Island Biogeography: Theory, Definition & Graph

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  • 0:00 What Is Island Biogeography?
  • 0:45 Influences on Diversity
  • 1:50 Equilibrium
  • 2:45 Useful Applications
  • 3:40 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In this lesson, you will learn about island biogeography, which is the species composition on an island. Because island habitats are so isolated and unique, the theory of island biogeography explains how many species can live there and why.

What is Island Biogeography?

Think of a large office building. It can hold many different people, possibly even several different companies. A smaller office building will hold far fewer people and may be limited to only one or two different companies. The limit is due to the amount of space - a larger space can hold more people and more companies, while a smaller space can't hold as many.

The same idea applies to island biogeography. The theory of island biogeography simply says that a larger island will have a greater number of species than a smaller island. For this theory, an island is any ecosystem that is remarkably different from the surrounding area. So, this could refer to an actual island in the ocean, or it may be an oasis that is surrounded by a desert.

Influences on Species Diversity

When trying to understand the species diversity within any of these ecological 'islands,' you will need to consider three main factors. First is immigration, which is the number of new species that move to the island. When there is a higher rate of immigration, there will be a higher number of species in the island ecosystem. However, immigration rates tend to slow when species diversity becomes higher on the island because of competition.

Next is emigration, which is the number of species that leave the island. Emigration produces results opposite of immigration. As more species emigrate, there is a lower species diversity on the island, and as fewer species emigrate, there will be a higher species diversity.

The third factor is extinction, which is the number of species on the island that become extinct. Extinction rates are related to the size of the island. The smaller the island, the higher the rate of extinction. This is because larger islands contain more resources and habitats, and are thus able to support more life.


When immigration rates and extinction rates are the same, the island is in equilibrium. This means that the number of species on the island stays roughly the same. However, while the number of species does not change, the composition of those species on the island may change. The rate at which one species is lost and another species takes its place is called the turnover rate.

Environmental factors also affect island biogeography. These factors may be time, weather, natural disasters, human interferences, and the amount of isolation. For example, the longer an island has been isolated, the less species diversity will be present. Likewise, the farther the island is from sources of immigration, the less species diversity will be found in that island. This is because the island is more isolated from other plants and animals that could influence the species composition.

Useful Applications

Island biogeography is a useful tool because it helps ecologists understand different species, how they interact with each other, and how they interact with their environment. Ecologists can look at potential mechanisms that lead to a decrease in species diversity within an island, and from this knowledge find ways to preserve habitat and resources.

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