The arachnofauna of Bos van Aa (Zemst, Vlaams-Brabant, Belgium): comparison between two faunistic studies, 25 years apart

Drastic changes reshaped Bos van Aa during the past 50 years. Once an extensive agricultural area, industrial activities turned these grasslands into an intensively used sand quarry. At present Bos van Aa is a nature reserve, both nationally and internationally protected. This paper presents the results of the arachnofauna based on an intensive invertebrate survey during 2011-2012. About 25 years ago, during the sand quarry period of Bos van Aa, VAN KEER & VAN KEER (1990) already revealed a diverse arachnofauna with 153 species (this amount axcludes species from the immediate surroundings and considers only the old sand quarry area). The present study revealed 189 species in the same area. Despite differences in methodology and efforts, some interesting trends are noticed. The total amount of species, the percentage of Red List species, southern species, and species of more stable environments increased. Our results show that Bos van Aa still harbours a species rich arachnofauna. Seemingly, spider communities now are more varied than 25 years ago. This might be explained by the increased habitat diversity. On the other hand, mainly species from open, dry and wet oligotrophic grasslands disappeared or strongly decreased. Because of natural succession these grasslands are disappearing quickly throughout Bos van Aa. Their conservation is of prime importance, both on a local and a regional scale. The authors advise to conserve the present open vegetation and restore disappearing grasslands.