Differentiation of Ecological Niches in Trophic Specialists From a Disturbed Lacustrine Ecosystem: Insights from the Sympatric Lake Tana Labeobarbs (Cyprinidae)

Adaptive radiations of freshwater fishes are based on the partitioning of trophic resources as sympatric forms diverge and end up occupying unique and specialized trophic niches. It remained unknown how these specialists responded to recent human-induced ecosystem disturbances. Here, we tested how the species with various degrees of tropho-ecological specializations (generalists vs. specialists) of Lake Tana <i>Labeobarbus</i> spp. flock reacted to the ongoing detrimental human-induced ecosystem transformation and the resulting drastic population declines. In late 2022, we collected adults of the still remaining species to examine current niche partitioning within the assemblage based on the analyses of muscle stable isotope ratios and fatty acid compositions as time- and space-integrated tropho-ecological markers. The data revealed one niche for the generalized omnivorous labeobarb, four distinct niches for the nonpiscivorous, as well as three discernible niches for the piscivorous labeobarbs. Of the 11 species caught in the lake, only two pairs showed demonstrably large niche overlap. Of all the species in the Lake Tana species flock, the top (piscivorous) predators were the most strongly affected by the disturbance in ecological parameters, which resulted in altered and narrowed niches. This suggests that most of the endemic labeobarb species still retain their original, stable, and distinct diets and habitat preferences that appeared to have maintained the overall ecological relationships in Lake Tana. We also detected that some species apparently changed their diet, seemingly adapting to human-induced habitat disturbances and large population declines during the last decades.