The Theory of Island Biogeography

One of the most obvious observations you can make about factors affecting species diversity is that the larger the area, the more species it is likely to contain. This was first formalized in the 1920's, but it wasn't until the 1960's that the species-area curve became an object of study. Preston [10] and MacArthur & Wilson [4,5] provided the first explanation for the widely observed power relationship

$$S = CA^z \quad ,$$

where $S$ is the number of species, $A$ is the area, and $C$ and $z$ are species-specific constants. $C$ measures the overall species richness, and $z$ measures the extent to which increases in area have diminishing returns in terms of the number of species. Low values of $z$ indicate strongly diminishing returns. $z$ values tend to vary between 0.18 and 0.35, i.e., to double the number of species the area must be increased by a factor of between seven and 100.

Prior to Preston and MacArthur & Wilson, biologists who worried about the species-area relationship at all tended to explain it simply as a result of the greater habitat diversity associated with larger geographical areas. The equilibrium theory of biogeography proposes another explanation, that the species diversity of an area is the result of an equilibrium between colonization and extinction.

• If we compare large islands with small islands, we expect extinctions less frequently on large islands (because of larger local population sizes). If colonization occurs at the same rate on large and small islands, large islands will support a greater diversity of species.

• If we compare islands close to the mainland (a source of colonists) with those far away, we expect colonizations less frequently on distant islands than on near islands. If extinctions occur at the same rate on near and far islands, near islands will support a greater diversity of species than far islands.

In broad outline, both of these predictions seem to hold. In detail, there are many exceptions. Note, however, that the equilibrium theory implies a constant turnover in species, i.e., continual changes in species composition. An alternative interpretation of the phenomena is that the patterns seen are not equilibria at all. Even if they're not, they may provide some useful insight for conservationists.