At this moment more than 200 species of spiders have been recorded in the Flemish coastal dunes. Especially those species related to the open and sandy dune slacks and to the short grazed grasslands are threatened in Flanders. Since April 1997, two dune reserves (Westhoek & Houtsaegerduinen) along the Flemish westcoast are managed by means of extensive grazing by domestic livestock. Due to this grazing regime, we expect populations of threatened species to increase and assure their chances of survival in the Flemish coastal dunes. The University of Gent (Lab. Botany and Pedology) and the Institute of Nature Conservation are investigating the effect of grazing on the fauna, flora and soil characteristics. The project is supported by funds of the Life program (European community). Monitoring of surface active invertebrates (Carabidae & Araneae) is done by using pitfall traps at several sites, by means of which extensively grazed areas can be compared with ungrazed replicates (exclosures for the livestock). At each same site information is gathered about the hydro- logy, pedology and vegetation (species composition & structure). By studying aeronautic behaviour of spiders (better known as ballooning), we are trying to estimate the influence of this phenomenon on the population dynamics. In coastal dunes, ballooning is probably of major importance for the colonisation of habitats with short vegetation, where species cannot survive during winter. Indeed, our first data suggest that the small peak of aeronautic behaviour in early spring is related to the airborne invasion' of short vegetation (moss dunes, short grazed mesofile swards, pioneer dune slacks). Since spiders of habitats with a short vegetation, are only capable of surviving there under favourable conditions (high temperatures), they retreat during wintertime to more buffered habitats with higher and less fluctuating temperatures. Because spiders are vulnerable to freezing and they are not active during winter, this period is critical for their survival and determines the length of the species specific life cycle. The species specific critical minimum temperature, causes spiders to use different overwintering habitats, with different types of litter. First results suggest that especially litter of Salix repens shrubs is of great importance for the hibernation of species bound to pioneer and sandy dune slacks (Argenna subnigra, Typhocrestus digitatus and Xysticus sabulosus).
|Tijdschrift||Biologisch Jaarboek (Dodonaea)|
|Status||Gepubliceerd - 1998|