The theory of island biogeography, developed by Robert MacArthur and Edward O. Wilson, looks to explain the differences in species diversity with island size (for example, why large islands tend to have a greater number of species of a certain category than small islands). MacArthur and Wilson proposed that the number of species found on an island can be determined by a balance between the immigration rate (or the movement of species onto the island from other islands) and the extinction rate (or the rate at which species already on the island become nonexistent). [see graph 1]

 

This theory also supposes that immigration and extinction rates are affected by the size of the island and its distance from a non-island source of immigrant species [see graph 2]. In this regard, a larger island has higher species diversity for two reasons: it is a larger target, giving it a greater probability of becoming the home to immigrants, and it has a larger supply of resources necessary to prevent extinctions. 

 

Another prediction assumed by this theory is that an island's distance from a mainland source of new immigrants, despite its size, is an important factor in species diversity. Even if two islands are the exact same size and all other factors are constant, the island closest to the mainland is more likely to attract a larger number of immigrant species due to its proximity and convenience [see graph 3].