What determines how many species are living on an island? The equilibrium theory of island biogeography, first proposed by Robert MacArthur and Edward Wilson, postulates that the species richness on islands is determined by a balance of immigration of species from the mainland to the island and extinction of species on the island. A major factor influencing immigration rates is the distance from the mainland to the island: The further the distance, the lower the immigration. A major factor that influences extinction rates is the size of the island: A larger island should be able to support more species.
Below are simulations of species of lizards from the mainland species pool colonizing an island. In each of these simulations, the island begins with a species richness of zero, as would be the case for a new island or one that lost its fauna due to some extinction event. In these simulations, you will manipulate the size of the island, the distance from the island to the mainland, and the size of the mainland species pool, to illustrate how these three factors influence species richness on the island.
Open the simulation in a new window
Question 1
Set the initial island size to 2,500 hectares (ha), equivalent to a square 5 km on each side. The island should be located 17 km from the mainland, and there should be 50 species in the mainland species pool. Run the simulation by pressing start. At the end of 1,000 time periods how many species are on the island?
Question 2
Describe the time course of species number on the island.
Question 3
You can adjust the size of the island by moving the lower right-hand corner of the island. Shrink the island to 600 ha (+/– 30 ha), being sure that the island remains 17 km away from the mainland. Run the simulation again. How many species are on the island after 1,000 time periods? Describe the time course of the species number on the island.
Question 4
Adjust the island to 100 ha (+/– 5 ha), being sure that the island remains 17 km away from the mainland. How many species are on the island after 1,000 time periods? Has the time course of the species number changed?
Question 5
What general conclusions can you draw about the effect of changing the island size on the equilibrium species richness of islands?
Question 6
Now, we will examine the effect of distance from the mainland on island species richness at equilibrium. Keeping the island size at 100 ha, move the island to a distance of 4 km away from the mainland. How many species are on the island after 1,000 time periods?
Question 7
Keep the island 4 km away from the mainland and adjust its size to 600 ha. How many species are on the island after 1,000 time periods?
Question 8
Compare the results you had when the island was 4 km from the mainland to the results you had when the island was the same size but was 17 km from the mainland. What conclusion can you draw about the effect of distance from the mainland on species richness?
Question 9
The mainland species pool can also affect species richness. Islands near more species-rich mainland regions would likely receive more migrants than those near species-poor mainland regions. Consequently, one would expect that species richness of the island might correlate with the size of the species pools of mainland areas.
Adjust the species pool on the mainland to 100 species. Set the island size to 600 ha and the distance from the mainland to 17 km. How many species are on the island after 1,000 time periods? How does this compare with the results you obtained when the mainland species pool was 50 species and the island was the same area and distance from the mainland?
Question 10
Adjust the mainland species pool to 25, keeping the other variables the same. How many species are present on the island after 1,000 time periods?
Question 11
What conclusion can you draw about the effect of the size of the mainland species pool on island species richness?
Question 12
What other factors may influence the equilibrium species richness of islands?
