Trait-mediated effects between predators: ant chemical cues induce spider dispersal

Predators can induce changes in prey phenotype such as dispersal, activity and foraging rate. Such trait-mediated effects (TMEs) can strongly affect prey populations and generate trophic cascades, rivaling the importance of predation in communities. However, the relevance of TME on intraguild interactions has rarely been addressed. Ants and spiders are widespread generalist predators in terrestrial habitats. Ants influence arthropod assemblages and disrupt top-down effects of spiders on herbivores by killing spiders and/or by inducing spider emigration. Here, we examined whether ants induce dispersal behaviour in spiders. We tested the effect of chemical cues of two ant species (Lasius niger, Formica clara) on the walking activity and the propensity for silk-based dispersal of spiders. Silk-based dispersal of the web-builder Phylloneta impressa increased by 80% with exposure to Lasius cues, whereas dispersal of the hunting spider Xysticus more than doubled when confronted with cues of both Lasius and Formica. In addition, Xysticus individuals showed a marked increase in walking activity when exposed to Formica but not Lasius cues. Our results show for the first time that perceived predation risk influences spider dispersal. The strong effect of ant chemical cues on spider dispersal demonstrates that TMEs contribute to the impact of ants on arthropod communities.