Macroevolutionary dynamics of Atlantic island avifaunas support MacArthur & Wilson's equilibrium prediction

Half a century ago, MacArthur & Wilson proposed that the number of species on islands tends towards a dynamic equilibrium diversity around which species richness fluctuates. The current prevailing view in island biogeography accepts the fundamentals of MacArthur & Wilson's theory, but questions whether their prediction of equilibrium can be fulfilled over evolutionary time scales, given the unpredictable and ever-changing nature of island geological and biotic features. Here we conduct a complete molecular phylogenetic survey of the terrestrial bird species from four oceanic archipelagos that make up the diverse Macaronesian bioregion - Azores, Canary Islands, Cape Verde and Madeira. We estimate the times at which birds colonized and speciated in the four archipelagos, including many previously unsampled endemic and non-endemic taxa and their closest continental relatives. We develop and fit a new multi-archipelago dynamic stochastic model to these data, explicitly incorporating information from 91 taxa, both extant and extinct. Remarkably, we find that all four archipelagos have independently achieved and maintained a dynamic equilibrium over millions of years. Biogeographical rates are homogenous across archipelagos, except for the Canary Islands, which exhibit higher speciation and colonization. Our finding that the avian communities of the four Macaronesian archipelagos display an equilibrium diversity pattern indicates that a diversity plateau may be rapidly achieved on islands where rates of in situ radiation are low and extinction is high. This study reveals that equilibrium processes may be more prevalent than recently proposed, supporting MacArthur & Wilson's 50 year old theory