The distribution and abundance patterns of species and populations vary in their spatial extent across the landscape.

An example of this variability is the distribution map for the herbaceous perennial Clematis fremontii (FIGURE 9.5A). Clematis has a patchy distribution across Missouri, Kansas, and Nebraska, where its populations are restricted to dry, rocky, treeless meadows or glades formed on particular limestone outcrops within the region.

FIGURE 9.5 Many Populations Have a Patchy Distribution Thedistributionand abundance of the herbaceous perennial Clematis fremontii are patchy over different spatial scales. (A) Populations occur within limestone meadows. A group of populations makes up a metapopulation, and multiple metapopulations make up the geographic range (in this case, Missouri, Kansas, and Nebraska). (B) Individuals within a population show one of three different dispersion patterns. (A after R. O. Erickson. 1945. Ann Mo Bot Gard 32: 413-460.) View larger image

The aggregates of C. fremontii individuals found in the meadow populations provide an example of the dispersion, or spatial arrangement, of individuals within a population (FIGURE 9.5B). We can recognize three basic patterns in how the individuals of a population are positioned with respect to one another (see Figure 9.5A). In some cases, the members of a population have a regular dispersion, in which individuals are relatively evenly spaced throughout their habitat. In other cases, individuals show a random dispersion, similar to what occurs if individuals are positioned at locations selected at random. Finally, as in C. fremontii, individuals may be grouped together to form a clumped dispersion. In natural populations, clumped dispersions are more common than either regular or random dispersions. It is important to note that identifying dispersion patterns can depend on the spatial scale of the measured area. For example, clumped dispersion patterns might not be revealed except at large spatial scales.