Distribution vs Abundance Spatial Structure of Communities

Earlier sections treated the population of each species within an ecosystem as if it were a variable that depended only on time. However, their spatial distributions are interesting as well. Factors that control where an organism will be found include:

• Method of dispersal

• Physical factors

• Habitat selection

• Interactions with other species (predation, competition, disease)

Organisms disperse to colonize new areas in many ways. Plant seeds may be spread by wind, water, or animals, or they may be confined to the margins of the parent. Animals may disperse by their own motility or may be carried passively, like seeds, as in the case of insects. If the environment in the new area fits within their niche, they may thrive.

Some of the most general physical factors include climate, light availability, and the chemical environment. Geographically, these translate to such factors as latitude, altitude or depth (in soil or water), and position on a mountain slope. Soil or water chemistry may vary locally or regionally. Fire can alter the balance between grassland and forest, controlling the position of their border.

These factors vary in gradients throughout the world. For example, temperature, rainfall, depth of soil and light all vary with elevation along the slope of a mountain. Along the gradient various species rise and fall in abundance. In what are known as open communities, the population distributions overlap considerably, resulting in continuous variation in species along the gradient (Figure 14.19a). In the other extreme, known as closed communities, population abundances are strongly correlated (Figure 14.19b).

Closed communities

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