The Theory of Island Biogeography

The number of species on an island is determined by how fast new species arrive and old species become extinct, the island's size, and how far away it is from the mainland. Population generally follows the formula P = B + I - D - E. You can remember it by BIDE. It is the births plus the immigrations minus the deaths and the emigrations. Scientists Robert MacArthur and Edward O. Wilson first studied the Theory of Island Biogeography in the 1960s. They were trying to figure out the reason behind why larger islands tended to have more species of any categories such as insects or birds than do smaller islands.
The model that they developed predicts that at some point, the rate of immigration and extinction should reach equilibrium and perfectly balance each other out. This determines the island's average number of different species. Though this sounds like common sense, it was actually a phenomenal discovery back in the 1960s and a great contribution to the field of environmental science. According to the model, the two features of an island that affect its immigration and extinction rates and thus its species diversity are the size of the island and the distance of the island from the mainland. The size of the island is important because smaller islands tend to have fewer different species. This is because small islands generally have a lower immigration rate because it is a smaller target for any potential colonizers arriving by sea or air. Also, a small island may have higher extinction rates because it has fewer natural resources and fewer habitats for species that may colonize there. The island's distance from the nearest mainland is important because islands that are father away have lower immigration rates. This is because it is more difficult for colonizing species to find the island if the island is too far removed from the mainland.
The theory of island biogeography applies to other situations as well. Scientists have recently used this theory and have applied it to habitat islands such as parks surrounded by developed areas and also for habitat fragmentation.