Experimental cutting of reedmarsh vegetation and its influence on the spider (Araneae) fauna in the blankaart nature reserve, Belgium

Abstract

Experimental cutting has been carried out since 1978 to preserve botanical diversity in a reedmarsh subject to periodic desiccation. In order to study the influence of this treatment on spiders, two plots cut in winter, one in summer and seven control plots (without treatment) were sampled in 1984 by pitfall trapping and sweep-netting. The sampling plots were situated in two reedbelts and two plots were less in width than the others. In the case of winter cutting a tall species-rich reed vegetation with a considerable amount of bare soil has developed. Summer cutting favours the development towards grassy vegetation with a dense sod structure and only very thin and low reed. The controls are characterised by a thick litter layer, fewer plant species, thin reed and a dense structure due to the abundance of Calystegia sepium and Urtica dioica. Temperatures at ground level were more extreme in the plots with cutting treatment. A total of 13 923 adult spiders, belonging to 67 species, were captured by pitfall trapping. Kruskal-Wallis analysis of variance indicated that the distributions of most of the abundant species were aggregated. The spider coenoses of the two reedbelts differed from each other, the narrow reedbelt yielding more species from the surrounding habitats and less rare wetland spiders. Species richness and total number of individuals per plot were not significantly correlated with the act of cutting. At species level however, a significant increase or decrease was observed. Most sensitive to cutting were the rare wetland spiders and some widespread wetland species. On the other hand, a striking increase of ubiquitists, mostly with obvious pioneering abilities, and a few widespread wetland species was noticed in the cut plots. Differences were far more pronounced due to summer than to winter cutting. Sweep-netting yielded nine extra species. Optimal web-building conditions seem to be of primary importance for the occurrence of the most abundant species. It is argued that, for nature conservation purposes, sufficient untreated parts of reedmarsh should be preserved at all times where a litter layer can develop and vertical plant structures are present.