Human Population and Consumption: What are the Ecological Limits?


The phenomena of human population growth and its impacts are all too apparent; is the ecological community willing to ignore the most pressing social and scientific issue of all time?--Pulliam and Haddad, 1994

Whether it is because the subject lies at the interface of the physical and social sciences, or because the complexity and contention inherent in the issue provokes reluctance, the fact remains that human population growth is not a topic fully accepted for either research or debate. In an effort to more thoroughly integrate human population concerns into mainstream ecology, the Sustainable Biosphere Initiative Project Office and the Environmental Protection Agency Office of Research and Development organized a symposium titled "Human Population and Consumption: What Are the Ecological Limits?" at the 1996 Annual Meeting of the Ecological Society of America. Symposium participants were invited to rise to Pulliam and Haddad's challenge by presenting ecological research opportunities related to the effects of human population growth.

Though human population growth may not be a current ecological research priority, it was clear from the symposium's large attendance that a great deal of interest in the subject exists. Addressing the standing-room-only crowd were a number of prominent researchers. David Pimentel, a professor at the College of Agriculture and Life Sciences at Cornell University, discussed the natural resource constraints caused by expansion of the human population. Stressing the importance of interdisciplinary efforts in discovering how to best sustain earth's intricate life support systems was Gretchen Daily, a research scientist with Stanford University. Donald Ludwig of the Departments of Mathematics and Zoology at the University of British Columbia assessed the role of economics in human population growth and consumption. William Rees, also of the University of British Columbia, introduced the concept of the "ecological footprint," or the amount of land needed to sustain a population and its consumptive habits, contending that if the current consumption rates of industrial countries were to spread to lesser developed nations, an area equivalent to two extra planets would be needed to sustain human life. Joel Cohen, Professor of Populations at the Rockefeller University, examined the definition of human carrying capacity, emphasizing the role of human choices in determining how many people the earth can support. Ron Pulliam, Special Assistant to the Secretary of the Interior, concluded with a synthesis of many of the points raised by the speakers. During the symposium, three broad themes emerged: 1) the physical threats and constraints posed by human population growth and increasing consumption; 2) the need for more comprehensive, interdisciplinary efforts; and 3) new research opportunities for ecologists.

Physical Threats and Constraints

Though the opinions of symposium participants varied on many of the diverse issues surrounding human population, all acknowledged that existing evidence of global change caused by a continually increasing human population cannot be ignored. Speakers identified several constraints produced by an expanding populace and increasing consumptive habits, including a decreasing food supply, climbing energy usage, and disappearing water supplies. Additionally, many threats to human health, such as the emergence of infectious disease and antibiotic resistance to medication, were mentioned.

David Pimentel noted that approximately two billion of the six billion people inhabiting the world today are malnourished. Ensuring that people are fed is directly dependent on agricultural productivity, which is reliant upon the availability of land, energy, and water resources, as well as biodiversity. Unfortunately, according to Pimentel, each of these elements is in jeopardy.

Pimentel cited increasing levels of soil erosion as one of the main hazards to productivity, noting that 30% of the world's arable land has been lost or abandoned because of erosion. This land will not soon be recovered; though ten centimeters may disappear in a year, it takes 500 years to replace just one inch of top soil. Pimentel pointed out that this loss leads to increasing use of forested land for agricultural purposes, further exacerbating the problem of deforestation.

Furthermore, the amount of fossil fuel energy used in agriculture production continues to rise. Though during the early 1920's the United States was 91% dependent on wood for energy, today the U.S. is 96% dependent on fossil fuels. In fact, Pimentel noted that we burn 40% more fossil fuel energy than the total amount of energy captured by plants annually in the United States. As human populations continue to grow, greater amounts of energy resources will be needed to produce food, as energy is needed in many stages of agricultural production, including irrigation, fertilization, and the creation of pesticides. Unfortunately, fossil fuels are finite resources and once they are gone, they are gone for good.

A growing human population also poses a hazard to water quantity and quality. For example, Pimentel noted that massive quantities of water are required to grow crops, such as corn, which requires 4.2 million liters/ha of water. In fact, almost 87% of the fresh water pumped in the world is consumed by agriculture. This increases the threat to overdrawing already diminishing water supplies.

Stating that the "physical limits to human activities are increasingly evident in the deteriorating condition and growing scarcity of essential resources," Gretchen Daily addressed the human health risks created by unsanitary water supplies, including the emergence and spread of the many diseases transmitted by water. In addition to poor water quality, Daily noted several other factors threatening human welfare, such as the risks posed by new strains of infectious disease that are increasingly resistant to antibiotic treatment.

A Comprehensive Approach

Though any discussion of the increasing human population would be incomplete without a survey of the physical constraints to growth, symposium participants emphasized the importance of integrating social and economic factors as well. Rather than looking solely at the physical threats and constraints, the solution to a problem as complex as human population growth demands cooperation between physical and social scientists. The types of issues raised by each of the speakers contain inherently interdisciplinary elements.

Asserting that "human influence on the planet has increased faster than human population," Joel Cohen elaborated on the concept of human carrying capacity. Though carrying capacity is ultimately determined by natural constraints, Cohen stressed the role of human choices about lifestyle and consumption in determining how many people the earth can sustain within those constraints.

Emphasizing the role of economics, Donald Ludwig contended that the real problem with human population growth is not biological, but societal. Ludwig asserted that a fundamental conflict exists between economics and ecology; economists believe growth to be essential, leading to increased consumption, while ecologists say growth is inherently limited.

Reaffirming Ludwig's opinion about the importance of economics, William Rees introduced the concept of the ecological footprint, a model that puts the economy inside the ecosphere, intrinsically imposing limits on growth based on availability of resources. The ecological footprint recognizes the interplay of economics and ecology by measuring not just the natural resources a place uses of its own, but those consumed from other countries.

Though it is Gretchen Daily's opinion that "people have difficulty looking beyond their own discipline for solutions," she stressed the importance of crossing both societal and economic boundaries in exploring the issue of human population growth. Daily believes that not only must scientists cooperate with others, but cooperation within the sciences is essential; scientists worldwide must begin to share information.

Role of Ecologists

Rising to the challenge posed by Pulliam and Haddad to further elucidate ways in which the ecological community can contribute to the human population debate, many suggestions arose. In addition to cooperating with social scientists, symposium participants identified several other ways ecologists can become involved.

David Pimentel contended that in the future, management of the natural resources used for food production must be handled carefully. To this end, Pimentel added, ecologists must become involved in developing better agricultural technologies.

Raising the critical point that "although improved technology can assist in more efficient production, it will never be able to increase the supply of vital natural resources," Pimentel believes the role of education to be invaluable; ecologists must urge policy makers to take action before a crisis arises. Pimentel asserted that ecologists must inform governments of actions that can be taken to minimize damage, proposing the best way to do so is with widely published and available scientific reports.

Gretchen Daily maintained that ecologists can best contribute to the human population growth and consumption debate by answering the question: How do we best manage earth's life support systems? In achieving this goal, Daily believes scientists can "elevate the public understanding of life support systems; the value, function, and trends of biodiversity and ecosystem services such as production of goods, filtering and mitigation of pollution, and stabilization of communities." Specifically, Daily stated that ecologists can begin by:

  • identifying ecological services and the systems in which they operate, not leaving valuation of natural resources to economists alone;
  • evaluating the impact of human activities on ecosystems;
  • assessing the ways in which ecosystems are already impaired and which systems are amenable to repair;
  • emphasizing human dependence on biodiversity;
  • developing new technology for identifying and mitigating environmental problems;
  • monitoring to ensure that goals are achieved; and
  • developing broad consensus statements from the scientific community about specific issues.

It is Donald Ludwig's opinion that progressing toward a solution to the human population growth problem will require an assessment of the economic principles that guide our actions. Ludwig was critical of the "economics of overexploitation," stating that the way in which natural resources are currently valued distorts their true value, mainly because the future costs of current consumption rates are not considered. Asserting that special interests currently control this valuation, Ludwig emphasized the importance of grass roots actions in reversing this trend. Ludwig contended that a grass roots movement could become fundamental to ensuring that subsidies for natural resource exploitation are removed and that a carbon tax is established.
William Rees argued that most policy makers do not yet understand the "socioeconomic implications of shifting to ecologically sustainable development patterns." In his words, "better data and better facts do not necessarily make for better decision making." Rees contended that politicians must be educated about the ecologically necessary solutions to sustainability. Said Rees, "To the extent that ecologists remain aloof from the political arena, national and global development policy will be dominated by economic analyses and political agendas that are conceptually blind to biophysical reality."

In addition, Rees suggested that urban planners calculate the ecological footprint for a specific region to discover how much land is needed to support a community and its consumptive habits without having to import energy and materials and export wastes. For example, most cities in developed countries have an ecological footprint two to three times their size, some countries have footprints as high as 14 times their size. He believes urban planners must begin to take into account more factors than are currently being evaluated, including increasing self-reliance for food production and water supply, protecting local ecosystems, and striving for zero-impact development.

Employing the concept of carrying capacity, Joel Cohen stated that ecologists can play a role in the human population debate by "illuminating human carrying capacity by increasing and applying ecological knowledge to clarify both the trade-offs available to humans and the consequences of possible choices." Using Cohen's model leads to the realization that the future is dynamic and uncertain and will continually change with time, depending on the quality of life people desire. Though this type of model makes exact numbers difficult to measure, Cohen believes that models for estimating carrying capacity "focus attention on, and provide a framework in which to interpret, quantitative measures of the relationship between rapid population growth and changing human carrying capacity."

Specifically, Cohen challenged ecologists to:

  • measure the effects of current actions;
  • evaluate the trade-offs between different choices;
  • educate the public; and
  • defend biocentric values.


In recognition of the fact that issues of human population growth are fundamental to sustainability, the original Sustainable Biosphere Initiative report featured a segment on human population growth which stated that "to more fully understand how human populations affect and are affected by ecological processes, the complex interfaces between ecology and social and economic sciences and policy analyses must be developed to a much greater extent (Lubchenco et al. 1991)." This symposium provided a venue for further elucidating the role of ecologists in pursuing this goal.

There is general agreement throughout the scientific community that growth of the human population, and the resultant increase in consumption, is exerting an unsustainable amount of pressure on global systems. Ecologists are accustomed to identifying natural constraints to growth; however, issues of consumption become much more complicated. An interdisciplinary approach is critical to incorporating the social and economic factors that influence the growth rate of human population and consumption. In addition to fostering interdisciplinary communication, ecologists must provide basic information on trends, continue to research how natural systems work, and educate the public and policy makers.

Symposium participants identified many constraints to slowing human population growth which will require huge societal and economic change. Although this symposium was a good start, the issues raised by the participants must be used as stepping stones to more clearly defining the role of ecologists and identifying specific research opportunities related to human population growth and consumption.

Literature Cited

Lubchenco, J., A.M. Olson, L.B. Brubaker, S.R. Carpenter, M.M. Holland, S.P. Hubbell, S.A. Levin, J.A. MacMahon, P.A. Matson, J.M. Melillo, H.A. Mooney, C.H. Peterson, H.R. Pulliam, L.A. Real, P.J. Regal, and P.G. Risser. 1991. The Sustainable Biosphere Initiative: An Ecological Research Agenda. Ecology 72(2):371-412.

Pulliam, H.R., and N.M. Haddad. 1994. Human Population Growth and the Carrying Capacity Concept. Bulletin of the Ecological Society of America 75(3):141-157.

Dr. Elizabeth Stallman
Program Manager
Science Program
Ecological Society of America
Washington, DC

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