The distribution of spiders in the wooded riparian zone of three rivers in Western Europe

It has been hypothesised that river margin 'ecotones' are highly sensitive to changes in the hydrological regime. The distribution of spiders (Araneae) in the same eco-region ('Marine Division' of the 'Humid Temperate Domain') within the river margins of the rivers Helge, Garonne and Trent is examined. The analysis uses data for three patch types defined by elevation, a surrogate for flood frequency, with five samples from each (each sample comprising the combined collections from nine pitfall traps) for four sample dates (two in spring and two in late summer), each following 2-week collecting periods. The total capture was 7529 individuals, representing 147 species from 17 families. Multivariate analyses indicated a regional gradient along the principal axis of the ordination. At the local level, elevation is the most important variable, ahead of seasonal differences, with species diversity greatest at the intermediate levels. The data suggest that large-scale climatic and physical factors are driving regional assemblages, while at the reach level, flood duration and timing (directly or indirectly) become important in influencing the makeup of the resident community. The results of this study are allied with previous research in an attempt to define a benchmark riparian spider community for this eco-region.