Island Biogeography, the Species-Area Effect and Fragmentation

The species-area relationship

Across islands, forest patches and reserves, the number of species rises with area following a power law:

The exponent matters more than the constant. Because $z$ is well below $1$, species number does not fall in proportion to area lost — but it does fall predictably. With $z = 0{,}25$, cutting a reserve to $\frac{1}{16}$ of its area halves its species number, and cutting it to $\frac{1}{10\,000}$ leaves only a tenth of the original richness.

Island biogeography: two axes

MacArthur and Wilson explained island richness as a dynamic equilibrium between immigration and extinction. Two variables set that equilibrium:

• Size — larger islands support larger populations, so extinction rate is lower.
• Distance from the mainland — nearer islands receive more colonists, so immigration rate is higher.

Therefore a large, near island holds the most species and a small, far island the fewest. A habitat fragment surrounded by farmland or highway behaves exactly like an island.

Edge effects and the loss of core

Fragmentation does its damage even when no area at all is lost. Conditions near an edge differ from the interior: more light, more wind, drier soil, higher temperature, more nest predators and more invasive plants. Species that require interior conditions survive only in the core. Splitting one patch into four multiplies edge and can destroy nearly half the core while the map still shows the same total hectares.