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.

Predators unintentionally release chemical and other cues into their environment that can be used by prey to assess predator presence. Prey organisms can therefore perform specific antipredator behavior to reduce predation risk, which can strongly shape the outcome of trophic interactions. In contrast to aquatic systems, studies on cue‐driven antipredator behavior in terrestrial arthropods cover only few species to date. Here, we investigated occurrence and strength of antipredator behavior of the wood cricket Nemobius sylvestris toward cues of 14 syntopic spider species that are potential predators of wood crickets. We used two different behavioral arena experiments to investigate the influence of predator cues on wood cricket mobility. We further tested whether changes in wood cricket mobility can be explained by five predator‐specific traits: hunting mode, commonness, diurnal activity, predator–prey body–size ratio, and predator–prey life stage differences. Crickets were singly recorded (1) in separate arenas, either in presence or absence of spider cues, to analyze changes in mobility on filter paper covered with cues compared with normal mobility on filter paper without cues; and (2) in subdivided arenas partly covered with spider cues, where the crickets could choose between cue‐bearing and cue‐less areas to analyze differences in residence time and mobility when crickets are able to avoid cues. Crickets either increased or reduced their mobility in the presence of spider cues. In the experiments with cues and controls in separate arenas, the magnitude of behavioral change increased significantly with increasing predator–prey body size ratio. When crickets could choose between spider cues and control, their mobility was significantly higher in the presence of cues from common spider species than from rare spiders. We therefore conclude that wood crickets distinguish between cues from different predator species and that spiders unintentionally release a species‐specific composition and size‐dependent quantity of cues, which lead to distinct antipredator behavior in wood crickets.

Predators can indirectly enhance plant performance via herbivore suppression, with both prey consumption and changes in prey traits (e.g. changes in foraging behaviour) contributing to the reduction in herbivory. We performed a field experiment to determine the extent of such non-consumptive effects which consisted of repeatedly placing spiders (Pisaura mirabilis) on enclosed plants (Urtica dioica) for cue deposition. Control plants were enclosed in the same way but without spiders. After cue deposition, the enclosures were removed to allow arthropods to colonize the plants and feed on them. Arthropods were removed from the plants before the subsequent spider deposition or control enclosure. During six cycles of enclosure, we quantified leaf damage on the plants. After a seventh cycle, the colonizing arthropods were sampled to determine community composition in relation to the presence/absence of spider cues. We found that the presence of chemotactile spider cues reduced leaf damage by 50 %. In addition, spider cues led to changes in the arthropod community: smaller spiders avoided plants with spider cues. In contrast, the aphid-tending ant Myrmica rubra showed higher recruitment of workers on cue-bearing plants, possibly to protect aphids. Our results show that the risk of spider predation can reduce herbivory on wild plants and also demonstrate that non-consumptive effects can be particularly strong within the predator guild.

1. To avoid predation, prey often change their behaviour upon encountering cues of predator presence. Such behavioural changes should enhance individual survival, but are likely to be energy-demanding. This should deplete energy reserves of the prey, unless it increases food intake.2. These hypotheses were studied by conducting two microcosm experiments. In the first, crickets were kept on plants previously occupied by a spider or on control plants. After 3 days leaf consumption and weight gain of the crickets were quantified. In the second experiment, crickets were kept in the presence or absence of spider cues for 3 days. Spiders were then added and predation of the crickets was recorded during 24 h.3. Crickets that had previously experienced spider cues were more successful in avoiding predation. Moreover, crickets under predation risk tended to increase foraging in the first microcosm experiment and gained more weight in the second microcosm experiment.4. The results demonstrate that previous experience of predator cues decreases predation rate. Furthermore, they suggest that crickets are able to compensate for increased energy demands caused by antipredator behaviour. In more natural situations, moving to cue-free plants may play an addition role.

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