A SPIDER POPULATION IN FLUX - SELECTION AND ABANDONMENT OF ARTIFICIALWEB-SITES AND THE IMPORTANCE OF INTRASPECIFIC INTERACTIONS IN LEPTHYPHANTES TENUIS (ARANEAE, LINYPHIIDAE) IN WHEAT

Lepthyphantes tenuis, a small sheet-web-building linyphiid spider is o ne of the most abundant spider species of cereal fields in Europe. In the present study we examined the process of web-site selection and we b-site tenacity by adult females of this species in a winter wheat fie ld, Spiders were selective in their choice of web-site. Different immi gration rates into various manipulated web-sites, in field and laborat ory, suggested that structural support and suitable micro-climate (hig h humidity) are the most important factors in the selection. Small hol es dug in the ground were the most favoured web-sites. Web-site occupa tion was influenced by the presence of other conspecific spiders. Terr itorial contests occurred between spiders attempting to occupy the sam e web, these almost invariably led to the take-over of the web when th e intruder was heavier. interference, but also a certain level of tole rance, between spiders within the same web-site but in different webs was suggested by direct and indirect evidence. Many holes supported tw o or even three spiders in vertically stratified webs. Leaving probabi lity of marked spiders was significantly higher in multiply occupied h oles than in holes with a single web. Comparison with the results of a no-interference stochastic model showed that multiple occupancy in na ture is less frequent than predicted by the model. There was further e vidence for weak extra-web-interference between spiders in that multip le occupancy was even less frequent and overall occupancy was lower in web-sites which were packed close to each other. However, a level of tolerance for crowding is shown by the fact that closely packed hole c olonies supported a spider density 13 times higher than in natural web -sites in the field. A marking experiment was carried out to gain info rmation on web-site tenacity (i.e. the length of time a spider spends in a web-site) and abandonment. The average duration of tenacity was l ess than 2 days. A random loss function gave a good fit to the tenacit y distribution and suggested that spiders abandoned web-sites randomly with a fixed leaving probability of e. 0.5. individual webs were ofte n used consecutively by more than one spider, and some spiders built m ore than one web in the same web-site. Calculations showed that abando nment is the most frequent leaving mode, while take-over by contest be tween spiders and disappearance due to destruction were somewhat less frequent and equally likely modes of ending tenacity. It is suggested that the apparent contradiction be tween the selectiveness and competi tiveness of spiders for web-sites and the relatively short tenacity ob served can be resolved by hypothesising that spiders leave web-sites s oon because they apply the strategy of spreading risk: spiders by freq uently moving from one web-site to another distribute their reproducti ve efforts across several localities. This hypothesis is further suppo rted by changes in web-sire preference and ballooning behaviour at the onset of the reproductive stage in L. tenuis.