Climate and regional beta-diversity gradients in spiders: dispersal capacity has nothing to say?

Abstract. 1. The importance of dispersal capacity in structuring beta-diversity gradients has been pointed out in a number of recent studies, making geographical distance (GD) an important predictor of the composition of assemblages apart from environment. In this study, we analyse the relative importance of climate, habitat complexity and GD in four spider families with different dispersal tendencies in the centre of the Iberian Peninsula. 2. Regional beta-diversity was calculated for the four families and values were bootstrapped to test for significant difference between groups. Mantel tests assessed the explanatory capacity of four potential predictors – climatic similarity, similarity of vegetation complexity, GD and cost dis-tance (CD) considering topography – on faunistic similarity. Simple regressions were used to com-pare decay rates, and bootstrapping tested for significant differences in coefficients. 3. Regional beta-diversity values were higher in less vagile families. Climatic distance was strongly correlatedwith faunistic dissimilarity in the four families, whereasGDhad no explanatory power. Except in one family, CD was correlated with faunistic dissimilarity but its independent associationwas negligible when controlling for climate. 4. Both the pattern of regional beta-diversity among families and the pattern of variation in spe-cies composition explained by climate are consistent with the hypothesis that dispersion capacity influences the composition of the assemblages. However, climate distance was the main factor determining species sorting in space while GD had a weak effect. Differences in the degree of spe-cialisation or nonlinear relationships between species composition and space may obscure the sig-nal ofGDat the scale of work. Key words. Araneae, beta-diversity, dispersal, environmental gradient, geo-graphical distance