Population distributions have five important characteristics

When ecologists study the distribution of a population like the collared lizard in the Ozark Mountains, they typically need to know the characteristics of the population, including geographic range, abundance, density, dispersion, and dispersal. As we will see, each of these properties tells us something important about how individuals are distributed.

Geographic Range

We saw an example of geographic range in our discussion of the sugar maple tree (see Figure 11.1). The geographic range of a species includes all the areas its members occupy during their life. For example, the distribution of sockeye salmon includes not only the rivers of western North America and eastern Asia that are their spawning grounds, but also vast areas of the North Pacific Ocean where individuals grow to maturity before making the long migration back to their birthplace. The geographic range is an important measure because it tells us how large an area a population occupies. If a population is restricted to a small area, for example, it may be very susceptible to a natural disaster such as a hurricane or a fire that can wipe it out. This is a serious challenge for endemic species, which live in a single, often isolated, location, such as the Galápagos finches on islands off the coast of South America. Populations with a larger geographic range are less vulnerable to such events because much of the population would remain unaffected. Species with very large geographic ranges that can span several continents are known as cosmopolitan species.

Abundance

The abundance of a population is the total number of individuals that exist within a defined area. For example, we might count the total number of lizards on a mountain, the number of sunfish in a lake, or the number of coconut trees on an island. The total abundance of a population is important to ecologists because it provides a measure of whether a population is thriving or on the brink of extinction.

Density

The density of a population is the number of individuals per unit area or volume. If we know the abundance of a population in a given area and we know the size of the area, then we can calculate density by dividing the abundance by the area. Examples of density include the number of bears per km² in Alaska, the number of cattails per m² in a pond, or the number of bacteria per ml of water. Density is a valuable measure because it tells ecologists how many individuals are packed into a particular area. If a habitat can support a higher density than currently exists, the population can continue to grow in the area. If the population density is greater than what the habitat can support, either some individuals will have to leave the area or the population will experience lower growth and survival.

Although individuals only live in suitable habitats, not all habitats are equal in quality because the environment is inherently variable. Some habitat patches have abundant resources that support a large number of individuals while others have scarce resources and can only support a few individuals. Across a large geographic area, the highest concentrations of individuals are typically near the center of a population’s geographic range. As one moves closer to the limits of the geographic range, biotic and abiotic conditions become less ideal. As a result, the margins of the geographic range typically contain fewer individuals. Consider, for example, the geographic range of the dickcissel (Spiza americana), a small songbird related to the cardinal and found in North American prairies and grasslands. As illustrated in Figure 11.6, this bird has its highest densities in the center of its geographic range and its lowest densities near the periphery. However, because environmental conditions do not vary smoothly, the pattern of the dickcissel’s preferred habitat is highly irregular.

Figure 11.6 Densities across a geographic range. In the dickcissel bird, a relative of the cardinal, the highest densities are near the center of its geographic range and the lowest densities are near the periphery of its geographic range.

Data from B. McGill and C. Collins, A unified theory for macroecology based on spatial patterns of abundance, Evolutionary Ecology Research 5 (2003): 469–492.

Dispersion

Dispersion of a population describes the spacing of individuals with respect to one another within the geographic range of a population. As shown in Figure 11.7, dispersion patterns can either be clustered, evenly spaced, or random.

Figure 11.7 Three types of dispersion patterns. (a) Clustered dispersion is characterized by individuals that are aggregated. For example, sulphur tuft fungi (Hypholoma fasciculare) grow in clusters anywhere there is a rotting stump, such as this site in the United Kingdom. (b) Evenly spaced dispersion is characterized by each individual maintaining a minimum distance between neighbors, as is the case for these blue-eyed cormorants (Phalacrocorax atriceps) nesting in Patagonia, Argentina. (c) Random dispersion is characterized by each individual’s position being independent of the location of other individuals, such as these dandelions growing in a pasture in Bulgaria.

Photos by (a) Gary K. Smith/naturepl.com; (b) Juan Carlos Munoz/naturepl.com; (c) FLPA/Bob Gibbons/age fotostock.

Clustered Dispersion

In clustered dispersion, individuals are aggregated in discrete groups. Some clustered dispersions result from individuals living in social groups, as we discussed in Chapter 10. For instance, some birds live in flocks and some fish live in schools. Other clustered dispersions occur because individuals stay near clustered resources. For example, salamanders and sow bugs aggregate under logs because individuals of both species are attracted to dark, moist places. Another cause of clustered dispersions is offspring that remain close to their parents. Some species of trees, such as the quaking aspen, form clusters of stems because a parent tree gives rise to offspring by sending up new stems from its roots, a form of vegetative reproduction. As a result, we commonly see clusters of aspen trees that are composed of a parent tree surrounded by its offspring. Within such a cluster, however, the stems tend to be evenly spaced.

Evenly Spaced Dispersion

In evenly spaced dispersion, each individual maintains a uniform distance between itself and its neighbors. In agricultural settings we can observe even spacing in crops such as corn or apple trees because farmers want each plant to have sufficient resources to maximize crop production. In natural settings, even spacing most commonly arises from direct interactions between individuals. For example, plants positioned too close to larger neighbors often suffer from shading and root competition. In addition, some plants can emit chemicals from their leaves and roots that inhibit the growth of other plants around them. As these crowded individuals die, the remaining individuals become more evenly spaced. We can also observe evenly spaced distributions in animals that defend territories, such as birds and lizards. Because territory size commonly depends on the amount of resources available, neighboring territory holders commonly defend areas that are similar in size, which causes the territory holders to be evenly spaced.

Random Dispersion

In random dispersion, the position of each individual is independent of the position of other individuals in the population. Random dispersions are not common in nature, largely because abiotic conditions, resources, and interactions with other species are not randomly distributed. However, if we wished to know whether the dispersion pattern of a population is clustered or evenly spaced, we would have to demonstrate statistically that its distribution was significantly different from random.

Dispersal

Dispersal is the movement of individuals from one area to another. Dispersal is distinct from migration, which is the seasonal movement of individuals back and forth between habitats, such as birds flying north and south with the changing seasons. In contrast, dispersal involves individuals leaving their habitat of origin—where a seed was made or where a squirrel was born—and typically not returning. As we saw at the beginning of this chapter, dispersal is of great interest to ecologists because it is the mechanism by which individuals can move between suitable habitats and, in some cases, colonize suitable habitats that are not already inhabited by the species. Dispersal can also be a way to avoid areas of high competition or high predation risk. For example, when a fish arrives in a section of a stream, many of the aquatic insects in the stream disperse by floating downstream at night to avoid being eaten.