Recent progress toward understanding the global diversity gradient

ibs newsletter perspectives in biogeography spring 2008 - vol. 6, no 1 Recent progress toward understanding the global diversity gradient Bradford A. Hawkins, Department of Ecology & Evolutionary Biology, University of California, Irvine, CA, USA. bhawkins@uci.edu Four years ago I published a guest editorial opining that we are making rapid progress to- ward understanding global patterns of diver- sity (Hawkins 2004). I also claimed that we still did not have a complete answer, because although the evidence that climate strongly influences the distributions of currently exist- ing species is solid, there was little consensus about the mechanisms driving patterns over evolutionary time. Even so, the increasing availability of the right sort of data coupled with modern phylogenetic methods has us well on the way to finding the solution to the oldest problem in ecology and biogeography, and several important papers have appeared in the past four years. Although none of these provide incontrovertible evidence as to the final answer, in guest editorials the rules of evidence are more relaxed and one can make an argument without the formality that is re- quired in a peer-reviewed article. Based on these rules, and hoping that taking a strong position in an informal setting might stimulate debate, I suggest that we now have sufficient evidence to conclude that niche conservatism and time for speciation are the primary drivers of the contemporary diversity gradient. And while it remains true that additional data on geographic variation in speciation/extinction rates and the strength of biotic interactions are needed, these appear to be secondary effects that do not need to be invoked to explain the general patterns we see; that is to say, they are details, and any differences we may find in these processes will not change the overall explanation for why the tropics have more species. Clearly, this is a strong opinion, so on what basis can I claim that we know the answer? Although a large number of papers have been published on this topic, three re- cent ones stand out as critical pieces of the puzzle: Wiens & Donoghue (2004), Currie et al. (2004) and Mittelbach et al. (2007). Although the idea of niche conserva- tism did not origi- nate with Wiens & Donoghue (2004), this short paper reminded many ecologists of its existence and suc- cinctly described a potentially powerful explanation for higher tropical di- versity, based on three sets of obser- vations: (1) most very rich tropical groups originated there (giving them more time to speciate), (2) the world’s climates were mostly ‘tropical’ until the Oligocene (providing a very large area for new clades to arise and subse- quent radiations to occur), and (3) it has been difficult for species adapted to tropical cli- mates to break into the younger, cool temper- ate zones (tropical niches are conserved over long time periods in many clades). Once these three tenets are accepted, higher tropi- cal diversity is not only expected, it is virtually guaranteed. Numerous papers testing this idea have appeared in recent years (see Mit- telbach et al. 2007), and the power of niche conservatism to explain diversity patterns at a range of spatial scales is rapidly becoming evident. It is notable that most of the papers in the recent special feature of The American Naturalist on evolutionary approaches to un- derstanding diversity patterns discuss or test this (Harrison & Cornell 2007, Harrison & Grace 2007, Hawkins et al. 2007a, Roy & Goldberg 2007, Wiens 2007). One powerful aspect of niche conservatism is that it predicts diversity patterns based on the climatic condi- tions where (and perhaps when) groups arose and began radiating, and when niche conser- vatism is coupled with time for speciation it is