Carbon pathways and trophic attributes are conserved in carnivorous reef fishes across a major human disturbance gradient

Habitat degradation and overexploitation are key drivers of biodiversity loss globally. Negative, human-driven changes in habitat quality, species abundance, and community composition are well-documented across systems. While it is understood that human stressors destabilize consumer-resource interactions, how energy pathways and food webs may reorganize in hyperdiverse tropical systems in response to human disturbance remains poorly understood due to their complexity and past methodological limitations. Leveraging recent advances in molecular isotope techniques, we performed an ecosystem-scale natural experiment to evaluate how human disturbance reorganizes carbon pathways and trophic structure in a hyperdiverse tropical system, Kiritimati Island, the world's largest atoll. We specifically employed novel integrations of bulk tissue and amino acid-specific stable isotope analyses applied to six nominally generalist fish species sampled across Kiritimati's well-documented human disturbance gradient. Sampled fish species comprised 48% of carnivorous reef fish biomass. Essential amino acid stable carbon isotope (δ¹³C_EAA) fingerprinting and Bayesian stable isotope mixing models indicated that the proportional contribution of the carbon sources supporting five of the six sampled fish species did not vary across the disturbance gradient. Energy disproportionately (>80%) originated from planktonic production and microbially reworked detritus for most species, with only minor contributions of carbon sourced from coral and epilithic algal matrices. Reef fish trophic ecology was also consistent across the atoll, with species maintaining isotopic niches (size and position) and trophic positions across the atoll despite significant human disturbance-mediated changes in community composition and habitat complexity. Our findings suggest that the energy channels connecting basal resources to upper trophic level generalist consumers can be highly conserved following significant human disturbance in hyperdiverse tropical systems. On coral atolls, specifically, robust planktonic or detrital energy channels may buffer nominally generalist carnivorous reef fishes from some negative effects of chronic local human disturbance, promoting the maintenance of dominant energy fluxes in disturbed habitats. These results illustrate that disturbance-mediated changes in ecosystem structure and function do not universally destabilize broad energy fluxes and trophic relationships in hyperdiverse ecosystems. On the contrary, there appear to be mechanisms that promote stability, such as broad reliance on system-dominant production sources.