On a global scale, the distribution and diversity of organisms—the patterns of biogeography—vary among the different biomes, from continent to continent, and with latitude. On a smaller scale, other factors influence species diversity. Small islands tend to have fewer species than large islands, and islands close to a mainland (a species pool) tend to have more species than islands far away. The theory of island biogeography relates the size of an island and its distance from a mainland with the number of species an island maintains. The accompanying animation describes this theory as well as an experiment that tests it using mangrove islands in the Florida Keys.
Biogeographers documenting species diversity have repeatedly noticed a pattern on islands or islandlike habitats; that is, any area surrounded by a "sea" of dissimilar habitat. The pattern is called the species–area relationship and it states that as island size increases, the number of species on islands also increases.
In one example, biologist E. O. Wilson and mathematical ecologist Robert MacArthur plotted the number of species of birds against the size and distance of a number of islands from New Guinea. Note that the largest islands tend to have the most bird species. This is the species–area relationship. But Wilson and MacArthur also noted that for islands that are similar in size, such as in these groupings, the ones that are closest to the mainland (indicated by green) tend to have the most species, and those farthest away (indicated by orange) tend to have the fewest.
Wilson and MacArthur incorporated these facts about size and distance from the mainland into their theory of island biogeography. The theory states that the number of species on an island is a balance between immigration and extinction rates, which in turn depend primarily on island size and distance from the mainland, which contains the species pool.
If we consider an island that initially has no bird species, for instance, we would find that its immigration rate for new bird species would start out high, because the first immigrants are by definition new species. As the island becomes more populated with different species, the chance is lower that a new immigrant is also a new species to the island. In this way, immigration rates decrease as an island gains more species. Extinction rates, in contrast, start out low, because there are fewer species to go extinct on an island with a small number of species, and those species do not have as many competitors, but then the extinction rate rises as the island acquires more species. The point at which the immigration and extinction curves intersect predicts the equilibrium number of species on the island, indicated S. This is the number of species that an island tends to maintain. Let's say that this island is small and near to the mainland.
What about a large island also near the mainland? Larger islands provide greater habitat diversity than smaller islands and can sustain larger populations. In turn, larger populations tend to have lower extinction rates, which is why the extinction rate curve for a large island is drawn lower in the graph. In this theory of island biogeography, the equilibrium number for the large, near island is greater than for the small, near island.
For islands far from the mainland, the rates at which new species arrive will be lower overall, so an island that is small and far, and an island that is large and far will have equilibrium numbers lower than similar islands closer to the mainland. MacArthur and Wilson's theory emphasized that for each island there is a balance between immigration and extinction rates, resulting in an equilibrium number.
Between 1966 and 1969, Wilson and his student Daniel Simberloff conducted an experiment to test the theory of island biogeography, using arthropods living on mangrove islands in the Florida Keys. They identified four small, isolated clumps of red mangrove, all approximately the same size (11 to 12 meters in diameter). So in this experiment, the variable of island size is kept constant, but the variable of distance from the species pool is not. These mangrove islands, represented by the colored dots, were from two meters to 533 meters from the species pool. The islands were small enough to allow an accurate count of the arthropod species on each one. The island closest to the species pool, just two meters away, had the most species of arthropods, and the one farthest away had the fewest.
The islands were also small enough for the research team to enclose each island in a tent and gas it with methyl bromide to kill all the arthropods. Given concerns over environmental impacts, it should be noted that this experiment might not be approved today. The gassing reduced the number of arthropod species to zero for each island. The researchers had defaunated the islands.
Simberloff and Wilson monitored and tracked recolonization of the islands by arthropods. Recolonization was fastest on the closer islands and slowest on the one farthest from the mainland. After two years, the species number on all but the farthest island was close to what it was before defaunation. This observation is consistent with the theory of island biogeography—the idea that the number of species on the islands prior to defaunation represented an equilibrium number of species, balanced between immigration and extinction rates.
According to the theory of island biogeography, any particular island or isolated patch of suitable habitat (habitat island) will maintain a certain number of species, known as the equilibrium number. The theory is based on two processes: the immigration of new species to an island and the extinction of species already present on that island. The equilibrium number is a balance between these two processes.
In turn, two factors influence immigration and extinction rates: the size of the island and the distance of the island from the species pool (the source of immigrants). Smaller islands have fewer resources and greater potential for competition. Smaller islands therefore have higher extinction rates. Islands farther from the species pool will have lower immigration rates. In this way, smaller and farther islands typically have lower equilibrium numbers than larger and closer islands.
The theory of island biogeography has important applications for the conservation of endangered species. As habitat islands decrease in size because of human encroachment, more and more species become vulnerable to population declines, especially those that require large areas in order to live and breed successfully.