Drivers of -diversity along latitudinal gradients revisited

Aim Ecologists have generally agreed that -diversity is driven at least in part by ecological processes and mechanisms of community assembly and is a key determinant of global patterns of species richness. This idea has been challenged by a recent study based on an individual-based null model approach, which aims to account for the species pool. The goal of the present study is twofold: (1) to analyse data sets from different parts of the world to determine whether there are significant latitude-diversity gradients after accounting for the species pool, and (2) to evaluate the validity of the null model. Location Global. Methods A total of 257 forest plots, each being 0.1 ha in size and having 10 0.01-ha subplots, were used. We conducted four sets of analyses. A modified version of Whittaker's -diversity index was used to quantify -diversity for each forest plot. A randomization procedure was used to determine expected -diversity. Results The number of individuals per species, which characterizes species abundance distribution, alone explains 56.884.2% of the variation in observed -diversity. Species pool (-diversity) explained only an additional 2.615.2% of the variation in observed -diversity. Latitude explains 18.6% of the variation in raw deviation in Gentry's global data set, and explains 11.011.6% of the variation in standardized deviation in the global and three regional analyses. Latitude explains 33.246.2% of the variation in the number of individuals per species. Main conclusions Species abundance distribution, rather than species pool size, plays a key role in driving latitude-diversity gradients for -diversity in local forest communities. The individual-based null model is not a valid null model for investigating -diversity gradients driven by mechanisms of local community assembly because the null model incorporates species abundance distributions, which are driven by mechanisms of local community assembly and in turn generate -diversity gradients.