The theory of island biogeography states that the number of species found on a particular, undisturbed island is determined solely by the number of species immigrating to the island and by extinction rates. The theory also states that isolated species may follow evolutionary routes that are different than species on land masses that are not isolated.
There are five variables to consider whenever thinking about the island biogeography theory. These include the immigration rates of new species, the emigration rates, the extinction rates, the size of the island, and the distance the island is to another land mass. The land mass could be a mainland or another island. The theory states that all of these variables will play a part in how the species thrive, survive and evolve on the island. Other influencing factors that contribute to the island’s biodiversity include the degree of isolation, the length of isolation, the size of the island, the climate, ocean currents, and human activity. Ideally, the theory of island biogeography refers to an undisturbed island that has no human contact or activity. However, it is modifiable to include human activity but the biodiversity and the species' distribution and evolution will be different from that of an isolated island.