Population Attributes: Size, Density and Sex Ratio

A single organism has an age, a sex, and (while alive) a state of health, but it does not have a "birth rate" or a "density." These are emergent properties that appear only once individuals are considered together as a population -- a group of individuals of one species occupying a defined geographical area at a given time, sharing or competing for the same resources, and capable of interbreeding.

The most basic property of a population is its size, the total number of individuals present. In practice, ecologists usually work not with raw population size but with population density, the number of individuals per unit area or per unit volume of habitat (for example, the number of tigers per square kilometre of forest, or the number of diatoms per litre of pond water). Density is more informative than raw size alone, because it allows meaningful comparison between areas of different sizes and gives a sense of how crowded, and therefore how intensely competitive, a habitat is for its inhabitants.

Counting every individual in a population is not always straightforward, and ecologists sometimes have to use indirect or alternative measures of "how much" of a species is present. For very large, mobile, or hard-to-count populations, such as fish in the open ocean, actual density may be almost impossible to determine precisely, and an indirect index -- such as the number of fish caught per trawl -- may be used instead as a workable proxy. In other cases, counting discrete "individuals" is not even conceptually simple: a colonial organism such as a large banyan tree may, botanically, be considered a single, sprawling individual even though it appears to occupy the space of an entire small grove through its aerial prop roots, while some plants growing densely as a continuous mat are more sensibly measured by percentage area covered or by total biomass rather than by counting discrete "individual" plants. …