2.3 Island Biogeography

What is the Island Biogeography Theory?

After the formation of an island, it will be a certain amount of time before it is discovered and subsequently inhabitated by various different organisms. The immigration (arrival) and extinction (death) rate of these organisms help determine how quickly and how efficiently islands are taken over by ecosystems.

As extinction increases, immigration decreases. Larger islands or islands closer to land will have higher rates of immigration due to their proximity to other, larger ecosystems. As more and more species arrive, room runs out, and extinction rates will heighten. Eventually, this duality will balance out, and the point of equilibrium between immigration and extinction rates is ideal for the island. 

Size and distance matter greatly when discussing the species richness or accessibility of an island. Larger islands exhibit, on average, many more species than smaller islands. Islands closer to land are also easier to reach than islands too far for certain organisms to travel.

Invasives

Islands are home to many endemic species, or those that occur naturally in only one or certain parts of the world and are confined to that geographical area. Since these species are specialists, they require the specific ecosystem that they live in. This means that their ecological niche (more simply, their survivable environment) is quite narrow (or specialized). These specialists are sensitive to environmental changes as a result of this narrow niche and are often wiped out by invasive species. The invasive species are commonly generalists that have few to no native predators and are able to fill a broad niche. This often leads to the downfall of the endemics. To be clearer, endemic species with fewer livable environments can face extinction if an invasive species with less specific survival guidelines moves nearby.

Effects on Evolution

An excellent example of the IBT’s effects on evolution is Darwin’s Finches found on the Galapagos Islands. A small population was blown over to the island chain from Ecuador by a hurricane. The islands’ distance from the mainland affected the birds’ evolution. 

Since there was no further immigration from the mainland, the birds adapted to the islands and evolved away from their mainland cousins. Each island also had its own pressures (food type and habitat) that pushed evolution more. There are now fifteen species living on the Galapagos Islands.

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