2.3 Island Biogeography

Island biogeography is the study of how species are spread across islands. The theory, developed by Robert MacArthur and E. O. Wilson in the 1960s, applies to real islands in the ocean and also to habitat islands: patches of habitat surrounded by very different land. A forest fragment surrounded by farms, a mountaintop surrounded by desert, a lake surrounded by land and a city park are all habitat islands.

The number of species on an island reflects a balance between two rates. New species arrive by immigration, as animals fly or swim there and seeds float or blow in. Species disappear from the island through local extinction. Over time, the number of species settles where these two rates are equal.

• Island size: larger islands have more habitats, more resources and larger populations. Large populations are less likely to die out, so larger islands have lower extinction rates and more species.
• Distance from the mainland: closer islands are easier to reach, so more species arrive. Closer islands have higher immigration rates and more species.
• Putting them together: a large island close to the mainland has the most species; a small, distant island has the fewest.

Because islands are isolated, the species that reach them can evolve in new directions. When a few colonists arrive on an island with many empty niches, their descendants can split into several new species, each adapted to a different food or habitat. This is called adaptive radiation. Darwin's finches in the Galápagos and the Hawaiian honeycreepers are classic examples; honeycreeper beaks range from thick seed-crackers to long curved nectar-sippers.

Many island species end up as specialists with narrow niches and small populations. They often evolved without large predators or competitors, so they lack defenses. Some island birds even lost the ability to fly.

Island species are especially vulnerable to invasive species, which are non-native species that spread and cause harm. After brown tree snakes arrived on Guam in cargo after World War II, they wiped out most of the island's native forest birds, which had never faced a snake predator. Rats, cats, goats and pigs brought by people have caused many island extinctions.

The theory also guides conservation. When a forest is broken into fragments, each fragment acts like a small island and loses species over time. Larger reserves and wildlife corridors, which are strips of habitat that connect patches, help by raising immigration and lowering extinction.