Background Information

Ecology includes a broad and fascinatingly complex range of topics related to the interrelationships between living organisms and their nonliving environments. Ecologists classify these interactions into a hierarchy based on the complexity of interrelationships that occur between living (biotic) components, and nonliving (abiotic) components (such as soil, water, weather, temperature, pollutants, light, and nutrients). At the simplest level of this hierarchy, organismal ecology focuses on the interactions between individual organisms and the environment. Individual organisms can be grouped into a population, which is defined as a group of individuals of the same species that occupy and are adapted to a particular geographic area at the same time. Populations of different species that inhabit a particular area at the same time are grouped together into communities. All of the organisms and nonliving components within a given area are classified as an ecosystem. At the highest level in this hierarchy, all ecosystems on earth are part of the biosphere.

Population biologists are interested in the many factors that influence the size, health, growth, and distribution of a population. The scientific study of the factors that affect population size and growth over time is called demography. Two important factors that affect population growth are population density and the spacing or dispersion of individuals within the geological boundaries in which a population lives.

Ecologists have established a variety of methods for estimating the density of a population and for studying density-dependent and density-independent factors. Depending on the abundance and wariness of a species, sampling techniques can include signs of an organism such as nests, droppings, tracks, and reduction of specific forage.

Ecologists who study dispersion are particularly interested in the patterns of spacing that many species exhibit within a particular geographical range. Interactions of individuals within a population and the influence of abiotic factors will affect the spacing patterns of individuals within their geographical range. These patterns can involve grouped (clumped), evenly spaced (uniform), and unpredictable (random) dispersions of organisms.

Both density and dispersion of a population are directly influenced by three primary factors: (1) reproduction or birth rate (fertility), (2) death rate (mortality), and (3) migration. In migration, the addition of an organism to a population is known as immigration while the loss of an organism from a population is called emigration. Population growth remains the same (zero population growth; ZPG) if the number of individuals that join a population, either by immigration or by birth, is equal to the number of individuals that leave a population by either emigration or death. If the number of births and immigrating organisms exceeds the number of deaths and emigrating organisms, then population growth occurs. The reverse is true when a population is on the decline. In addition, both the birth rate and death rate of a population rely on the age of its members (age structure). For example, a population that has a higher percentage of older individuals that may be past their reproductive prime compared to younger individuals is likely to show a higher degree of mortality compared to birth rate.

Given the aforementioned factors and additional factors such as generation time and sex ratios, biologists, demographers, and sociologists can use statistical data and computer programs to predict future changes in the size of human populations. When studying populations of other organisms, factors such as competition and predator-prey relationships must also be considered.

Human population growth has wide-reaching impacts on human health and the infrastructure, resources, and carrying capacity of a given geographic region. The importance of understanding human population growth is exemplified by the following quote from the opening chapter of The Population Explosion, by Paul and Anne Ehrlich: "The population of the United States is increasing much more slowly than the world average, but it has more than doubled in only six decades; from 120 million in 1928 to 240 million in 1990. Such a huge population expansion within two or three generations can by itself account for a great many changes in the social and economic institutions of a society. It is also very frightening to those of us who spend our lives trying to keep track of the implications of the population explosion."

DemographyLab is designed to simulate how different demographic factors may influence human population size and growth, although most of the factors that you will consider in this lab have similar effects on the population growth of other animals and plants.


  1. Kates, R. W. "Sustaining Life on the Earth." Scientific American, October 1994.

  2. Raloff, J. "The Human Numbers Crunch." Science News, June 1996.

  3. Ehrlich, P. R., and Ehrlich, A. H. The Population Explosion. New York: Simon & Schuster, 1990.