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

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.

Automated summary

Individual specialization refers to differences in resource use within a population that are not related to factors such as growth, sexual dimorphism, or morphological variation. A wider niche promotes individual specialization as individuals can segregate in niche space. Trophic position plays a role in determining the level of individual specialization, with populations at intermediate and high trophic levels displaying higher levels of specialization. We found that regardless of trophic position, niche breadth positively affects the degree of individual specialization in fish populations from the upper Tocantins River. Omnivory did not have a significant impact on individual specialization. These findings highlight the relationship between individual specialization, trophic position, and niche breadth in tropical fish.