Optimal Foraging Strategy of the Sheet‐Web Spider Lepthyphantes Flavipes Under Perturbation

Field work concerning a sheet—web spider, Lepthyphantes flavipes (Linyphiidae), found on the upper surface of a deciduous forest litter indicates that a physical perturbation like falling leaves has a major effect on spider movement. The proposed optimal foraging model using stochastic dynamic programming includes perturbation movement as opposed to foraging movement between web sites, with variable prey availability in space and time. The two possible strategies (fixed—site strategy or mobile strategy) are constrained by different energy costs and predation risks. Decision is base on prey availability of an occupied web site and on internal fat reserves. It is found that a fixed—site strategy is optimal in good web sites (better than the habitat mean) and in poor web sites when spiders have high fat reserves. A mobile strategy is optimal in poor web sites at low reserves. An optimal mobile strategy is much less profitable than an optimal fixed site strategy, but the latter is interrupted by perturbation. The model may explain the prevalence of the fixed—site strategy in L. flavipes and in Linyphiidae in general.