28.2: Distribution and Dispersion
To understand intra-specific interactions in populations, scientists measure the spatial arrangement of species individuals. This geographic arrangement is known as the species distribution or dispersion. Highly territorial species exhibit a uniform distribution pattern, in which individuals are spaced at relatively equal distances from one another. Species that are highly tied to particular resources, such as food or shelter, tend to concentrate around those resources, and thus exhibit a clumped distribution pattern. Lastly, organisms unaffected by the placement of resources or other individuals exhibit a random distribution pattern.
Modes of Dispersion
A species distribution spans the entire area that is used by individuals of the species. Organisms can be equally spaced apart, clustered in groups or dispersed randomly throughout the landscape.
In species that exhibit uniform dispersion, individuals are spaced equidistantly from one another. Plants that release chemicals into the ground to prevent the growth of other individuals nearby display uniform distributions. Territorial species also tend to be more uniformly distributed. For example, mountain lions are a highly territorial species in which males defend space around females and resources.
Organisms that do not move around much or live in social groups or clusters often display a clumped distribution. This type of distribution is the most common and can occur when resources are patchy or regionally concentrated. A clumped distribution can also result from small population size, as mating may be difficult for rare animals that do not stay in groups.
Lastly, organisms that exhibit a random distribution are relatively rare but are more common in habitats with plentiful resources. Examples of randomly distributed organisms include plant species with wind-dispersed seeds. These organisms settle wherever conditions are viable, independent of the placement of resources and other individuals.
Species distribution can also be distinct at different spatial levels. For example, within a population of reproducing shearwaters (a species of seabird), individuals display a uniform distribution. However, at the level of the entire species, populations show a clumped distribution, clustered on islands that are suitable for nest-building and protected from predators.