# 3.2.2: Population Size

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Populations are dynamic entities. Their size and composition fluctuate in response to numerous factors, including seasonal and yearly changes in the environment, natural disasters such as forest fires and volcanic eruptions, and competition for resources between and within species. The study of populations is called demography.

## Population Size and Density

Populations are characterized by their population size (total number of individuals) and their population density (number of individuals per unit area; figure $$\PageIndex{a}$$). A population may have a large number of individuals that are distributed densely, or sparsely. There are also populations with small numbers of individuals that may be dense or very sparsely distributed in a local area. Population size can affect potential for adaptation because it affects the amount of genetic variation present in the population. Density can have effects on interactions within a population such as competition for food, the ability of individuals to find a mate, and diseases spread. (Dispersion patterns can also affect these factors; for example, a solitary species with a random distribution might have difficulty finding a mate when compared to social species clumped together in groups.) Smaller organisms tend to be more densely distributed than larger organisms (figure $$\PageIndex{b}$$).

Figure $$\PageIndex{a}$$: Both populations shown contain seven plants, so their populations size is the same. However, the population on the left has a higher population density (2 plants per square) than the population on the right (density = 0.5 plants per square). Image by Melissa Ha using Green Plant (public domain).

Figure $$\PageIndex{b}$$: Australian mammals show a typical inverse relationship between population density and body size. Log mass in grams is on the x-axis, and log density in square kilometers (km2) is on the y-axis. Each data point represents a different species, including the quoll, four bandicoot species, wombat, rat-kangaroo, potoroo, four possom species, tree kangaroo, three wallaby species, kangaroo, bear cuscus, and six glider species. As this graph shows, population density typically decreases with increasing body size. For example, quolls have the lowest body mass and the highest density. The heaviest wallaby species has the lowest population density.