Trophic position of the species and site trophic state affect diet niche and individual specialization: from apex predator to herbivore

Abstract Intra-species variability in a diet niche, specifically total niche width (TNW), individual niche width (INW), and individual specialization (IS), was studied using an innovative approach without sacrificing the animals. Stable isotopes ( δ 13 C, δ 15 N) in four body tissues differing in isotopic half-life were analyzed from four freshwater fish species representing different trophic positions. TNW was widest for the apex predator (European catfish) and narrowest for the obligate predator (Northern pike). INW exhibited a polynomial trend for European catfish, Northern pike and Eurasian perch (mesopredator), decreasing with body mass and increasing again after exceeding a certain species-dependent body mass threshold. Thus, for ectotherms, apex predator status is linked rather to its size than to the species. In herbivores (rudd), INW increased with body mass. IS of predators negatively correlated with site trophic state. The trends can be generalized to other species at similar trophic positions in either aquatic or terrestrial systems.