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Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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The Theory of Island Biogeography

I. The observations

For a long time ecologists have observed that the larger an area which is studied, the more species will be observed.
z
This species-area relationship has been mathematically expressed as:  S= cA

where:  S= number of species
c= the number of species in a single location
A= area sampled
z= the rate at which species number falls as area decreases

When this analysis is conducted on continents or large islands, the rate at which diversity falls with decreasing sample size ranges between 0.15-0.20.

However, when these same analyses are conducted on isolated islands (ocean, mountain tops, etc), it is found that the number of species in a single location (c) is lower, and that the rate of decrease in species number with decreasing island size (z) is twice as high.

Because of this, single islands always have fewer species than areas on the mainland of similar size.

Why is this the case?

II.  The theory

In the mid-1960's the Theory of Island Biogeography was proposed to help answer this question.

According to this theory, the number of species found in any given location is due to the rates of immigration and extinction present at that location.

The theory states that:

As the number of species increases in an area, the rate of extinction will rise, as it becomes more likely that someone will be able to competitively exclude another.

Because of this, the immigration an distinction curves will cross at some number of species.  Where this happens, immigration and extinction will be balanced, and the number of species will remain stable.

This, by the way, is identical to the way supply and demand interact to determine the price of goods in economics.

How does this help us explain why single islands will always have fewer species than areas of the same size on the mainland?

A.  Because islands are isolated, it will be harder for species to immigrate to them, lowering the rate of immigration.

B.  Because of the limited amount of resources on islands, carrying capacity will be lower, decreasing population sizes and increasing extinction rates.

As immigration falls and extinction increases, the two lines will intersect at a lower number of species than is present on the mainland.

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