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

Abstract

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 nichebased metrics. In this study, individual-level diet patterns were quantified for a meso-predatory fish (Gobiomorphusbreviceps) intwocontrasting lakes, onewithstablemacrophytebedsandtheotherafilamentous green algae dominated system. Metabarcoding of gut contents and stable isotope data from 48 fish were compared to abundance andisotopiccompositions of macroinvertebrate prey communities. We examinedwhether: 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 lowmacrophyte 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 ref lects diet diversity between individuals. Resource partitioning analysis within populations demonstrated that the meso-predator population diet composition exhibited a non-random distribution of prey. Dietspecialization between individuals, or lack of, determined population trophic structure suggesting that food webstructure was driven by diet compartmentalization. These findings illuminate food webs respond to disturbancessuchasmacrophytelossandsuggestobservedattributesoffoodwebstructuremaybescale-independent emergent properties occurred-occurring at lower levels of trophic organization.