Question

Why is population relevant to the environment?

Answer 1

Humans have sought to understand the relationship between population dynamics and the environment since the earliest times (1, 2), but it was Thomas Malthus’ Essay on the Principle of Population (3) in 1798 that is credited with launching the study of population and resources as a scientific topic of inquiry. Malthus’ famous hypothesis was that population numbers tend to grow exponentially while food production grows linearly, never quite keeping pace with population and thus resulting in natural “checks” (such as famine) to further growth. Although the subject was periodically taken up again in the ensuring decades, with for example George Perkins Marsh’s classic Man and Nature (1864) (4) and concern over human-induced soil depletion in colonial Africa (5, 6), it was not until the 1960s that significant research interest was rekindled. In 1963, the U.S. National Academy of Sciences published The Growth of World Population (7), a report that reflected scientific concern about the consequences of global population growth, which was then reaching its peak annual rate of two percent. In 1968, Paul Ehrlich published The Population Bomb (8), which focused public attention on the issue of population growth, food production, and the environment. By 1972, the Club of Rome had released its World Model (9), which represented the first computer-based population-environment modeling effort, predicting an “overshoot” of global carrying capacity within 100 years.

Clearly, efforts to understand the relationship between demographic and environmental change are part of a venerable tradition. Yet, by the same token, it is a tradition that has often sought to reduce environmental change to a mere function of population size or growth. Indeed, an overlay of graphs depicting global trends in population, energy consumption, carbon dioxide (CO2) emissions, nitrogen deposition, or land area deforested has often been used to demonstrate the impact that population has on the environment. Although we start from the premise that population dynamics do indeed have an impact on the environment, we also believe that monocausal explanations of environmental change that give a preeminent place to population size and growth suffer from three major deficiencies: They oversimplify a complex reality, they often raise more questions than they answer, and they may in some instances even provide the wrong answers.

As the field of population-environment studies has matured, researchers increasingly have wanted to understand the nuances of the relationship. In the past two decades demographers, geographers, anthropologists, economists, and environmental scientists have sought to answer a more complex set of questions, which include among others: How do specific population changes (in density, composition, or numbers) relate to specific changes in the environment (such as deforestation, climate change, or ambient concentrations of air and water pollutants)? How do environmental conditions and changes, in turn, affect population dynamics? How do intervening variables, such as institutions or markets, mediate the relationship? And how do these relationships vary in time and space? They have sought to answer these questions armed with a host of new tools (geographic information systems, remote sensing, computer-based models, and statistical packages) and with evolving theories on human-environment interactions.

This review explores the ways in which demographers and other social scientists have sought to understand the relationships among a full range of population dynamics (e.g., population size, growth, density, age and sex composition, migration, urbanization, vital rates) and environmental changes. With the exception of the energy subsection, the focus is largely on micro- and mesoscale studies in the developing world. We have surveyed a wide array of literature with an emphasis on peer-reviewed articles from the past decade, but given the veritable explosion in population-environment research, we hasten to add that this review merely provides a sampling of the most salient findings. It then proceeds to provide a state-of-the-art review of studies that have examined population dynamics and their relationship to the following environmental issue areas: land-cover change and deforestation; agricultural land degradation and improvement; abstraction and pollution of water resources; coastal and marine environments; and energy, air pollution, and climate change. In the concluding section, we relate population-environment research to the emerging understanding of complex human-environment systems.