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Introduction to Populations

Author(s): Deanne Erdmann, MS

Population Density

Population density is a measure of the number of individuals of the same species living in a designated unit of space. It is influenced by relationships among organisms, movement of individuals in and out of the habitat, resources, and abiotic environmental factors (such as climate). Fluctuations in population density can be indications of changes in the environment.

Carrying capacity is the maximum number of organisms in a population that can be supported by a particular habitat. Many factors determine carrying capacity, some of which are influenced by the density of the population, while others are not. Density-dependent factors in an environment might be influenced by available food, water, and shelter. Density-independent factors include all facets of weather and climate, such as droughts, storms, and volcanic eruptions.

It often is difficult to determine the size of a population because of the wide range of the habitat or mobility of the organisms. In such cases, ecologists use a variety of sampling methods. For instance, a designated area of study might be sectioned into grids or plots. Numbers of organisms counted in selected grids are extrapolated to estimate the total population size. Mark-and-recapture is another method used to estimate population size in large geographic areas. Traps are set in the study area. Trapped organisms are tagged and released. After a period of time, traps are set again, and calculations are made based on the number of marked organisms that are recaptured.

Total population = total size of 2nd sample  X  marked # in 1st catch
                                      marked  # recaptured in 2nd catch