Seasonality and interspecific competition shape individual niche variation in co‐occurring tetra fish in Neotropical streams

The drivers of intraspecific niche variation and its effects on species interactions are still unclear, especially in species‐rich Neotropical environments. Here, we investigated how ecological opportunity and interspecific competition affect the degree of individual trophic specialization and the population niche breadth in tetra fish. We studied the four ecologically similar species (Psalidodon aff. gymnodontus, P. aff. paranae, P. bifasciatus, and Bryconamericus ikaa) in subtropical headwater streams (three sites with two co‐occurring species and three sites with only one species). We sampled fish in two contrasting seasons (winter/dry and summer/wet) and quantified their trophic niches using gut content analysis. Psalidodon bifasciatus was the only species distributed over all the sampled streams. We observed seasonal differences in population trophic niche breadth of P. bifasciatus just when this species co‐occurred with P. aff. gymnodontus. These findings confirm the complex nature of the effects of interspecific competition, depending, for instance, on the identity of the competitor. The degree of individual specialization of P. bifasciatus was higher in the winter, and it was not influenced by the presence of another species. Conversely, the other two Psalidodon species studied presented greater individual specialization in the summer, when fish consumed a higher proportion of allochthonous items (terrestrial insects and seeds), and there were no effects only for B. ikaa. Herein, our results suggest that seasonality in food‐resource availability is a major driver of niche variation and it has the potential to play an important role in how these similar tetra species interact and coexist. in Portuguese is available with online material. RESUMO Os fatores determinantes da variação intraespecífica de nicho e seus efeitos nas interações entre espécies ainda são incertos, especialmente em diversos ambientes neotropicais. Aqui, nós investigamos como a oportunidade ecológica e a competição interespecífica afetam o grau de especialização individual e a amplitude de nicho populacional em lambaris. Nós estudamos o nicho trófico de quatro espécies de lambaris ecologicamente similares (Psalidodon aff. gymnodontus, P. aff. paranae, P. bifasciatus e Bryconamericus ikaa) em seis riachos de cabeceira subtropicais (três riachos com pares de espécies de co‐ocorrência e três locais com apenas uma espécie). Nós amostramos os peixes em duas estações contrastantes (inve