Population size, density, & dispersal
- A population consists of all the organisms of a given species that live in a particular area.
- The statistical study of populations and how they change over time is called demography.
- Two important measures of a population are population size, the number of individuals, and population density, the number of individuals per unit area or volume.
- Ecologists estimate the size and density of populations using quadrats and the mark-recapture method.
- The organisms in a population may be distributed in a uniform, random, or clumped pattern. Uniform means that the population is evenly spaced, random indicates random spacing, and clumped means that the population is distributed in clusters.
What is a population?
Demography: describing populations and how they change
Population size and density
- Larger populations may be more stable than smaller populations because they’re likely to have greater genetic variability and thus more potential to adapt to changes in the environment through natural selection.
- A member of a low-density population—where organisms are sparsely spread out—might have more trouble finding a mate to reproduce with than an individual in a high-density population.
Measuring population size
Example: using the mark-recapture method
- Uniform dispersion. In uniform dispersion, individuals of a population are spaced more or less evenly. One example of uniform dispersion comes from plants that secrete toxins to inhibit growth of nearby individuals—a phenomenon called allelopathy. We can also find uniform dispersion in animal species where individuals stake out and defend territories.
- Random dispersion. In random dispersion, individuals are distributed randomly, without a predictable pattern. An example of random dispersion comes from dandelions and other plants that have wind-dispersed seeds. The seeds spread widely and sprout where they happen to fall, as long as the environment is favorable—has enough soil, water, nutrients, and light.
- Clumped dispersion. In a clumped dispersion, individuals are clustered in groups. A clumped dispersion may be seen in plants that drop their seeds straight to the ground—such as oak trees—or animals that live in groups—schools of fish or herds of elephants. Clumped dispersions also happen in habitats that are patchy, with only some patches suitable to live in.