8-1 Community Structure and Species Diversity

What Is Community Structure? Appearance, Diversity, and Niches

Ecologists use three characteristics to describe a biological community:

• Physical appearance: the relative sizes, stratification, and distribution of its populations and species.

• Species diversity: a combination of its number of different species (species richness) and the abundance of individuals within each of its species (species evenness).

• Niche structure: the number of ecological niches, how they resemble or differ from each other, and how species interact with one another.

What Determines the Number of Species on Islands? Island Size and Isolation

The species equilibrium model and the theory of island biogeography were formulated as a way to explain the differences in species richness with respect to island size. According to this widely accepted model, a balance between two factors determines the number of different species found on an island: the rate at which new species immigrate to the island and the rate at which existing species become extinct on the island.

According to the model, two features of an island affect its immigration and extinction rates and thus its species diversity. One is the island’s size, with a small island tending to have fewer different species than a large one.

A second factor is an island’s distance from the nearest mainland. According to the model, if we have two islands about equal in size and other factors, the island closer to a mainland source of immigrant species should have the higher immigration rate and thus a higher species richness.