Individual specialization is the highest in generalist populations from intermediary to high trophic positions in tropical freshwater fishes

Abstract

Individual specialization (IS) refers to intrapopulation variation in resource use unrelated to ontogeny, sexual dimorphism, or discrete morphological variation. A broad niche increases individual specialization because individuals might segregate in niche space. If trophic position influences niche breadth, it would indirectly constrain the degree of individual specialization. Intermediate trophic levels, usually associated with omnivory, might display fewer constraints on their trophic niche, leading to high levels of individual specialization. We investigate the degree of individual specialization and its relationship to the trophic position in 121 fish populations belonging to 32 species from the upper Tocantins River, Central Brazil. We calculated IS using the index V based on the stomach contents of individuals, whereas values close to 1 indicate strong IS. IS ranged from 0 to 0.87 and greatly varied among species. We showed that niche breadth positively affects a population's individual specialization regardless of its trophic position. Populations occupying intermediate and high trophic positions displayed the highest degrees of individual specialization. Omnivory has no significant effect over individual specialization values. We argue that the intricate relationship among individual specialization, trophic position, and niche breadth relates to the great ecological opportunity and trophic plasticity in tropical fishes.