Island Biogeography Theory

Island biogeography also known as insular biogeography is a field within biogeography that examines the factors that affect the species richness of isolated natural communities. MacArthur and EO Wilson in 1967 explained the species richness of actual islands, principally oceanic.

The assumptions of island biogeography theory are:

  1. Species richness is a balance between immigration rates and extinction rates.
  2. Immigration rates are modified by island size and distance from the mainland.
  3. Immigration and extinction rates are random events and all species have an equal ability to immigrate or become extinct

Thus, the theory explains about two variables that affect the extinction rate and immigration rate on an island which are,

  • Size of island
  • Distance from the mainland

a) Size of island

As the size of the island increases, immigration rates also increases slightly as the island will be a bigger target for dispersing individuals. And similarly, the extinction rate will be lesser because larger islands support a large population of species and vice-versa.

b) Distance from the mainland

This theory predicts that distant islands will have lower immigration rates than those close to the mainland, and equilibrium will occur with fewer species on distant islands. Similarly, closer islands will have high immigration rates and support more species.

Island biogeography theory


Island biogeography theory can be applied in the design of protected areas/biodiversity conservation.

a. Reserve size

If we consider reserves as islands, then the species-area relationship, as well as the immigration-extinction curves, argues forcefully for a single large reserve.

b. Reserve isolation

The relative isolation of an island from its mainland species source affects the rate of successful immigration. A large island close to the source will sustain more diversity
than an identically-sized island far from the source simply as more species reach it.

A common technique to increase the success of migrations is to provide a buffer zone around the reserve, the establishment of new PAs nearby the reserve, etc.

c. Connectivity

Connections are critical to wildlife health because they allow for genetic sharing among populations, which in turn increases species’ likelihood of survival. Connectivity can be,

  • Corridors: Corridors are the terrestrial equivalent of land bridges for oceanic islands.
  • Stepping Stones: A stepping stone is essentially a “rest stop” within the matrix which improves the chances of successful immigration.

d. Circuitry

If the reserves are linked in more or less a straight line pattern, the movements occur in only two possible directions. However, if the reserves are clustered and linked with many loops
or alternate routes, movement can occur in many different directions.
Increased circuitry speeds the interaction between the patches, which in turn allows more effective genetic sharing and a more realistic simulation of natural ecosystems

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