Trends in Scorpion Diversity Along the Baja California Peninsula

The scorpion fauna of Baja California, Mexico, was analyzed to identify the processes responsible for the observed patterns of diversity. We considered both the probability that equilibrial processes operate on the present population, by testing for a "peninsular effect," and the historical, ecological, and evolutionary characteristics of the scorpions. The most diverse scorpion fauna in the world occurs in Baja California, with at least 53 species on the peninsula and an additional 8 species on the associated islands. Fewer species are found at the northern base of the peninsula (5) than either at the southern tip (8) or in the lower mid-peninsular region (10-13). This pattern is opposite to the one expected from the peninsular effect, which predicts that an equilibrium between extinction and (re)colonization will lead to fewer species at the tip of the peninsula than at the base. In general, equilibrial processes contribute little to an understanding of the patterns of scorpion diversity; evidence suggests that colonization is a relatively unimportant process among Baja California scorpions. Diversity is high in the south because in situ speciation produces a large number of endemic species. Scorpion diversity along the Florida and Italian peninsulas also was analyzed. More species live at the tip in southern Florida than in the north. Potential (re)colonization comes from a direction opposite to that posited by the peninsular effect: from the rich Caribbean fauna rather than the species-poor north. Although the distribution of scorpions in Italy is consistent with that predicted by the peninsular effect, Italian scorpion biogeography can be explained more parsimoniously by examining the autecology of each species rather than by invoking the more complex equilibrial scenario of extinction and (re)colonization. In conclusion, neither the present analyses nor previous studies in the literature on peninsular vertebrates provide much evidence for the peninsular effect in Baja California or on other peninsulas. Other processes (speciation, autecology, paleogeographic legacy) appear to contribute more to the biogeographic patterns of species living along peninsulas.