Trophic Position of the Species and Site Trophic State Affect Diet Niche and Individual Specialization: From Apex Predator to Herbivore

SIMPLE SUMMARY: Niche widths and individual specialization were studied based on the isotopic signals, but using the innovative and non-lethal approach. We analyzed four different body tissues with different isotopic half-lives, and revealed crucial results on trophic interactions of fish. We assume that the observed trends will occur in other food webs with similar trophic positions. For example, the apex predator status of ectotherms is linked to the individual size, not to the species. In addition, thanks to the consideration of the site trophic state (total phosphorus content), which has been little studied in relation to niche width, we observed a significant impact on the individual specialization of species in higher trophic positions. Thus, eutrophication can significantly change the foraging behavior. ABSTRACT: Intra-species variability in isotopic niches, specifically isotopic total niche width (ITNW), isotopic individual niche width (IINW), and isotopic individual specialization (IIS), was studied using an innovative approach without sacrificing the vertebrates. 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. ITNW was widest for the apex predator (European catfish) and narrowest for the obligate predator (Northern pike). IINW exhibited a polynomial trend for the 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), IINW increased with body mass. The IIS of predators negatively correlated with site trophic state. Therefore, eutrophication can significantly change the foraging behavior of certain species. We assume that the observed trends will occur in other species at similar trophic positions in either aquatic or terrestrial systems. For confirmation, we recommend conducting a similar study on other species in different habitats.