Contingency and Determinism in Mammalian Biogeography: The Role of History

Although history is commonly thought to create unique and contingent consequences, historical effects in biogeography are often strikingly deterministic. The role of history in shaping deterministic biogeographic outcomes is evaluated over four temporal scales and against four biogeographic paradigms. First, history is shown to represent an important parameter for island biogeography; over ecological time spans, trenchant differences arise in patterns of species richness and species composition as a reflection of island history. Second, predictions arising from Quaternary studies of palynology and fossils can be refined by history; over millennia, dramatically different biogeographic responses are shown by historically differentiated faunas inhabiting the same mountain tops in the American Southwest. Third, phylogenetic analyses recover historical signals not in categories but rather in sequence; while “phylogeographic” studies provide unique explanations for specific taxa, the series of range expansions and dis-junctions that are implied by these relationships offer “general” explanations for many other taxa. Finally, over tens of millions of years, taxa respond differentially to the complex scenarios created by plate tectonics. For sigmodontine rodents invading South America, this response has involved differentiation and diversification at a continental scale. Over time, this effect has involved fundamental transformations of the invading lineage and reinvasions of formerly occupied range. Widespread evidence for historical influences in these and other biogeographic scenarios suggest that few, if any, systems are actually in equilibrium.