The question of which factors determine the geographical change in species composition and abundance (turnover and β diversity) has typically been studied at a single scale, so that, e.g., regional-scale factors are used to explain regional turnover. But cross-scale effects are also important to understand species turnover and the spatal distribution of biodiversity in general. Here, we explored how regional richness, local forest-structure, and regional climatic factors interact to influence local species turnover. We analysed a dataset that includes the distribution of 916 fern species recorded in 1227 plots in the Bolivian Andes, forest-structure variables collected in the feld, and climatic variables extracted from global databases. We used path analyses to: (i) select the best models explaining the variaton in local species turnover and (ii) identify the factors that have a direct effect on species turnover and those with only indirect effects. We contrasted our results against those obtained from a null model analysis. The most important variable explaining variaton in species turnover was regional species richness. We consider that this is the result of inter specific competition resulting in narrower realized ecological niches of species, although further studies are needed to confirm the mechanism. We also found that the relationship between climatic variables and local species turnover is best described by the indirect link between climatic factors and regional species richness. Our results might appear to be in conflict with previous studies finding that climatic and edaphic factors are direct predictors of local and regional variaton in fern turnover. However, this is due to the different scales at which turnover was analysed. In contrast to previous studies, ours reflects the cross-scale effect of the variaton in regional factors on local species turnover. Our study supports the idea that in regions with high species richness, biotic interactions strongly determine local community composition.
