In accordance with Wilson and MacArthur's theory of island biogeography, the following equations represent the immigration and extinction rates of species of beetles on a small island. (I = immigration rate of species per month; E = extinction rate of species per month; S = number of species currently on the island.) I = 9 ? 0.2S E = 0.3S - Based on these equations, what is the expected equilibrium species diversity on that island?