No experimental evidence for the role of trait-mediated interactions in a marine tri-trophic cascade

In multitrophic systems, top predators influence the abundance and biomass of lower trophic levels. Top-down control through predation is important for regulating ecosystems. In addition to consumption, predators also alter the behaviour, life-history, and morphology of prey species. This trait-mediated interaction is a response to predation risk and can indirectly reduce reproductive fitness as prey species change patterns of energy uptake and allocation. These combined direct and indirect predator-control pathways are understood to be widespread. Here, we investigated the role of trait-mediated interactions in a marine tri-trophic food chain: an echinoderm top predator (Crossaster papposus) and echinoderm mesopredator (Asterias rubens), and a molluscan basal prey species (Mytilus edulis). We exposed the mesopredator to cues from the top predator and the prey species and analysed movement patterns in a mesocosm experiment. We found no statistically significant differences between different cue combinations (control, top predator only, prey only, combined predator and prey cues). This suggests that trait-mediated interactions can be context-dependent, and that “fear effects” may not always be the primary driver of movement, though caution is warranted when extrapolating beyond the present experimental conditions.