The Twenty-First Century: The World at Carrying Capacity BY GARY W. BARRETT AND EUGENE P. ODUM
uch has been written in recent years regarding the need to live within a society that sustains its resources for the future, a goal that requires implementing plans for the future based on the concept of sustainable development (e.g., Lubchenco et al. 1991, Huntley et al. 1991, NCR 1991, Heinen 1994, Goodland 1995). A forum on "Perspectives on Sustainability," which appeared in Ecological Applications (November 1993), attempted to summarize many of the earlier perspectives surrounding this topic. Unfortunately, considerable confusion remains, especially among the citizenry, as to what is meant by sustainable development. Dictionaries define "to sustain" as "to hold;' "to keep in existence;' "to support;' "to endorse without failing or yielding;' "to maintain;' or "to supply with necessities or nourishment to prevent from falling below a given threshold of health or vitality." Given these definitions, the businessperson often views sustainability as sustaining profits based on everincreasing consumption of limited natural resources or sustaining rapid economic growth forever! At the other extreme, the definition in the widely cited Brundtland report (WeED 1987)-namely, that "sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs" (p. 8)-is so vague as to be impossible to quantify or implement. In terms of human affairs over the long term, sustainability may more effectively be understood and dealt with in terms of a two-dimensional carrying capacity concept, which considers not only numbers but also per capita impact at the ecosystem and ecosphere levels (Ehrlich and Holdren 1971, Catton 1987). "Carrying capacity" is a concept that was originally developed by ecologists with reference to the number of animals of a given species that can be supported without injury to the habitat (i.e., carried over long periods of time without reducing the capacity of the habitat to support that many animals in the future; for details, see Pulliam and Haddad 1994). When referring to humans, however, just considering numbers (density) is not enough because individuals can differ greatly in the intensity of their impact on the environment. An American citizen, for example, may consume 100 or more times
M
Gary w: Barrett (e-mail:
[email protected]) is Odum Professor of Ecology, and Eugene P. Odum is Director Emeritus, at the Institute of Ecology, University of Georgia, Athens, GA 30602-2202. © 2000 American Institute of Biological Sciences.
as much energy and resources as a citizen of a developing country. In other words, affluence reduces the number of people that can be supported by a given resource base. Thus, ecologists have to be concerned not only with density but also with per capita demands. From another viewpoint, ecosystem development parallels societal development in that both will inevitably end up in a non-equilibrium pulsing stable state (Odum et al. 1995) in which respiration (R; i.e., maintenance) on the average does not exceed production (P). If the energy, money, or technology available to maintain a complex system are inadequate, then the system becomes disorderly and soon defaults or dies. From this functional or energetic viewpoint, carrying capacity is reached when PIR is approximately 1. To use carrying capacity as an index for sustainability, two major concerns must be addressed: the tendency for growth to overshoot carrying capacity and the possibility that the optimum carrying capacity is less than the maximum. Figure 1 depicts the two contrasting growth forms: the sigmoid, in which growth levels off as limits (K) are approached, and the exponential, in which the momentum results in overshooting limits, creating a "boom-andbust" pattern. As pointed out by Wiegert (1974), these forms represent the slowest and fastest growth forms, with many populations exhibiting intermediate patterns of growth. The overshoot pattern occurs in nature and, increasingly, in human affairs (Catton 1980). For example, Barkley and Seckler (1972) listed four factors that force or encourage urban development to overshoot the optimum: detrimental self-crowding effects (e.g., pollution, congestion, and rising costs of schools, taxes, and police protection) are not felt until sometime after the optimum density has been exceeded; money is seldom made available for growth management or land-use planning until congestion and traffic become major problems; political power is concentrated within a few wealthy or "keystone" groups who benefit more from growth in size or quantitative growth than the average-income citizen; and the mystique of growth persists from the pioneer days, when rapid growth and development were necessary and desirable. Although the self-crowding problems that large, rapidly growing cities are experiencing are generally blamed on poor fiscal management, in reality such cities have overshot not only their economic support base but also their regional life support base. Wackernagel and Rees (1996) present several regional examples of populations that April 2000 / Vol. 50 No.4' BioScience 363
Downloaded from https://academic.oup.com/bioscience/article-abstract/50/4/363/270776 by guest on 31 July 2018
Roundtable Figure 1. The contrasting sigmoid (S-shaped) and exponential (Jshaped) growth form models in relation to the maximum (Km) and optimum (Ko) carrying capacity concepts. In this case, growth refers to increase in numbers of humans.
Maximum Carrying Capacily( Km)
Optimum Carrying Capacity (Ko)
1
Sigmoid (S-Shaped) Growth
Exponential (J-Shaped) Growth
Time
•
require an area that is much larger than their home territory to support their present consumer lifestyles. For example, the "ecological footprint" of Vancouver spans the Lower Fraser Valley and is 19 times larger than the area of Vancouver itself. With rapid urbanization occurring throughout the world, these predicaments are becoming global in extent. Thus, land-use planners, policymakers, economists, and resource ecologists need to consider the merits of downsizing based on a carrying capacity that is more sustainable over the long term. In Figure 1, we show the overshoot followed by a downsizing to a lower carrying capacity level (KJ, which we term "optimum carrying capacity." We present this pattern as an optimistic prediction for the future. In other words, after experiencing the trauma and disorder of many booms and busts, societal thinking and planning will reorient to emphasize qualitative rather than quantitative development (Odum 1975). Qualitative development would prevent the kind of living on the edge in which, for example, a I-year regional crop failure would cause widespread starvation, as is now occurring in North Africa. A related concept, "safe carrying capacity," comes out of long-term studies of animal populations. For example, Paul Errington, who is well known for his lifelong studies of muskrat and mink populations in Iowa freshwater marshes, observed that muskrats that had secure dens near feeding areas were much less vulnerable to predation by mink than muskrats that did not have such high-quality housing (Errington 1963). The number of muskrats with safe dens represented the safe carrying capacity (KJ, which was almost always less than the maximum number that could be supported by the food supply in the absence of predators (i.e., the maximum carrying capacity, Km). 364 BioScience • April 2000 I Vol. 50 No.4
Downloaded from https://academic.oup.com/bioscience/article-abstract/50/4/363/270776 by guest on 31 July 2018
Such examples from natural populations could serve as models for human population growth as well. Indeed, not too long ago, politicians used to talk about "the greatest good for the greatest numbers" as a goal for society. But this slogan is rarely heard now because society is finding out by experience that the greatest good, in terms of quality of life for the individual, comes when the numbers are not as high as they can possibly be-and when the per capita impacts are not maximized, either. Fortunately, human population growth seems to be beginning to level off in sigmoid fashion, with a projected plateau at 904 billion by the year 2050 and IDA billion by the year 2100 (Bongaarts 1998, UN 1998). Current world human population density may, in fact, be just beyond the point of inflection in the sigmoid growth model (see Figure 1). The earth can probably feed more than 10 billion people, even with increasing affluence, although if such a density proves to exceed the optimum carrying capacity for a quality lifestyle, as many ecologists believe will be the case (e.g., Smail 1999), then there will have to be a period of negative growth in population and per capita consumption. It seems likely that such a period of negative growth and per capita consumption will come soon. People in developed countries are increasingly becoming concerned about overconsumption, waste, unwanted babies, and the increasing gap between rich and poor worldwide. However, the momentum of rapid population growth is such that downsizing is not likely to occur until after the overshoot. Of course, some "cornucopian optimists" believe that developing technologies (hydrogen economy, wasteless industries, landless agriculture) will enable 10 billion people to coexist with enough natural environment to provide for the necessary life support, preservation of endangered species, and enjoyment of nature (see AusubeI1996). New technologies will certainly increase the carrying capacity of the earth for humans, but, as Paul Gray, former president of the Massachusetts Institute of Technology, has said, "A paradox of our time is the mixed blessing of almost every technological development" (Gray 1989, p.
192). Many technologies, such as fission atomic energy or deep-sea drilling for oil, cost more than they are worth (i.e., they have provided little or no net energy). Even Donella Meadows and colleagues, whose 1972 book Limits of Growth predicted overshoots unless economic policies changed (Meadows et al. 1972), are now somewhat optimistic that the worst "booms and busts" can be avoided, even though they document cases where limits have been exceeded (Meadows et al. 1992). We contend, therefore, that there exist limits (e.g., net energy and feedback controls) to human popUlation growth, especiallyas related to the quality of human existence, and that these limits and regulatory processes are increasingly coming into play. Rather than expecting continuing unregulated human population growth well into the new century, society should plan for a future based on a sustainable optimum carrying capacity (Ko )' Such planning would benefit from a better understanding of how to integrate economic capital with nature's capital.
140
130
120
110
S
~
0" .... en
100
><
"
."
.E
90 Economic Welfare (Quality of Life)
80
70
1950
1955
1960
1970
1965
1975
1980
1985
1990
Year
Figure 2. Recent trends in the gross national product (GNP) index and the Index of Sustainable Economic Welfare (ISEW) in the United States, with the threshold suggested as the optimum carrying capacity. Open circles, GNP index; solid boxes, ISEW index. Figure modified from Max-Neef (1995), with terms in parentheses (on the curves) added by the authors.
The carrying capacity concept as related to economics The urgent need to integrate nonmarket goods and services (natural capital) into mainstream economic systems (see Barrett et al. 1998) is the focus for this special issue of BioScience. This need has long been recognized but poorly implemented. For example, in the 1960s, economist Kenneth Boulding wrote about the need to move from quantitative (getting bigger) to qualitative (getting better) economics. Although his books (e.g., A Reconstruction of Economics; Boulding 1962) and papers (Boulding 1966a, 1966b) were admired and he was one of the first economists to be elected to the National Academy of Sciences, mainstream economists paid little attention. The time was not yet right to implement his ideas. In the 1970s, Howard and Eugene Odum suggested using energy as a common denominator to evaluate and combine both market and nonmarket values (Odum 1971, 1973, Odum and Odum 1972, Gosselink et al. 1974). Because it takes energy within an economy to make money, energetic values can be converted to monetary values. Dollar values for wetlands calculated on this basis were impressive enough to the citizenry to playa major role in coastal marsh protection legislation of Atlantic and Gulf Coast wetlands in the early 1970s. However, economists of that time objected strenuously to the conversion of energy values to monetary values. They contended that value and price were determined by people's "willingness to pay" and not by the amount of energy required to produce a prod-
uct or service (a quantity now labeled "eMergy"; Odum 1996). The Odums and economists Leonard Shabman and Sandra Batie engaged in a point-counterpoint discussion of this difference in the pages of the Coastal Zone Management Journal (Shabman and Batie 1978, Odum 1979a, Odum 1979b). Again, the time had not yet come for any kind of reconstruction of economics. A weak ecological economics infrastructure during the 1970s most likely impeded the integration of ecology and economics at that time. However, during the 1980s and 1990s, the time was right for a serious dialogue between ecologists and economists. Ecologist Robert Costanza and economist Herman Daly collaborated with many others to establish the International Society for Ecological Economics and a new journal, Ecological Economics. For example, economist Manfred Max-Neef (1995) compared trends in the gross national product (GNP) index with the Daly-Cobb (1989) Index of Sustainable Economic Welfare (Figure 2). The two indexes track one another and then separate at a time known as the economic welfare threshold. Figure 2 represents the United States' situation, but Max-Neef (1995) presented graphs depicting similar trends for all of the countries in western Europe, although the threshold point comes a bit later in time in some of these countries. We suggest that the economic welfare threshold is equivalent to optimum carrying capacity because it represents the point at which increasing returns of scale (bigger is better) changes to decreasing returns of scale (bigger is April 2000 / Vol. 50 No.4' BioScience 365
Downloaded from https://academic.oup.com/bioscience/article-abstract/50/4/363/270776 by guest on 31 July 2018
Figure 3. Converting a bifurcated aesthetic perspective of nature's capital and human market capital to an integrative landscape perspective in which a coevolutionary (urban-rural) system emerges that includes all goods and services necessary to sustain a quality life for all of Earth's inhabitants.
...
-
HUMANKIND'S GOODS AND SERVICES
..
... NATURE'S GOODS AND SERVICES
1/ COEVOLUTIONARY (URBAN-RURAL) LANDSCAPE DEVELOPMENT
no longer better). It would seem, therefore, that economic growth measured by GNP is beginning to overshoot what might be called "quality of life carrying capacity" in the developed countries. It is important to distinguish between economic growth and economic development if society is to achieve a quality of life carrying capacity. A 1991 report from the United Nations Educational, Scientific and Cultural Organization (Goodland et al. 1991) makes a distinction between economic growth, which involves getting larger (quantitative growth), and economic development, which involves getting better (qualitative growth) without increasing the total consumption of energy and materials beyond a level that is reasonably sustainable. The report concludes that "a five-to-tenfold expansion of anything remotely resembling the present economy [which some economists say is necessary to reduce poverty worldwide 1 would simply speed us from today's long-run unsustainability to imminent collapse" (pp. lO-ll). Therefore, the report goes on, the economic growth required for poverty reduction (especially in the less-developed countries) "must be bal366 BioScience • April 2000 I Vol. 50 No.4
Downloaded from https://academic.oup.com/bioscience/article-abstract/50/4/363/270776 by guest on 31 July 2018
anced by negative throughput growth for the rich." A noteworthy roundtable discussion of carrying capacity and ecological economics by Daly (1995) and Mark Sagoff (1995) has helped to continue this dialogue among ecologists and economists. Costanza et al. (1997) have estimated that the value of the total biospheric natural ecosystem services in monetary terms is greater than the value of the total world market goods and services. Although mainstream economists were quick to challenge the Costanza et al. (1997) initial estimate, it is encouraging that there now exists a serious discussion focusing on the quantitative (monetary) value of nature's goods and services. It is also encouraging that during the past 5 years there has been a flood of new books that might be classified under the heading of "the greening of business." A good example is Anderson (1998). Ray Anderson is CEO of a carpet company named Interface that leases, rather than sells, carpets. Carpets taken back to the factory when they are worn out are completely recycled and reconstituted into new carpets with almost no waste and pollution. Anderson says that he was inspired to make his "mid-course correction" after reading Paul Hawken's Ecology of Commerce: A Declaration of Sustainability (Hawken 1994).
Overview: An integrated (ecologic/economic) capitalism Human society is rapidly approaching, and in some aspects already overshooting, global carrying capacity, which we define as two dimensional (density and intensity of per capita use). We have made a case for the proposition that the optimum carrying capacity (Ko) is less than the maximum (K m ), and that carrying capacity is reached when the P/R ratio approximates 1. Accordingly, there exists an urgent need to bring together essential procedures and approaches that in the past were considered separate and unrelated operations (the "one problem-one solution" mind set). Some of these needs or strategies are as follows: • A need to bring together market and nonmarket goods and services as a basis for a more holistic economic and ecosystem/landscape management perspective. Odum
(l997a, 1998) suggests creating an integrated "dual capitalism" (i.e., natural and market capital; Figure 3) as a way to capture the substantial benefits that, as summarized by Daily et al. (1997), natural ecosystems supply to human societies. Cairns (1997) expresses this need in terms of promoting a co evolutionary, mutualistic relationship between human society and natural systems, rather than an individualistic, competitive view of ecology and economics. • A need to promote appreciation of the aesthetics, as well as the utility, of a diverse landscape that includes natural areas, clean streams and lakes, well-managed farmland, and attractive villages, towns, and cities all operating together to maintain a high quality of life (i.e., an integrative landscape perspective; Barrett 1985, 1992, Barrett et al. 1998). Many people view heavily fertilized and watered gardens, mowed lawns or clipped grasses, and yards of carefully swept soil (i.e., something to be weeded, manicured, or managed for the short term in accordance with a traditional economic purpose or a cultivated taste) as "beautiful" and natural, self-sustaining meadows, grasslands, or woodlands as "ugly." Figure 3 illustrates how this previously bifurcated aesthetic perspective is now being integrated into a holistic, coevolutionary landscape perspective. An informed and educated society that understands the goods and services provided by an integrated urban-rural landscape will likely conduct its business based on an educational incentive rather than a regulatory mandate (Barrett 1989). • A need to bridge the communication gap between science and society, which requires improving the dialogue between decision-makers (politicians, corporate managers, economists, lawyers)-who are, by and large, not scientists-and research scientists, who are increasingly more specialized (reductionist) and isolated from the realities of public policy. • Reorganizing colleges and universities to promote more interdisciplinary courses, including an integrative science approach (Barrett and Odum 1998), as is now under way in many colleges and universities, will help to develop C. P. Snow's "third culture;' in which the communication gap between the humanities and the sciences would be closed (see Odum 1997b). For this concept to come to fruition, a closer liaison must be established between basic and applied disciplines (Barrett 1984, 1994), which are often segregated in separate schools in academia, with little communication among them. • A need to unite rural and urban planning initiatives into comprehensive landscape-level planning (Barrett et al. 1998) that acknowledges the interdependence of the low-powered, rural, natural life-supporting ecosystems and the high-powered, urban, technoecosystems (term suggested by Naveh 1982). This
task is difficult because political lines often separate city from country. Ian McHarg pioneered this approach to planning three decades ago (e.g., McHarg 1969), but society was not as ready for this approach as it is now. Finally, society could learn from the study of the pioneer to mature natural ecosystem development (r to K selection). (Resources are diverted to high growth rate and fecundity, rather than persistence, in r-selected species; resources are directed to persistence, rather than high growth rate, in K-selected species under carrying capacity conditions.) Natural selection favors opportunistic, r-selected species in pioneer or early successional communities but K-selected species in mature systems, where survival and quality of the individual are more important than the quantity of production. Society could find ways to make a similar transition. In addition to developing these strategies to bring together scholars and practitioners of ecology and economics, it is important to do so quickly. Unfortunately, many of the ideas and concepts about sustain ability and carrying capacity being espoused in recent reviews and books have remained traditional and disciplinary in concept and implementation. The time has come to find ways to develop truly integrative ideas about sustainability and carrying capacity and to implement these as soon as possible, in view of the increasing threats to the health of the cohabitants of Earth. Indeed, Lester Brown recently noted that human society appears to be approaching a threshold regarding an environmental awakening, including shifting views concerning energy, urban transportation and planning, material use, and human population growth (Brown 1999). We sense that human society is ready to usher in new integrative (transdisciplinary) and ecological definitions of carrying capacity and sustainable growth as we enter the twenty-first century.
Acknowledgments w. B. thanks Almo Farina for inviting him to co-chair the plenary roundtable discussion entitled "Integrating Ecology and Economics" at the VII International Congress of Ecology during July 1998 in Florence, Italy. This plenary session served as the catalyst for this special issue of BioScience. We are indebted to Terry Barrett and three anonymous reviewers for their critical review of the manuscript and to Rebecca Chasan, editor-in-chief of BioScience, for her editorial skills in bringing this issue to fruition.
G.
References cited Anderson RC. 1998. Mid-course Correction: Toward a Sustainable Enterprise: The Interface Model. Atlanta (GA): Peregrinzilla Press. Ausubel JH. 1996. Can technology spare the Earth? American Scientist R4: 166-178. Barkley PW, Seckler DW. 1972. Economic Growth and Environmental Decay. New York: Harcourt Brace Jovanovich. Barrett GW. 1984. Applied ecology: An integrative paradigm for the 1980s. Environmental Conservation 11: 319-322.
Aprii2000 / Vol. 50 No.4· BioScience 367
Downloaded from https://academic.oup.com/bioscience/article-abstract/50/4/363/270776 by guest on 31 July 2018
Roundtable ~~~~~==~~~:=====~~" ___.1985. A problem-solving approach to resource management. BioScience 35: 423---427. ___.1989. A sustainable society. BioScience 39: 754. ___.1992. Landscape ecology: Designing sustainable agricultural landscapes. Pages 83-103 in Olsen RK, ed. Integrating Sustainable Agriculture, Ecology, and Environmental Policy. New York: Haworth Press. ___. 1994. Restoration ecology: Lessons yet to be learned. Pages 113-126 in Baldwin AD, De Luce J, PIetsch C, eds. Beyond Preservation: Restoring and Inventing Landscapes" Minneapolis (MN): University of Minnesota Press. Barrett Gw, Odum EP. 1998. From the President: Integrative science. BioScience 48: 980. Barrett GW, Barrett TA, Peles JD. 1998. Managing agroecosystems as agrolandscapes: Reconnecting agricultural and urban landscapes. Pages 197-213 in Collins WW, Qualset CO, eds. Biodiversity in Agroecosystems. Boca Raton (FL): CRC Press. Bongaarts J. 1998. Demographic consequences of declining fertility. Science 282: 419---420. Boulding K. 1962. The Reconstruction of Economics. New York: Science Editions. ___.1966a. The economics of the coming spaceship earth. Pages 3-14 in Jarrett H, ed. Environmental Quality in a Growing Economy: Resources for the Future. Baltimore: Johns Hopkins University Press. ___. 1966b. Economics and ecology. Pages 225-234 in Darling FF, Milton JP, eds. Future Environments of North America. New York: Natural History Press. Brown LR. 1999. Crossing the threshold. WorldWatch 12: 13-22. Cairns J Jr. 1997. Global coevolution of natural systems and human society. Revista de la Sociedad Mexicana de Historia Natural 47: 217-228. Catton WR Jr. 1980. Overshoot: Ecological Basis of Revolutionary Change. Urbana (IL): Illinois University Press. ___.1987. The world's most polymorphic species. BioScience 37: 413419. Costanza R, et al. 1997. The value of the world's ecosystem services and natural capital. Nature 387: 253-260. Daily GC, et al. 1997. Ecosystem Services: Benefits Supplied to Human Societies by Natural Ecosystems. Washington (DC): Ecological Society of America. Daly HE. 1995. Reply to Mark Sagoff's "Carrying capacity and ecological economics." BioScience 45: 621-624. Daly HE, Cobb JB. 1989. For the Common Good: Redirecting the Economy toward Community for the Environment and a Sustainable Future. Boston: Beacon Press. Ehrlich PR, Holdren J. 1971. The impact of population growth. Science 171: 1212-1217. Errington P. 1963. Muskrat Populations. Atnes (IA): Iowa State University Press. Goodland R. 1995. The concept of environmental sustainability. Annual Review of Ecology and Systematics 26: 1-24. Goodland R, Daly H, Serafy SE, von Droste B. 1991. Environmentally Sustainable Economic Development: Building on Brundtland. Paris: United Nations Educational, Scientific and Cultural Organization. Gosselink JG, Odum EP, Pope RM. 1974. The Value of the Tidal Marsh. Baton Rouge (LA): Center for Wetland Resources at Louisiana State University. Gray PE. 1989. The paradox of technological development. Pages 192-205 in Ausubel JH, Sladovich HE, eds. Technology and Environment. Washington (DC): National Academy Press. Hawken P. 1994. Ecology of Commerce: A Declaration of Sustainability. New York: Harper Business.
368 BioScience . April 2000 / Vol. 50 No.4
Downloaded from https://academic.oup.com/bioscience/article-abstract/50/4/363/270776 by guest on 31 July 2018
Heinen )T. 1994. Emerging, diverging and converging paradigms on sustainable development. International Journal of Sustainable Development and World Ecology 1: 22-33. Huntley Bj, et al. 1991. A sustainable biosphere: The global imperative. Ecology International 20: 5-14. Lubchenco j, et al. 1991. The sustainable biosphere initiative: An ecological research agenda. Ecology 72: 371---412. Max-Neef M. 1995. Economic growth and quality of life. Ecological Economics 15: 115-118. McHarg IL. 1969. Design with Nature. New York: Natural History Press. Meadows DH, Meadows DL, Randers j, Behrens WW. 1972. The Limits to Growth. New York: Universe Books. Meadows DH, Meadows DL, Randers J. 1992. Beyond the Limits: Confronting Global Collapse, Envisioning a Sustainable Future. Post Mills (VT): Chelsea Green Publishing. [NRC] National Research Council. 1991. Toward Sustainability: A Plan for Collaborative Research on Agricultural and Natural Resource Management. Washington (DC): National Academy Press. Naveh Z. 1982. Landscape ecology as an emerging branch of human ecosystem science. Advances in Ecological Research 12: 189-237. Odum EP. 1979a. Rebuttal of "Economic value of natural coastal wetlands: A critique!' Coastal Zone Management JournalS: 231-237. ___. 1997a. Ecology: A Bridge Between Science and Society. Sunderland (MA): Sinauer Associates. ___. 1997b. Can ecology contribute to C. P. Snow's third culture? Bulletin of the Ecological Society of America 78: 234. Odum EP, Odum HT. 1972. Natural Areas as Necessary Components of Man's Total Environment. Washington (DC): Wildlife Management Institute. North American Wildlife and Natural Resources Conference Proceedings no. 37. Odum HT. 1971. Environment, Power, and Society. New York: Wiley-Interscience. ___.1973. Energy, ecology, and economics. Ambio 2: 220---227. ___. 1975. Energy quality and the carrying capacity of the earth. Tropical Ecology 16: 1-8. ___. 1979h. Principle of environmental energy matching for estimating potential value: A rebuttal. Coastal Zone Management JournalS: 239241. ___. 1996. Environmental Accounting: Emergy and Environmental Decision Making. New York: John Wiley & Sons. Odum WE, Odum EP, Odum HT. 1995. Nature's pulsing paradigm. Estuaries 18: 547-555. Pulliam R, Haddad NM. 1994. Human population growth and the carrying capacity concept. Bulletin of the Ecological Society of America 75: 141-157. Sagoff M. 1995. Carrying capacity and ecological economics. BioScience 45: 610-620. Shabman LA, Batie SS. 1978. Economic value of natural coastal wetlands: A critique. Coastal Zone Management Journal 4: 231-247. Smail JK. 1997. Beyond population stabilization: The case for dramatically reducing global human numbers. Politics and the Life Sciences 16: 183-192. [UN] United Nations. 1998. World Population Prospects: The 1996 Revision. New York: United Nations. Wackernagel M, Rees W. 1996. Our Ecological Footprint: Reducing Human Impact on the Earth. Gabriola Island (Canada): New Society Publishers. Wiegert RG. 1974. Competition: A theory based on realistic, general equations of population growth. Science 185: 539-542. [WCED] World Commission on Environment and Development. 1987. Our Common Future. Oxford: Oxford University Press.
