haub and Yinger 1992; McNicoll 1992), population then grows fairly slowly, stabilizing at around 12 billion early in the 22nd century. The medium variant, however, is only one element of the projections.

To generate the other elements, the low and high variants, the UN adheres to the same model used to generate the medium variant but adjusts the fertility end point and the path of fertility to that point. In the low variant—a lower bound for plausible scenarios—each country's fertility end point is reset to achieve 1.6 children per woman before 2050 and held constant thereafter. To fix an upper bound of plausibility, the high variant applies the same schedule to settle at 2.6 children per woman.

Those are not error limits. Instead, the low and high variants are distinct, but extreme, scenarios of demographic change applied to every country. The low variant mimics the behavior of many European and several East Asian populations that over the last 2 decades have dipped below replacement fertility (1.2–1.9 children per woman). The high variant mimics a number of Central American and South American countries that have momentarily stabilized at levels somewhat higher than replacement (Haub and Yinger 1992). For example, total fertility of both Uruguay and Argentina has fluctuated erratically below 3.5 children per woman for at least 50 years without ever having reached replacement levels. In

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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image

Figure 2
Fertility curves for three countries (UN 1996b)—past data and projected high-, medium-, and low-variant
scenarios and corresponding national population projections (UN 1996c).

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Costa Rica and Chile, fertility declined rapidly during the 1970s but stalled at similar levels.

By generating low and high variants, the UN projections present an envelope of plausibility, suggesting that a range of population futures is possible. Those two scenarios project a 2050 world population between 7.9 billion and 11.9 billion (figure 3).

A Separate Demographic Reality

The demographic experience of the world suggests that total fertility is dynamic and highly responsive to the circumstances of women and couples. The UN series of projections, however, must necessarily remain mechanical and thus reproducible every 2 years. Perhaps the most mechanical feature is the UN's assumption of a stable fertility end point for each variant. But even the paths drawn to those end points often appear inconsistent with past data.

image

Figure 3
Past and projected world population from UN estimates and
projections (1996c) and annual increment of world population
growth (annual change in growth) derived from these data and
projections. The trough in the world-population growth increment
that began in the middle 1970s was caused by irregularities
in China's growth increment. China's irregular growth
was due to an age structure shaped by high mortality
(NRC 1984) and low fertility (Coale and Li 1987) that occurred
in the wake of famine and political upheaval during the
Great Leap Forward from 1958 to 1960.

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Three examples illustrate these points (see figure 2). In the case of Nigeria, where there is still little evidence of substantial demographic change, all three fertility variants seem highly speculative and contrived. For Japan's projected fertility, at least two of the variants seem difficult to reconcile with past trends. In the case of Colombia, however, high-, medium-, and low-variant fertility curves all seem similarly consistent with past data.

Although there are good reasons to expect fertility decline to continue where families are typically large, there is no particular reason to assume that fertility rates will settle between 2.0 and 2.1 or at 2.6 or 1.6 children per woman. In fact, Sweden, Luxembourg, the United Kingdom, and France—each below replacement-level fertility today—have been there before (UN 1996b; Livi-Bacci 1992), bobbing back up above replacement-level during national baby booms and moving downward during political and economic turmoil.

Recently, the UN low-variant population projection has been used by several analysts and journalists (Buchanan 1997; Eberstadt 1997; Wattenberg 1997) as evidence that UN demographers are predicting alarming declines in global population beginning as “soon” as 2040. That is either a gross misunderstanding or a misuse of the projections. The low variant does, in fact, trace a downward path after that date. But eventually it must, by its very nature. With each national population in the world fixed forever at 1.6 children per woman—about a half-child below the replacement level—there is ultimately nowhere for the calculated population to go but down. The high variant, just as artificially, forces the trajectory upward, and the medium variant ultimately forces stability. Clearly, it makes little sense to use one variant without reference to the others.

There is no good reason to assume that the below-replacement-level fertility experienced in some industrialized countries today will be sustained long enough to lead to a substantial net population decline in the long run. Fertility rates might well rise again if the direct costs or opportunity costs of childrearing decline or if larger families regain social approval. Nor does it make sense to assume that below-replacement-level fertility returns to and stabilizes exactly at replacement-level fertility. Realistically speaking, we do not know.

In practice, most journalists and analysts take the UN's “medium variant,” or middle trajectory, to be the most probable one, whether for national, regional, or global population figures. It is often expressed inaccurately as the “expected” population future. That hardly makes the medium projection the “most likely” scenario within the wide range of plausible paths described by the high and low variants. True, neither the high nor the low extreme could be properly considered as “likely”—they are extremes, after all—but neither is there any special center of gravity midway between them. In fact, the medium projection uses a reasonable, repeatable method that cuts a path through a future without surprises, one in which demographic change is gradual and limited.

A relative absence of demographic surprise, however, has not always been the rule. Until the 1950s, demographers most often underestimated population growth. The largest cause of failure among early population projections was that their authors missed the fact that mortality was falling at an increasing rate in the developing world. The country-by-country triumphs of sanitation, clean water

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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supply, antibiotics, and vaccinations were the big surprise. Of less impact, but still important, were increases in fertility in the industrialized countries after World War II—the so-called baby boom—that were difficult to foretell.

Strictly speaking, no population growth—not even tomorrow's—is really certain. Obviously, in the unlikely event that a nuclear war breaks out or a comet hits, all demographic bets are off. But leaving aside those unlikely possibilities, it is sobering to remind ourselves that infectious disease, war, and economic disruption still strongly influence population dynamics of individual nations and could do so more forcefully in the future. Clearly, words like inevitable and certain overstate the case.

More important, such language lulls observers into a conviction that no action in the present can influence the near demographic future. With much of the developing world exposed to television and computers in the span of a mere decade or two, a revolution in fertility patterns cannot be ruled out either. The likelihood of such changes is discounted in projections, perhaps reasonably, but such assumptions receive no discussion when the projection results are released to the public.

Nature's Place in Humanity

Humanity is pushing our planet across a series of important environmental thresholds at a time when our institutions—even in democratic societies—seem disinclined to take such threats seriously. This is the case whether the need is to secure the future or to help those whose well-being is most threatened today (Cincotta and Engelman 1997). What nonhuman genetic endowment shall we strive to preserve for future generations? Although the question is largely ethical and biological, we can be sure that demographics and economics will ultimately provide history with much or most of the answer (Morowitz 1991).

Among the complex factors that drive these changes in our ecology, human population growth is arguably the most easily addressed. Ten years ago, that statement would have seemed absurd. Five years ago, it might have been considered bravado. Now demographers tell us that the cessation of human population growth is within reach during the 21st century. We stand on that century's door-step.

How, then, should scientists view and represent the prospects for world population? Certainly not in terms of any inevitable figure. In peering into the future, it is useful to consider the UN population projections—the entire range described by the variants, not just medium variants—as a reasonably sound basis for describing a demographic future without substantial surprises.

We must loosen the grip that the medium projections have on the limited attention of policy-makers and the public. We need at least to bring attention to the range of growth suggested by the low and high projections for the next century and beyond. And, despite its necessarily artificial quality, we should hold forward the realistic hope offered by the low variant. Population growth might well slow further within the next few decades. Population size might peak before adding more than 2 billion people to our current numbers. And the world might

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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someday experience a degree of population decrease before attaining relative long-term stability.

If the scenario becomes real, it will not be the product of “population control” or coercive government family-size targets. Rather, such a world will grow out of consistently pursued development initiatives that focus largely on the capacity of women to manage their own lives, especially their reproductive options. Such initiatives slow population growth while serving more immediate human needs. And in slowing and eventually easing to a halt the growth of human population, such a strategy can help to ensure that nature and its myriad ecosystems and species do not recede forever from their rightful place on the planet.

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Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Population Growth, Sustainable Development, and the Environment

Sergey Kapitza
Population Action International, Washington, DC
(Current address: Institute for Physical Problems, Russian Academy of Sciences, 2 Kosygina St., Moscow 117334, Russia)

The long-term state of the biosphere—the conservation of species and of biodiversity—will depend to a great extent on the population growth of the world and the demographic pressure on the environment. At present, the population of the world is 5.9 billion, and it is growing by 1.5% a year; 250,000 inhabitants are added every day. Practically all population growth occurs in the developing world. At the same time, much of the industrial development also happens there. On the other hand, in the foreseeable future, because of transition in the population, global population is expected to level off at 12–14 billion. It is in these terms that one should consider the effect of humankind on the future of our planet, taking into account the trends in development seen in the larger context of the dynamics of the world-population system.

The links of population growth and development were the subject of a seminal statement of the Royal Society of London and the US National Academy of Sciences titled Population Growth, Resource Consumption, and a Sustainable World and signed by the presidents of the two societies (Atiyah and Press 1993). In that statement, probably for the first time, two great academies voiced their opinion on this all-important and sensitive subject. They came to the following conclusions:

The applications of science and technology to global problems are a key component of providing a decent standard of living for a majority of the human race. Science and technology have an especially important role to play in developing countries in helping them to manage their resources effectively and to participate fully in worldwide initiatives for common benefit. Capabilities in science

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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and technology must be strengthened in LDCs [less-developed countries] as a matter of urgency through joint initiatives from the developed and developing worlds. But science and technology alone are not enough. Global policies are urgently needed to promote more rapid economic development throughout the world, more environmentally benign patterns of human activity, and more rapid stabilization of world population. The future of our planet is in the balance. Sustainable development can be achieved, but only if irreversible degradation of the environment can be halted in time. The next 30 years may be crucial.

The issues raised in that statement have grown in importance, and a major international debate has followed. The problem of global population growth has been reviewed thoroughly, and it is probably best summed up in the book The Future Population of the World: What Can We Assume Today?, edited by Lutz of the International Institute for Applied Systems Analysis, IIASA (Lutz 1994). An extensive study by Cohen (1995), How Many People Can the Earth Support?, reviews a vast amount of data and many ideas and misconceptions. Perhaps the complexity and difficulty of these subjects, which are essential to discussions of sustainable development, can be seen in the fact that the titles of both books are questions.

The book by Cohen and chapter 10 of the book by Lutz, “How Many People Can Be Fed on Earth?”, show that the idea of carrying capacity is counterproductive, if not misleading or even wrong. That point is well illustrated by the “limits” suggested by various writers since 1600, when fewer than a half-billion people lived. Until 1900, the limits indicated are rather similar and mostly reasonable, around 10–15 billion; these figures compare well with modern assessments. The huge discrepancies—from 1 billion to 1 trillion—seen in projections from the last 50–100 years more likely indicate the ups and downs of the subjective mood, private and public, that in its own way expresses the turbulent history and transitory nature of 20th century, rather than a trend toward a greater understanding of human destiny.

Projections of Population Growth by Demographic Methods

Those projections of the world's population were based on assumptions of the availability of resources, mainly land. Most modern estimates, however, are the result of extensive studies of population growth that look at population dynamics rather than resources. Results can be obtained with standard demographic methods and are valid for 1 or 1.5 generations. For periods of longer than about 30–50 years, demographic calculations become computationally unstable. All extrapolations farther into the future come from plausible hypotheses regarding the future development of humankind, which set guidelines for the calculations made. For the year 2100, the most probable projections by a team at IIASA indicate a population of 11.5 ± 1 billion. That estimate indicates that in the next 100 years, the population of our planet will barely double (Lutz and others 1994).

Of critical importance is that the future population of the world will be determined not by the incessant growth that has marked development until the present

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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but by a complex transition to a stabilized world population. This demographic transition, as it has become known, is the crucial feature of modern human population growth. The phenomenon, first recognized by Notenstein in 1950, now is determining the lack of population growth in the developed countries; in the next 50 years, it undoubtedly will lead to a decrease in the rate of growth and the final leveling off of the world's population. For the more-distant future, demographers can make only plausible guesses, estimating a stabilized world population of some 12 billion.

The demographic transition will be accompanied by a marked change in the age structure of the population. Currently, people younger than 15 years make up 32% of the world population and people older than 65 years, 7%; after the transition, the numbers will be 18% and 22%, respectively. At the same time, a huge number of people will move to towns in a global pattern of urban development. Ultimately, the demographic transition will lead to major changes in the lifestyle and values of billions of people. It is certainly the most significant event in human history, seen on a large scale. That is why it is well worth the effort to study the demographic transition with methods other than those provided by modern demography.

Modeling the Growth of Humankind

I have developed an alternative approach (Kapitza 1994, 1996a,b), which interprets the global population as an interactive dynamic system. The entire population of the world is the object of study. The global-population system is considered to be an entity, coupled with interactions that determine its long-term growth and development, rather than as a mere sum of countries and regions, each following its own pattern of growth. This is the next step in generalizing population growth. In fact, when describing the demographics of China or India, we already are summing up, in a single measure of growth, a vast country that has many regions and cultures of great ethnic diversity and that encompasses 17–20% of all humanity.

In treating the whole of humankind in such a general way, it is also possible to expand the time scale considerably. One must break away from the unit of a generation that is used customarily in demography and even go beyond the millennia of history to the millions of years that provide the scale for human development as seen in anthropology. For shorter intervals, this way of describing the growth of human population must merge with and rely on the methods and concepts of demography. Thus, the two ways of describing our growth and development complement each other and provide mutual support and justification of their results.

If we consider the long-term pattern of the growth rate of the human population, we see that the rate is proportional to the square of the total number of people. This nonlinear growth corresponds to a hyperbolic growth curve and is well known for describing explosive systemic development. For humankind, quadratic growth is valid for more than 1 million years into the past, right from the appearance of Homo habilis, the primeval tool-maker. The quadratic law of growth

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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is applicable only to the total number of people; it cannot be applied to describe regional growth. But every region and country participates in and is influenced by it because we are dealing with a nonlinear law that implies a global interaction in the complex population system of the entire world.

However, the quadratic law leads to a divergence in a finite time. The divergence has begun already; if the growth rate thBat has been valid for so long persists, the runaway into infinity will happen in 2025. The pattern of quadratic growth can describe the first stage of the population transition—the population explosion. This is a new way of looking at the demographic transition as a phase transition, describing it in terms and methods that come from nonlinear physics of systems.

For explosive divergences, it is well known that a cutoff must be brought in, taking into account factors of less importance during the main period of growth. As we approach the singularity, the concepts of demography become significant. They indicate that if we take into account the finite human life span and reproductive time, we can expect the whole pattern of growth to change. The development of this phenomenon, following well-established methods of systems analysis and physics, envisages an asymptotic transition to a stabilized world population of 12–14 billion, with 12 billion reached in 2100. A time constant of 42 years, characterizing the human life span, has to be incorporated into the calculations.

This model has developed into a theory that describes the gross features of the growth of humankind. It provides an estimate of the beginning of human development, some 4–5 million years ago, and an estimate of the number of people who ever lived, of 100 billion. As we approach the critical year of 2007 (it shifts from 2025 when the 42-year cutoff is taken into account), the dynamics of growth indicate a logarithmic compression of the time scale of history. In 2007, the maximal annual growth of 85–90 million is expected, but the relative growth rate already reached its peak, 17% per year, in 1989.

My approach now reconciles well with the methods of demography and can be seen as complementary in treating the same problem on a larger scale: the way human population has led to the concept of the population imperative. This means that growth has not been limited globally by resources, but is governed by the inherent nature of the systemic interactive dynamics of the global population system. On a large scale, growth has been systemically stable, although local and temporal variations have occurred.

Factors that Limit Population Growth.

At this point, it is appropriate to mention the fundamental differences between my model and the Malthusian population principle. According to Malthus and those who developed the same ideas later into the “limits-to-growth” concept, growth is limited by resources (Meadows and others 1972). In the case of Malthus, the lack of food led to hunger, which decreased the birth rate. But this way of treating the origin of growth and the factors that control it does not consider that humankind is a highly interactive system. In the case of my model, the rise in the number of people, observed over the ages, is the outcome of all factors

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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of a biological, technological, economic, social, and cultural nature relevant to growth in society and expressed by the quadratic-growth law. In other words, we must treat humankind as an entity, an open system, and integrate everything that is going on inside it. That is the meaning of systemic development, as opposed to the reductionist approach that is pursued in most global models, in which all relevant processes and resources are purportedly taken into account separately. With great expertise and effort, this is done in demography and is the reason for its limited temporal horizon in describing growth, although in this case we get insight into the details of growth and its distribution in age groups and space.

It should be noted that humans are not only qualitatively different but also quantitatively different from any mammals of comparable size and position in the food chain; humans are 100,000 times more numerous. Only domestic animals that accompany humans outnumber by far their relatives that live in comparative equilibrium in the wild. Humankind has broken away from the rest of nature and has developed a habitat of its own. On the other hand, in the last million years, humans have scarcely evolved biologically at all since the appearance of Homosapiens. These are the basic reasons for considering the development of humankind as a separate entity, a system of its own, in which, in the process of sapientation, social, technological, and economic development have determined our incessant growth.

In the global interaction of all people, the exchange of information and the transfer of knowledge are instrumental in how growth becomes the outcome of all processes in the complex nonlinear global system. The interaction described by the quadratic growth rate can be seen as a collective phenomenon, an expression of consciousness, peculiar to humans and making them fundamentally different from all other animals. By speech and language, information is transferred vertically from the past and into the future. At the same time, information is spread horizontally, synchronizing human development globally. The nonlinear systemic theory of the growth of humankind indicates the synchronous development of the large-scale features of history and prehistory that are well substantiated by observations of historians and anthropologists.

Finally, this theory indicates that our development over the vast period of growth can be seen best on a logarithmic scale. This has been intuitively done by anthropologists, who otherwise could not accommodate on a single chart the million years of the lower Paleolithic age with the 10,000 years of the Neolithic age. Calculations show that the time of the development of all humankind should be shown logarithmically, reckoning time from the year 2007—the peak of the demographic transition—so that the whole human story can be shown in the same table (see figure 1). A table like this also offers an explanation of the nonuniform way in which time has passed during the course of our development. This change in the relative duration of events is a direct kinematic result of the accelerated growth of humankind proportional to the population of the world. As we approach the singularity of the demographic transition, the transformation and compression of time in history are striking. In the theory of growth, the largescale features appear as epochs and periods of population growth. The first epoch, A, which lasted 2.8 million years, corresponds to the time it took for Homo

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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image

Figure 1
The Development of Humankind on a Logarithmic Scale.

habilis to emerge during the evolution of early hominids. Epoch B, the time of quadratic growth, began 1.6 million years ago and culminated in 1965 in the advent of the demographic transition. This led to epoch C, the transition to a stabilized population of the world. The periods traditionally identified by anthropology and history subdivide epochs A and B into 12 intervals of DT years. These intervals become shorter and shorter as we approach 2007, the critical date of the transition.

This novel way of presenting human development is the result of a consistent and straightforward mathematical model for interpreting the general features of the development of humankind. It comes from applying methods of sciences that arrogantly call themselves exact to problems of the humanities—an effort that is far from easy, because the sides need to learn to understand each other but have

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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long been separated in our culture. It seems that this can be done only through an interdisciplinary endeavor, and global problems likely are best suited for this purpose. Population growth now practically has reached the peak of the transition to a stabilized world population for the foreseeable future, and the period 1965–2050 is the time of this transition. The transition is remarkably short if we compare it with the million years of our development, but 10% of all the people who ever lived will experience this period of rapid change. The pace and width of the transition are the result of interactions in the global population and the outcome of the complex behavior of a highly nonlinear dynamic system. During this eventful period of 85 years, the population of the world will become 3 times larger and much older. It is the most critical and singular period ever to be experienced by humankind. All through the ages, humankind has followed a stable, persistent pattern of growth; this pattern is changing rapidly now to a stabilized global population. In fact, it cannot change faster (barring an all-out nuclear war or extraterrestrial intervention!), and it is this rapid change—from blowup to saturation—that must be kept in mind in any attempt to understand the global problems, that now face the world.

Sustainable Development

Since the Conference on Development and the Environment was held in Rio de Janeiro in 1992, the concept of sustainable development has emerged as an important landmark in the international debate on world affairs. In the summer of 1997, a review conference in New York showed the difficulties—even a split in the attitudes toward development and the environment—between the developed and developing nations. The consensus reached in Rio de Janeiro is being questioned now, and the origin of the differences in attitudes needs to be investigated, taking into account the population transition.

Because the transition first happened in the so-called developed world and now is proceeding to the developing world, it would be better to speak in terms of the countries where the population has already stabilized and the greater part of the world, which now is passing through the demographic transition. To see the magnitude of these events, the population undergoing the transition now is 15 times larger than, and the rate of change is twice as great as, in the developed world. Today's annual growth rate in China is 1.1% in a population of 1.3 billion; in India, the annual growth rate is 1.9% in a population of 930 million. The respective economic growth rates of these countries are 12% and 6–7%. What should be of greatest concern for the world community is stability during this remarkable time of development (Kapitza 1996c). On the other hand, the differences in the stages of the demographic transition provide the demographic and economic backdrop against which the concept of sustainable development must be examined.

In the developed world, the demographic transition already has led to a stabilized and rapidly aging population of predominantly senior citizens. The process of urban development is slowing down. Indications of such a stable, affluent, and highly developed society can be seen in many ways. Extensive service sectors of

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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the economy—of health, education, and social security—are developing. The change in values of the fundamental paradigm of development—from growth on all counts in terms of children, cars, or soldiers to that of limited growth and concern for the environment—is important. We, hopefully, can see a trend to abate consumption, making it a subject of public awareness. As an expression of a new consciousness, global responsibility for the environment is gaining ground. It is from these premises that the idea of sustainable development has sprung (Kapitza 1997).

The other side, that is, the developing world, has quite different circumstances. The younger generations are predominant. There is a vast migration from villages to towns, leading to rapid urbanization. The young migrants are the new working class, who are active and unsettled and who can man armies or leave the country or, in the case of unemployment, become a source of unrest. The possible scenarios are well known. In the developed world, we need to look back only 100 or 150 years to find a similar situation, but we must keep in mind that growth and everything that accompanies it occur twice as fast today as they did then.

In assessing changes and development in the world system, one also needs to think in terms not of averages, but of distributions—distributions by sex and age in populations, in wealth and income, in education and health, and in the very nonuniform distribution of people in towns and villages. Without studying the evolution of these distributions, it is practically impossible to describe the changes that are happening. Because of a lack of understanding of the statistical origins and social relevance of these distributions, ideas have evolved on drastic cuts in world population, of a “golden billion”, and of extrapolating the southern California lifesryle worldwide.

All distributions of land, food, energy, and wealth show that the world population system is far from equilibrium. The origin of these distributions is most important; it indicates rapid growth, which increases as a country approaches the demographic transition. On the other hand, the evolution of these distributions shows that, in processes of growth, the world population system was dynamically sustainable—otherwise it could not have evolved consistently for 1 million years as it has. In this context, Vishnevsky made an interesting observation in interpreting the dynamic model. He remarked that the history of humankind preceding the demographic transition can be seen as a rapid passage, a nonequilibrium, a transitory state of growth and self-organization toward a stabilized world population, which will be the long-term asymptotic and stable state of humankind. This point is important to remember when we are addressing global problems of the present age.

We must look into the meaning of sustainability in a world of zero or very low population growth. We should assume that the world population is moving rapidly toward stabilization and that, in promoting and propagating the idea of a sustainable world, this must be taken into account. But will we run out of global resources at the expected levels of consumption? That is what matters and what led to the split at the 1997 New York conference.

The point is often made that we are living in a common world and that we must consider the common heritage that we all share—be it the atmosphere, the

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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oceans, or the complexity of the biosphere. That is certainly true, but where are the limits of demanding a common policy on these issues? In this new, stabilized world, the population by the end of the next century will be twice as large as it is today. The energy produced—the best way of estimating the use of resources—will be 4–6 times as large as it is now (Holden 1991). Can our planet carry this load without collapsing? Probably so, but great changes will take place. It is best to remember that the environment in every populated part of the world—from Europe to China, India, and much of North America—has a highly transformed natural habitat. There are still large sparsely populated spaces that escape our attention. A comparison of Argentina and India is instructive. India's area is about 40% larger than Argentina's, but its population is 30 times greater. India is one of the oldest civilizations, if not the oldest, whereas Argentina, as a nation, is only 200 years old. But Argentina reportedly could feed the entire world.

As long as such large discrepancies exist, it can be assumed that the global population system is open and has enough resources to support its development in the foreseeable future. The first indication of a global shortage will be a more uniform pattern of the use of resources. On this scale of events, the next century will be crucial for humankind to negotiate the last stage of adaptation to the stabilized state of its future, when, we hope, we can carry out a pattern of sustainable development. At that stage, all progress will need to be reckoned with by means that do not involve numerical growth, the stereotype of development that has dominated humankind for 1 million years and tens of thousands of generations. History and our present experience show that our “software”—our ideas and values—evolves much more slowly than our “hardware”, which for ages was geared for maximal growth and productivity. Under the pressure of rapid development, these long-entrenched attitudes will have to change. Of all factors, this probably is central to resolving the issue of sustainability.

Sustaining Biodiversity

These ideas provide the historical context for considering the sustainability of biodiversity. As recent environmental research has shown, we can expect to lose biodiversity mainly during the period of rapid growth, as happened in the developed world two or three generations ago, during the first stage of the demographic transition—the stage of rapid growth. Today, many see the very fast growth of the developing world as the primary menace to the global environment, with biodiversity in first place over the short term, compared with long-term environmental issues. The sheer rate of growth and the rapid transition to a stabilized new world are competing factors that will determine the outcome and the state of the world in the foreseeable future. What can and will resolve these issues to some extent is a change of values that will determine our patterns of social behavior. At the peak rate of the present stage of development, material growth by far outstrips the development of humankind's “software”.

The differences in our values, ideas, and material development are influenced to a great extent by the processes of globalization. If the spread of technology, money, and industrial know-how is accelerating development, the appropriate

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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diffusion of ideas and values is lagging. The sheer complexity of global society is complicating matters, for it takes much time for our social habits and customs to be established and even longer for international institutions to evolve. The time scales involved can be traced to the fact that it takes only 9 months to produce a human's “hardware” but at least 20 years to program a human's “software”. These are the fundamental biological and human constants that finally determine both our personal development and the fate of humankind. Ultimately, it is the interplay and balance of matter and mind that will resolve our predicament.

References

Atiyah M, Press F. 1993. Population growth, resource consumption, and a sustainable world. Statement of the Royal Society of London and US National Academy of Sciences. Washington DC: National Academy Press.

Cohen J. 1995. How many people can the world support? New York: Norton.

Holdren J. 1991. Population and the energy problem. Population and environment. J Interdisc Stud 12(3):231–55.

Kapitza SP. 1995. Population dynamics and the future of the world. In: Towards a war-free world. Proceedings of the 44th Pugwash conference on science and world affairs. Singapore: World Scientific.

Kapitza SP. 1996a. The phenomenological theory of world population growth. Physics—Uspekhi 39(1):57–72

Kapitza SP. 1994. The impact of the demographic transition. In: Schwab K (ed). Overcoming indifference: ten key challenges in today's world. New York NY: New York Univ Pr.

Kapitza SP. 1996b. Population: past and future. A mathematical model of the world population system. Science Spectra 2(4).

Kapitza SP. 1996c. Population dynamics and the West-East Development. Annals of the 7th Engelberg Forum.

Kapitza SP. 1997 Population growth and sustainable development. The 47th Pugwash Conference on Science and World Affairs. Lillehammer, Norway: World Scientific.

Lutz W (ed). 1994. The future population of the world: what can we assume today? London UK: IIASA and Earthscan Press.

Lutz W, Sanderson W, Scherbov S. 1997. Doubling of world population unlikely. Nature (387):803–5.

Meadows D and others. 1972. Limits to growth. New York NY: Universe Bk.

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Nonindigenous Species—A Global Threat to Biodiversity and Stability

Daniel Simberloff
Department of Ecology and Evolutionary Biology, University of Tennessee, 569 Dabney Hall, Knoxville, TN 37996

The world's biota is being rapidly homogenized. This global change constitutes a major threat to biodiversity and to our ability to extract resources sustainably from many ecosystems. The threat was first recognized 50 years ago, but its extent is only now being realized as burgeoning tourism and unfettered international trade expand the opportunity for species to get from one region to another. In the past, a desired immigrant species or one furtively hitching a ride often had to survive a sea voyage of months. Now, over 280 million passengers use commercial airliners each year worldwide, as do millions of tons of cargo. The brown tree snake (Boiga irregularis) occasionally arrives in Honolulu in wheel wells and cargo bays of planes from Guam, where it has devastated forest birds after introduction from the Admiralty Islands (Rodda and others 1992). Similarly, mosquitoes arrive in Great Britain from Africa in airliner passenger cabins (Bright 1996), and the giant African snail (Achatina fulica), which has ravaged agriculture on many Pacific islands, was carried by a boy from Hawaii to Florida as a gift to his grandmother (Simberloff 1997a).

Of course, on every continent, many of the most venerated plants and animals were introduced intentionally. In many parts of the world, the major crop plants are almost all introduced, as are livestock. For example, of nine crop plants in the United States classified as “major” (USDA 1997), one (corn) is native and five were introduced from the Old World, one from the Andes, and two from Central America. Pets and ornamental plants are also usually of exotic origin. So what is the threat, exactly?

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Effects of Nonindigenous Species

The biggest threat posed by introduced species is the disruption of ecosystems, often by invasive plant species that replace the native species. The Australian tree Melaleuca quinquenervia, until recently increasing its range in southern Florida by more than 20 ha/day, replaces cypress and other native plants. It now covers about 200,000 ha, provides poor habitat for many native animals, affects the fire regime, and causes water loss (Schmitz and others 1997). South American water hyacinth (Eichhornia crassipes) now blankets many near-shore areas of Africa's Lake Victoria, blocking light and killing plants at the bottom of the food chain. The death and decay of plants that make up the water hyacinth mat remove still more oxygen from the water, and the major fisheries are in drastic decline. In addition to the ecological damage, water hyacinths are an economic nightmare, fouling engines and propellers of cargo ships and ferries, preventing docking, and clogging power-plant pipes and so causing numerous blackouts (McKinley 1996).

Introduced plants can also change an ecosystem without smothering the native plants. For example, on the island of Hawaii, the eastern Atlantic island shrub Myrica faya has invaded nitrogen-poor lava flows and ash deposits. A nitrogenfixer, it favors other introduced species over the native plants adapted to low nitrogen (Vitousek 1986). In much of the American West and in Hawaii, Old World grasses, such as cheatgrass (Bromus tectorum), increase the frequency and intensity of fires to the great detriment of native plants and the animals that use them (D'Antonio and Vitousek 1992; Macdonald and others 1989).

Entire marine ecosystems can be radically changed by the invasion of a single plant species. The Pacific seaweed Caulerpa taxifolia, released from the Oceanographic Museum of Monaco into the Mediterranean about 15 years ago, now covers over 4,000 ha and has locally smothered native seagrass beds that harbor many native animals (Boudouresque and others 1994; Simons 1997). Introduced red mangrove (Rhizophora mangle) trees from Florida on the coasts of the Hawaiian islands and Australian “pine” trees (Casuarina spp.) on the Florida coast have come to dominate their new homes, displacing native plants and animals (Schmitz and others 1997; Walsh 1967).

Just as an introduced plant can modify an ecosystem, a species that eliminates a plant can have a drastic effect. The Asian chestnut blight fungus (Cryphonectria parasitica), which arrived in New York City on nursery stock in the late 19th century, spread over about 100 million ha of the eastern United States in less than 50 years, destroying almost all chestnut trees (von Broembsen 1989). Chestnut had been the most common tree in many forests, making up one-fourth or more of the canopy trees, so the cascading ecosystem effects of this invasion were substantial. For example, several insect species that were host-specific to chestnuts were extinguished (Opler 1979); that chestnut leaves decompose faster than leaves of the oaks that largely replaced them suggests that the invasion greatly affected nutrient cycling (K. Cromack, Oregon State University, pers. comm.), although systematic data were not gathered. The North American pine wood nematode (Bursaphelenchus xylophilus) reached Japan in timber and spread

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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among the islands, killing more than 10 million pine trees and affecting 25% of Japan's pine forests (von Broembsen 1989). The effects on other forest species must have been dramatic.

In addition to ecosystem effects, nonindigenous species have myriad effects on particular native species or groups of them. They can eat them, for example. The Nile perch (Lates niloticus), after introduction into Lake Victoria, eliminated many species of endemic cichlid fishes, which had undergone perhaps the greatest evolutionary radiation that scientists have studied (Goldschmidt 1996). Introduced rats (Rattus spp.) on many islands have destroyed at least 37 species and subspecies of island birds (Atkinson 1985; King 1985). The impact of the brown tree snake on the Guam avifauna is noted above. Introduced herbivores can similarly drive species to extinction, especially on islands where plants are less likely to have a refuge, an area that herbivores cannot reach. For example, goats introduced to St. Helena in 1513 almost certainly eliminated over 50 endemic plant species, although only seven were scientifically described before they disappeared (Groombridge 1992).

Introduced pathogens, often carried by introduced plants and animals, can also devastate native species. The chestnut blight was noted above. As another example, in the Hawaiian islands, the extensive introduction of Asian songbirds has brought avian pox and avian malaria, which have contributed to the decline and extinction of numerous native forest-bird species (van Riper and others 1986). The introduction into Africa of the virus rinderpest, native to India, in cattle in the 1890s led to the infection of many native ungulate species; mortality in some species reached 90%, and the distribution of some species is still affected by the virus (Dobson 1995).

Nonindigenous species can compete with native ones, although competition for resources is often difficult to demonstrate. Some well-studied examples provide good evidence. The house gecko (Hemidactylus frenatus) has invaded many Pacific islands; this has led to drastic declines in the population of some native gecko species. Experiments suggest that at least one of the natives, Lepidodactylus lugubris, avoids the larger house gecko, thereby suffering food shortage (Petren and others 1993), and that the invader depletes the insect food base sufficiently to reduce the food available for the native (Petren and Case 1996). The continuing replacement in the United Kingdom of the native red squirrel (Sciurus vulgaris) with the introduced American gray squirrel (S. carolinensis) is now attributed largely to the greater foraging efficiency of the invader and concomitant lowering of food available to the native (Williamson 1996).

Many instances are known in which introduced species affect native ones by interfering with them directly rather than indirectly through resource depletion. The South American fire ant (Solenopsis invicta), which has spread throughout the southeastern United States, attacks individuals of native ant species and is replacing the latter in many habitats (Tschinkel 1993). In a plant analogue of aggression, the African crystalline ice plant (Mesembryanthemum crystallinum) accumulates salt, which remains in the soil when the plant decomposes. In California, this plant thus excludes native plants that are intolerant of such salty soil (Vivrette and Muller 1977).

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Nonindigenous species also eliminate native species by mating with them; this threat is especially strong if the native species is much less numerous than the introduced one. For example, the New Zealand gray duck (Anas superciliosa superciliosa) and the Hawaiian duck (A. wyvilliana) are threatened with a sort of genetic extinction because of rampant hybridization and introgression with the introduced North American mallard (A. Platyrhynchos) (Rhymer and Simberloff 1996). Likewise, Europe's rarest duck, the white-headed duck (Oxyura leucocephala), is threatened in its last redoubt in Spain by hybridization and introgression with North American ruddy ducks (O. jamaicensis), which were introduced into England as an amenity, escaped, and made their way to Spain (Rhymer and Simberloff 1996). This sort of threat is far more common in regions that exchange closely related species (such as Europe and North America) than in those whose species are so distantly related that they are unlikely to be able to mate and exchange genes (such as Australia and either Europe or North America). A native species can be threatened by hybridization with an introduced one even if no genes are exchanged, simply by the reproductive reduction effected by fruitless matings. Females of the endangered European mink (Mustela lutreola) mate with male introduced American mink (M. vision); although the embryos are aborted, the loss of reproduction by the European mink exacerbates their population decline (Rozhnov 1993).

Slowing the Flow

The first line of defense against nonindigenous species is to keep them from being introduced. There are both practical and legal impediments to doing so. The sheer volume of tourism and trade dictates that inspection is destined to miss many inadvertent immigrants. Agricultural pests insinuate themselves into foodstuffs, woodboring beetles into timber, rodents into cargo containers—virtually any product shipped in bulk can carry many hitchhikers. Routine purging of ship's ballast water has released hundreds of nonindigenous species in waters throughout the world (Carlton and Geller 1993). Tourists can easily import species inadvertently in baggage, even if they heed warnings about which items are the most likely carriers of immigrants. In 1990, about 333 million nonindigenous plants were imported into the United States through Miami International Airport alone (OTA 1993). Economic resources are insufficient to examine everything that crosses a nation's borders.

Furthermore, liberalization of trade through such treaties as the General Agreement on Tariffs and Trade (GATT) and the North American Free Trade Agreement (NAFTA) is bound to increase the flow of nonindigenous species, and not only as a result of the increased volume. Under GATT and NAFTA, restrictions claimed as environmental measures can be challenged on the grounds that they are protectionist. The relevant regulatory authority must then adjudicate the dispute. Aside from the overwhelming appeal of free trade, both GATT and NAFTA require that species exclusions be based on risk assessments. However, risk-assessment procedures for introduced species are in their infancy and

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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do not appear to be scientific, often resting on undefended judgments by experts and on arbitrary algorithms for combining risks (Simberloff and Alexander 1998). Furthermore, these risk assessments are expensive; one conducted by the US Department of Agriculture (USDA) on risks associated with importing larch from Siberia into the United States (USDA 1991) cost $500,000 (Jenkins 1996). It is difficult to imagine finding funding sources sufficient to mount risk assessments for all the challenges that might appear even to an educated layperson to be justified on prima facie grounds.

Virtually every specialist in invasion biology who has examined the matter concludes that aspects of the ecological impact of a nonindigenous species are inherently unpredictable (for example, Hobbs and Humphries 1995), and many scientists argue that every species should be considered a potential threat to biodiversity and sustainability if it were to be introduced (for example, Ruesink and others 1995). That implies that every species proposed for deliberate introduction, whether or not it appears superficially to be innocuous, necessitates some formal risk assessment. The cost would be staggering if the USDA process (USDA 1991) were the model.

In addition, many parties introduce species not inadvertently, but deliberately. These range from the boy smuggling giant African snails to his grandmother, who released them in her yard in Miami (Simberloff 1997a), to such large industries as the pet and ornamental-pet trades, which lobby vigorously against many restrictions. In the United States, recommendations that all species proposed for introduction must be on “white lists”—lists of species whose invasive potential has been assessed and has been approved for introduction—have been systematically attacked by those interest groups. Rather, the major laws that restrict entry of species use “black lists”lists of species that have already been shown to be damaging or are strongly suspected of being dangerous; a species is prohibited only if it is on such a list (Schmitz and Simberloff 1997). Rarely is blacklisting forward-looking.

Thus, there will always be a flow of nonindigenous species. However, the flow can be lessened. Undoubtedly, increased public education as to the risks would lead to fewer deliberate and inadvertent introductions as people strive to be good environmental citizens. The Convention on Biological Diversity mandates that its signatories “as far as possible and as appropriate . . .prevent the introduction of . . .those alien species which threaten ecosystems, habitats, or species.” Bean (1996) suggests that this statement reflects widespread recognition that nations are obliged to attempt to prevent introductions, and he cites as an example New Zealand's 1993 Biosecurity Act, which subjects all incoming persons and goods to rigorous inspection and prevents the importation of any species not already cleared by government authorities for inclusion on a white list. He also notes the increasing international and national regulation of purging of ballast water and points out that the considerable legal framework and effort that many nations use to attempt to prevent agriculturally harmful introductions could be adapted and expanded to prevent ecologically harmful ones. The problem is educating the public sufficiently that they demand regulation of nonindigenous species.

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Managing Nonindigenous Species

Once a species enters a new region, there are several options for managing it. The most obvious one is to attempt to eradicate it. This approach is often feasible if the invasion is recognized and targeted early enough (Simberloff 1997a), but several factors militate against its success. Perhaps foremost, almost no countries have an early-warning system in place that is charged with determining when an invasion has occurred, much less a procedure to generate a rapid, coordinated response while the invasion is still restricted geographically. The reaction is usually only after an invasion has existed for so long that it has become noticeable, and by then eradication is often impossible (Schmitz and Simberloff 1997). Second, for species deliberately introduced, the same forces that conspired to allow introduction in the first place act to prevent eradication. In addition, many invasions appear innocuous for long periods (Crooks and Soulé 1996; Williamson 1996); by the time they are recognized as ecologically or economically damaging, they are so widespread that they cannot be eradicated.

Minimizing ecological and economic damage if eradication proves impossible is usually attempted by one or more of three routes (Simberloff 1996): chemical, mechanical, and biological control. The environmental and human health effects of broad-spectrum pesticides are legendary. Although some newer chemicals have far fewer side effects, their high cost and the necessity of repeated application and the frequent evolution of resistance by the target pest have led to great interest in alternative methods. Also, if pesticides were used to prevent damage by introduced species both to vast areas of natural habitats and to agriculture, all the above problems would be exacerbated.

Mechanical methods, either alone or in concert with pesticides, are sometimes feasible. For example, water hyacinth has been successfully controlled in Florida for over 20 years by a combination of mechanical harvesting and treatment with the herbicide 2,4-D (Schardt 1997). However, mechanical devices are often expensive and would be less likely to work on widespread invasions.

Biological control—the introduction of a natural enemy of the pest—has seemed an extremely attractive alternative to chemical and mechanical control on both ecological and economic grounds. Many biological-control projects have provided continuing suppression of a pest to acceptably low levels with the sole costs being those of the initial exploration to find natural enemies and the testing for efficacy and safety. Odour (1996) cites the control of water hyacinth in Sudan by three South American insects, of prickly pear cactus (Opuntia inermis and O. stricta) in Australia by the moth Cactoblastis cactorum from Argentina, and of the South American cassava mealybug (Phenacoccus manihoti) in Africa by a South American encyrtid wasp. In each instance, the natural enemies maintain populations in perpetuity without further human intercession.

More recently, biological control has been subjected to critical scrutiny on the grounds that nontarget species, some of conservation concern, have been attacked and even driven to extinction (Howarth 1991; Simberloff 1992). Early biological control projects using vertebrates, such as the small Indian mongoose or the cane toad, and the widespread dissemination of the New World predatory snail

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Euglandina rosea to control the giant African snail were disastrous, and biological control professionals now eschew the use of vertebrates, except for fishes. However, insects tested for host specificity have also attacked nontarget species. For example, the Eurasian weevil Rhinocyllus conicus, introduced into North America to control musk thistle (Carduus nutans), is now attacking native nonpest thistles, including narrowly restricted endemic species in nature reserves (Louda and others 1997). Although the extent of such problems is controversial, the fact that biological control agents can both disperse and evolve, just as any other introduced species can, suggests great caution in their use and extensive preliminary testing before their release.

Action Needed Now

Burgeoning international interest in invasive nonindigenous species has led to several international meetings (for example, Sandlund and others 1996), new monographs (for example, Williamson 1996; Simberloff and others 1997), increased news coverage (for example, McKinley 1996; Simons 1997), and widespread appeals for action (for example, Glowka and de Klemm 1996). Nevertheless, there is no evidence that the flow of exotics is decelerating under the pressures of increased trade and tourism described above. What else must be done?

Glowka and de Klemm (1996) feel that inclusion of nonindigenous species as a priority item for the Conference of Parties to the Convention on Biological Diversity, which has been ratified by 172 nations, is necessary to prevent a fragmented approach to the problem. Schmitz and Simberloff (1997) see the effort in the United States as also bedeviled by fragmentation. In short, as long as one program deals with aquatic plants, another aquatic animals, another agricultural weeds, and yet another bird introductions, the effort is bound to be frustrated if only because species often interact synergistically to generate an environmental or economic problem (Simberloff 1997b). Furthermore, because nonindigenous species do not recognize political boundaries, both regulatory and management responsibility must also cross for them to be effective. Thus, the Convention on Biological Diversity, an international instrument, is highly appropriate as one locus of action. It is important to observe, however, that, even if no species were henceforth able to cross a national border, introduced species would still be a major problem. In the United States, for example, interstate movement of introduced species has the same effect as importing such species from other countries: ecosystems are subjected to invasion and disruption by species that have evolved elsewhere. And, within-country transport can threaten invasion of neighboring countries.

A major current lacuna is a comprehensive database on introduced species that is associated with an early-warning system and a rapid-response team. For most taxa in most countries, someone who finds a species suspected to be nonindigenous and potentially invasive has nowhere to turn to examine this possibility. There is no emergency telephone number to use to determine whether it is a newly recorded species or a species that is spreading after introduction. Even if

Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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there were an organization charged with receiving such queries, there is no list of species to which it could turn to give an answer. For most species, there is no systematic effort to record where they have been introduced or their suspected effects. And there is rarely a procedure in place to respond rapidly to a newly recorded invasion, partly because of the fragmentation of authority described above.

The white-list approach advocated by Ruesink and others (1995) and Wade (1995) and discussed above needs to be adopted in some form both nationally and internationally. Black lists have never worked well, and the inherent unpredictability and idiosyncrasy of introductions dictate that all potential introductions be subjected to scrutiny—with no blanket exceptions. That requirement, of course, would mean that funding would be needed to process applications and to give them the necessary attention. Whether the costs of white listing are borne by the party wishing to import a species or by society as a whole will have to be addressed. For that matter, so will the costs of an unforeseen disaster if a white-listed species turned out not to be innocuous. Should an applicant be required to post a bond? Should an applicant be able to be indemnified by purchasing disaster insurance? Should society as a whole bear the cost? These matters have barely been broached.

How a species proposed for introduction should be assessed is yet another crucial issue that has been at best cursorily considered. As noted above, standard risk-assessment procedures for chemical and physical stressors do not appear to work well for biological introductions, for which the probabilities of such events as evolution and long-distance dispersal are so difficult to evaluate as to be mere guesses (Simberloff and Alexander 1998). The concatenation of guesses and arbitrary assignment of risk categories that pervades the current USDA risk-assessment procedure (see, for example, USDA 1991) hardly seems scientific, but no general alternative has been widely considered (O'Brien 1994). Having agreed that risk assessment will be the appropriate procedure to adjudicate disputes, we must determine how to do risk assessments en masse for nonindigenous species.

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Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Page 329
Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Page 330
Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
×
Page 331
Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
×
Page 332
Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Page 333
Suggested Citation:"5 Threats to Sustainability." National Academy of Sciences and National Research Council. 1997. Nature and Human Society: The Quest for a Sustainable World. Washington, DC: The National Academies Press. doi: 10.17226/6142.
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Nature and Human Society: The Quest for a Sustainable World Get This Book
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From earliest times, human beings have noticed patterns in nature: night and day, tides and lunar cycles, the changing seasons, plant succession, and animal migration. While recognizing patterns conferred great survival advantage, we are now in danger from our own success in multiplying our numbers and altering those patterns for our own purposes.

It is imperative that we engage again with the patterns of nature, but this time, with awareness of our impact as a species. How will burgeoning human populations affect the health of ecosystems? Is loss of species simply a regrettable byproduct of human expansion? Or is the planet passing into a new epoch in just a few human generations?

Nature and Human Society presents a wide-ranging exploration of these and other fundamental questions about our relationship with the environment. This book features findings, insights, and informed speculations from key figures in the field: E.O. Wilson, Thomas Lovejoy, Peter H. Raven, Gretchen Daily, David Suzuki, Norman Myers, Paul Erlich, Michael Bean, and many others.

This volume explores the accelerated extinction of species and what we stand to lose--medicines, energy sources, crop pollination and pest control, the ability of water and soil to renew itself through biological processes, aesthetic and recreational benefits--and how these losses may be felt locally and acutely.

What are the specific threats to biodiversity? The book explores human population growth, the homogenization of biota as a result in tourism and trade, and other factors, including the social influences of law, religious belief, and public education.

Do we have the tools to protect biodiversity? The book looks at molecular genetics, satellite data, tools borrowed from medicine, and other scientific techniques to firm up our grasp of important processes in biology and earth science, including the "new" science of conservation biology.

Nature and Human Society helps us renew our understanding and appreciation for natural patterns, with surprising details about microorganisms, nematodes, and other overlooked forms of life: their numbers, pervasiveness, and importance to the health of the soil, water, and air and to a host of human endeavors.

This book will be of value to anyone who believes that the world's gross natural product is as important as the world's gross national product.

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