The Equilibrium Theory of Island Biogeography is a scientific concept that explains how species diversity on islands is determined by the balance between immigration and extinction rates. According to this theory, larger islands closer to the mainland tend to have more species because they can receive more immigrants and have lower extinction rates compared to smaller, more isolated islands. This theory was developed by ecologists Robert MacArthur and Edward O. Wilson in the 1960s. It suggests that as the number of species on an island increases, the rate of new species arriving decreases, while the rate of extinction increases, eventually reaching an equilibrium point where the number of species stabilizes.