Regional and local variation of spider assemblages (Araneae) from coastal grey dunes along the North Sea

This study aims to determine the underlying causes of local and regional patterns of variation in community structure of spiders in coastal grey dunes, and especially whether ecological time constrains the species composition in young and isolated grey dune habitats.The study was conducted in coastal dunes from northern France (Boulonnais, Nord-Pas-de-Calais), Belgium (Flemish coastal dunes) and the Netherlands [Amsterdam Water Supply (AWD) dunes and Dunes from the Provincial Water Company North-Holland]. Spiders were collected with pitfall traps in twenty-eight grey dune patches in the four areas under investigation. Species composition and environmental parameters (vegetation structure, distance to the sea and the patch-edge, eolic dynamics and lime richness) were determined. Assemblage composition was related to the regional and local environmental factors with the Primer software package to determine the assemblage-determining parameters. Differences in species presence were analysed as a function of their habitat preference and distribution range. Differences in grey dune spider assemblage structure can mainly be attributed to differences in local sand dynamics and the region. Species from dynamic dunes are mainly present in grey dunes from Belgium and France, while species from non-dunal xerothermic habitats (chalk grasslands and heathland) occur in both the Boulonnais and the north Holland dune region. These species are absent from geologically young or other xerotherm habitat isolated Flemish coastal dunes. The data show that regional variation in spider assemblage composition results from local landscape characteristics (dynamics in the dune area), the latitude and the connectivity to non-dunal xerothermic habitats. The strong and moderate geological isolation of dune areas from the Flemish coast and the AWD dunes, respectively, results in the absence of (at least some) species that are primarily bound to heathland and/or chalk grassland. This indicates the importance of ecological time for the assemblage structure. The limited dispersal capacity of the heathland and/or chalk grassland species is probably the main reason for their absence.