The equilibrium model of the island biogeography states that the number of species on islands or the species richness is determined by a dynamic balance between immigration and extinction of species.
The rate of immigration and extinction has been found to be influenced by number of factors such as the number of species on the island, extent of isolation of the island from the source land, and area of the island.
The ecologists have shown that rate of immigration increases with decrease in isolation.
The rate of extinction has been shown to increase with decrease in area of the island.
Hence, large islands near to the source land have been found to have the highest species richness. The small far islands have been found to have the lowest species richness.
The islands those are far and large along with islands that are near and small showed intermediate number of species.
The islands to be studied are illustrated below:
Island 1 and island 2 are stated to lie at equal distances from the mainland. The islands differ in the area with island 1 larger than island 2. As per the equilibrium model of island biogeography, island 1 and 2 should experience approximately equal rate of immigration of birds due to similar extent of isolation. However, other ecologists have suggested that area of island can influence rate of immigration.
As per the equilibrium model of island biogeography, the rate of extinction is more in smaller islands compared to larger islands. Hence, island 2 should show high rate of extinction and island 1 should show lowest rate of extinction.
However, other ecologists have suggested that high rate of immigration reduces the rate of extinction. Hence, the distance of the islands from the mainland may also govern the rate of extinction on both the islands.
Hence, the equilibrium model of island biogeography is being extended by ecologists to account for the other environmental factors affecting species richness.
The equilibrium model of island biogeography states that the species richness on islands is governed by the dynamic interplay of immigration and extinction.
As per the model, the rate of immigration and the rate of extinction are dependent on many factors such as the number of species on the island, the area of island, and the isolation of island.
As the number of species goes on increasing on the island, the rate of immigration from the source land to the island has been found to decrease.
As the number of species goes on increasing on the island, the rate of extinction of species has been found to increase.
The rate of immigration and the rate of extinction can become equal leading to equilibrium. This leads to a constant number of species on the island. However, this equilibrium is found to be dynamic as there is a constant change in the composition of species which is called species turnover.
The model also explains the variation in number of species on different islands depending on the area of island and the isolation.
A larger island closely situated to the source island has been shown to harbor high number of species at equilibrium. A smaller island situated far from the source island harbors a comparatively lower number of species at equilibrium.
The islands assessed by Diamond might be near equilibrium. Hence, the rate of extinction and immigration observed might have been approximately equal. Hence, his results are consistent with the equilibrium model of island biogeography.
The plot of the logarithms of area and species number should yield the tightest relationship between area and species richness.