Biogeographic patterns of spiders in coastal dunes along a gradient of mediterraneity.

The Iberian Peninsula is recognized for its high levels of species richness, rarity and endemicity. Our main aim was to study biogeographic patterns of spiders in coastal dunes along a gradient of mediterraneity. Distance-decay of similarity, nestedness and co-occurrence metrics were used to explore spider' distribution patterns. A similarity analysis was performed in order to obtain a hierarchical classification of sites. Indicator species analysis was conducted to identify indicator species for the various clustering levels of the site typology. The differentiation among assemblages was further explored by non-metric multidimensional scaling (NMDS) and analysis of similarities (ANOSIM). Assemblages' similarity among sites decreased with climatic/geographic distance. The observed values of nestedness metrics (T and NODF) were not significant, while the co-occurrence metrics (C-score and Checkerboard units) were higher than expected by chance. Cluster analysis showed that spider' assemblages were structured along a gradient from North to South, forming four geographically distinct clusters. ANOSIM tests and NMDS supported the biogeographic patterns identified by cluster analysis. Several indicator species were found for the different levels of the hierarchic site typology. Spider assemblages revealed a high degree of biogeographical structure along the Atlantic coast of the Iberian Peninsula. The coast is a 'biogeographic crossroad', encompassing faunistic elements of different origins. The hierarchic typology of sites is generally consistent with the major biogeographic regions and the thermoclimatic belts recognized for the region. Our results indicate that the climatic gradient and historic factors played a key role in the current spiders' biogeographic patterns. [ABSTRACT FROM AUTHOR]