Trophic trait plasticity in response to changes in resource availability and predation risk

1. Animals can mitigate the consequences of conflicting food-web constraints (e.g. resource availability vs. predation risk) through plasticity in the expression of functional traits. Because functional trait expression in turn determines a species impact on ecosystem functions, e.g. its trophic function, there is a growing need to develop a predictive theory that links trait plasticity to ecosystem functioning. But there is currently an incomplete empirical foundation on which to develop such theory. 2. To address this issue, we explored the link between plasticity in a suite of traits (e.g. head morphology, gut size, bite size) that determine the trophic function of a generalist grasshopper herbivore facing predation risk. We reared grasshoppers in the field under two single-forage environments where the plant resources had different physical and nutritional properties and in intermediate mixed-forage environments with or without risk of spider predation. 3. We found that in the single-forage environments, grasshoppers became more efficient at ingesting the resource they were reared on. 4. In mixed-resource environments, grasshoppers developed a phenotype that was not intermediate to the phenotypes in the two single-forage environments, regardless of predation risk. 5. We propose that the suite of phenotypic changes observed in our study represent two alternative feeding strategies that vary with the possibility of using behavioural resource selection in different environmental contexts. 6. Our results suggest that plasticity in trophic function may not emerge from a direct link between the primary traits that determine consumer consumption efficiency given the physical properties of its resources (as is customarily presumed) but rather indirectly from secondary traits that may remain latent until such time that they are drawn into play when the species faces a particular challenge. This means that fully understanding how functional traits influence a species' role in an ecosystem requires consideration of a broader suite of traits than simply those assumed to constrain resource ingestion.