  Whether the species-area curve (rather than the mere qualitative relation) has any empirical support remains a matter of  contention.[60]  In the 1950s, Preston was a strong advocate of the power law model which he believed to be the result of a dynamic equilibrium of the exchange of species between isolated habitat  patches.[61]  (The same idea had been worked out in some detail much earlier by Munroe but did not receive any  attention.)[62]  Preston's work was extended by MacArthur and Wilson to construct the theory of island  biogeography.[63]  According to this theory, the number of species in islands with the same habitat (at the same latitude) depends only on the size of the island and its isolation. There is a dynamic equilibrium in the sense that this number does not change over time though there is a turnover of species which changes the composition of the  community.[64]  The equilibrium is supposed to be a result of a balance between immigration and extinction. The immigration rate varies inversely with the degree of isolation while the extinction rate decreases with area. Thus, this theory incorporates the second mechanism for the species-area relation mentioned in the last paragraph. While some initial experimental evidence seemed to support the theory, by the mid-1970s its status had become  controversial.[65]