Effects of the presence of a predatory fish and the phenotype of its prey (a shredding shrimp) on leaf litter decomposition

The role of inducible defences in trophic cascades is poorly understood. The shrimp shredder Xiphocaris elongata has a short rostrum in the absence of predatory fishes and a long rostrum in their presence. The long rostrum in X. elongata is a morphological defence that is inducible by the predatory fish Agonostomus monticola (mountain mullet). Our objective was to measure the effect of fish presence and absence and shrimp phenotype (long rostrum versus short rostrum) on the foraging behaviour of X. elongata and on the decomposition of leaves of the riparian tree Cecropia schreberiana, including their potential for direct or trait‐mediated top‐down trophic cascades effects. Experiments were carried out in wading pools in which leaf discs of C. schreberiana were exposed for 24 days to processing by either microbes or short‐rostrum or long‐rostrum X. elongata in either the presence or absence of the predatory fish A. monticola. Shrimp foraging activities, shrimp survival and leaf decomposition were quantified. Shrimp density was not affected by fish presence. However, fish presence decreased foraging activity of long‐rostrum X. elongata presumably by increasing their use of refugia throughout the experiments. The foraging of short‐rostrum shrimps did not respond to fish presence. The fraction of leaf mass remaining was significantly higher when shrimps were in the presence of fish than when fish were absent, particularly when short‐rostrum shrimps were shredding leaves. Shrimps decreased the fraction of mass remaining of C. schreberiana more than the effect of microbial colonisation alone. These results demonstrate that the presence of the predatory fish A. monticola can create trait‐mediated top‐down trophic cascades in the presence of short‐rostrum X. elongata. This study provides evidence that predators have different effects on prey with morphological inducible defences than prey without defences and that these interactions may have critical consequences for other trophic levels.