The sign of cascading predator effects varies with prey traits in a detrital system.

Theory and experiments show that the nature of 'green' trophic cascades, between predators, herbivores and plants, varies with several key species traits: predator hunting mode and predator and prey habitat domains. Meanwhile, 'brown' cascades between predators, environment-modifying detritivores and plants have been largely overlooked and the roles of species traits, particularly prey traits, in determining the nature of these cascades remains unclear. We hypothesize that, in predator-detritivore-plant interaction chains, the burrowing ability of plant-facilitating detritivores determines their response to predators and thus the sign of indirect effect transmitted. In the dung-decomposer food web of an alpine meadow, we predicted that in the presence of above-ground predacious beetles: (i) non-burrowing detritivores will suffer mortality due to predation and transmit negative indirect effects to plants, whereas (ii) burrowing detritivores will escape predation by retreating deeper into the soil, transmitting positive indirect effects to plants. In support of predictions, experiments showed that a single species of predacious beetle (i) reduced the density of the non-burrowing species and indirectly reduced dung loss rate, soil nutrient concentrations and plant biomass, but (ii) drove the burrowing species deeper, indirectly improved soil conditions and increased plant biomass. These results show that the burrowing ability of a detritivore can determine whether it transmits a negative indirect effect mediated by a reduction in its density, or a positive indirect effect mediated by its behavioural response to predation risk. We call for further tests of our detritivore-trait hypothesis in different regions and ecosystems to further develop a general trait-based framework for trophic cascades in detrital food webs. We further advance the general hypothesis that the locomotion traits of prey species (e.g. burrowing/flying ability) may help explain their behavioural response to predation risk and the nature of indirect effect they transmit from predators to plants.