Individual diet specialization drives population trophic niche responses to environmental change in a predator fish population

Isotopic niche indices are increasingly used for assessing food web responses to ecological stress. Understanding the trophic mechanisms that result in ‘smaller’ or ‘larger’ isotopic niche space limits the utility of isotopic niche-based metrics. In this study, individual-level diet patterns were quantified for a meso-predatory fish (Gobiomorphus breviceps) in two contrasting lakes, one with stable macrophyte beds and the other a filamentous green algae dominated system. Metabarcoding of gut contents and stable isotope data from 48 fish were compared to abundance and isotopic compositions of macroinvertebrate prey communities. We examined whether: i) niche contraction occurs in response to reduced basal resource breadth and, ii) trophic niche contraction is caused by individual fish transitioning from diet-specialization to diet-generalism. Isotope niche contraction within the meso-predatory fish diet reflected altered macroinvertebrate food web structure within the low-macrophyte lake and additional population-level diet patterns. Niche contraction was associated with increased diet generalism; fish had more taxa (sequenced taxa) in their diet resulting in population-level diet homogenization. Isotopic and diet taxa composition dissimilarity were positively related indicating the isotopic niche reflects diet diversity between individuals. Resource partitioning analysis within populations demonstrated that the meso-predator population diet composition exhibited a non-random distribution of prey. Diet-specialization between individuals, or lack of, determined population trophic structure suggesting that food web structure was driven by diet compartmentalization. These findings illuminate food webs respond to disturbances such as macrophyte loss and suggest observed attributes of food web structure may be scale-independent emergent properties occurred-occurring at lower levels of trophic organization.