Spiders as bio-indicators of anthropogenic stress in natural and semi-natural habitats in Flanders (Belgium): some recent developments

Until recently, spiders were used only as ecological indicators. The term “ecological indicator” was defined by Blandin (1986) as a special case of bio-indication in which the absence or presence of a species and, in the latter case, its abundance are the bio-indicator. We have used spiders as ecological indicators to evaluate the nature conservation value and the biodiversity of particular sites or to evaluate the effects of changes in habitat structure brought about by nature-management measures. The approach is straightforward and simple (Maelfait & Baert, 1988; Maelfait, 1996). Situations differing in features assumed to be of importance for nature conservation or nature management are sampled for their spider fauna by means of pitfall traps. By means of multivariate techniques, the different sampling sites are ordinated or classified. From this analysis, the factors determining the composition of the spider communities are deduced. This approach has been applied for spiders of dune habitats (Maelfait et al., 1997), woodlands (Maelfait et al., 1991, 1995), marshlands (Maelfait et al., 1992, 1993), grasslands (Maelfait & Seghers, 1986; Maelfait et al., 1988; Maelfait & De Keer, 1990) and heathlands (Maelfait et al., 1990; Maelfait, 1993)—all in the northern part of Belgium. Here we want to illustrate how we have tried to use spiders for other kinds of bio-indication, in which individuals and populations of spiders are analysed in more detail. Two anthropogenic disturbance factors which are generally considered to have important effects on wild organisms in densely populated and economically highly developed regions are pollution by heavy metals and habitat fragmentation. Although at first apparently of a quite different nature, they are both suspected of being able to cause bottlenecks in populations of wild organisms. That is why we have investigated the effects of both factors at the same time and why we report here the first results of these combined studies. The occurrence and the type of problems these two disturbance factors may cause to, amongst others, spider populations (as bio-indicators) are described in some detail.