Size-related competition and predation risk drive shifts in trophic traits

Background/Question/Methods

The function of animals in food webs depends on their ability to mitigate conflicting demands such as finding resources and avoiding predators. This may lead to an alteration of the phenotypic expression of functional traits that can further feedback on the structure of the community. If the strength of competition and predation is size related we expect the magnitude of phenotypic responses of a competitor or a prey to be scaled with size. To test this we used the size-structured Eurasian perch (Perca fluviatilis) and manipulated competitor size and density with and without predation risk in an enclosure experiment. We tested resource use of the prey fish and their phenotypic responses to competition and predation separately and combined. Perch undergoes size related ontogenetic niche shifts suggesting stronger intra-cohort than inter-cohort competition. Predation risk is also strongly size related and was assumed to decrease with size. Thus, we expected the magnitude of morphological responses of the prey to decrease with size in relation to competition and predation.

Results/Conclusions

The results showed that the diet of large perch prey shifted to more benthic resources at high competitor density but there was no shift in diet in response to predators. Small perch showed more complex responses in diet to competitors and predators: in the presence of predators, they included more benthic resources at low competitor density but less benthic resources at high competitor density. Still, competition and predation had a small influence on diet breadth, diet specialization and diet overlap between prey size-classes. The morphological responses to competitors and predators were relatively strong and in different directions depending on the strength of competition and the risk of predation. This suggests that the phenotypic response to the simultaneous changes in competition and predation results in an expression of phenotypic traits that are likely a result from conflicting demands of finding resources and avoiding predators.