One key challenge in the study of species richness gradients is that the traditional approach to testing hypotheses – essentially documenting correlations between species richness and hypothesized explanatory variables (such as temperature) – is insufficient to conclusively discern between competing hypotheses. Firstly, many potential explanatory variables exhibit very similar patterns of geographical variation, and thus are similarly correlated with species richness. Secondly, often, we have no theory that constrains the functional form or potential magnitude of the relationship between species richness and different explanatory variables. And finally, multiple explanatory variables almost certainly interact to influence species richness. We address these challenges in two ways.
Firstly, we develop models that characterize the establishment of species richness gradients more mechanistically, as a function of processes that determine range limits. This sort of approach allows us to derive more explicitly whether particular causal relationships imply particular constraints on the shape or magnitude of species richness gradients.
Secondly, we have shifted our focus away from species richness itself as a (univariate) response variable, and have instead shifted to more information rich response variables. For instance, species richness gradients arise from species turnover in geographical space – the distribution of species’ range limits. Can the variables invoked to explain species richness gradients explain where species’ ranges begin and end? Do hypothesized drivers of species richness gradients imply that species with particular traits should be more or less susceptible to faunal breaks?
Connolly, S. R. 2005. Process-based models of species distributions and the mid-domain effect. American Naturalist 166: 1-11.
Bellwood, D.R., T.P. Hughes, S.R. Connolly, and J. Tanner. 2005. Environmental and geometric constraints on Indo-Pacific coral reef biodiversity. Ecology Letters 8: 643-651.
Connolly, S. R. 2009. Macroecological theory and the analysis of species richness gradients. Pages 279-309 in Witman, J. and K. Roy, editors. Marine Macroecology. University of Chicago Press, Chicago, USA.
Gotelli, N.J., M.J. Anderson, H.T. Arita, A. Chao, R. K. Colwell, S. R. Connolly, D.J. Currie, R.R. Dunn, G.R. Graves, J.L. Green, J.-A. Grytnes, Y.-H. Jiang, W. Jetz, S.K. Lyons, C.M. McCain, A.E. Magurran, C. Rahbek, T.F.L.V.B. Rangel, J. Soberón, C.O. Webb, M.R. Willig. 2009. Patterns and causes of species richness: a general simulation model for macroecology. Ecology Letters, 12: 873–886.