Environmental and functional dissimilarity shape fish co-occurrence in streams with different vegetation types in Eastern Amazon

Abstract Context Landscape heterogeneity and environmental gradients operating across spatial scales shape species distributions and community assembly in freshwater ecosystems. In stream networks, these landscape processes mediate both environmental filtering and dispersal, generating species co-occurrence patterns that reflect spatial aggregation or segregation. Objectives Our aim is to understand how environmental variables and functional and phylogenetic dissimilarity influence fish species co-occurrence patterns in tropical streams. Methods Fishes were sampled using circular hand nets at 18 headwater streams, distributed across two contrasting Amazonian vegetation types: savannas and forests. Environmental variables were assessed at three spatial scales: local, catchment, and drainage network. Local habitat structure and physicochemical parameters were assessed in situ, using standardized protocols. Catchment-scale variables and distances to the main river channel were obtained through remote sensing analyses. Results Fish assemblages differed markedly between forest and savanna streams, reflecting distinct environmental settings and coexistence patterns. Co-occurrence analyses revealed non-random associations among species, encompassing both positive and negative relationships. These relationships were significantly influenced by local habitat structure, catchment vegetation cover, and spatial structure. Species pairs with greater functional dissimilarity tended to co-occur more frequently than expected by chance, whereas phylogenetic dissimilarity showed no relationship with co-occurrence patterns. Conclusions Environmental variation between forest and savanna streams drives differences in species coexistence mechanisms. Riparian vegetation and forest cover at the catchment scale are key determinants of species co-occurrence and community structure. Maintaining riparian buffers and preserving forest cover are therefore essential to sustain biodiversity, ecological interactions, and functional integrity in Amazonian freshwater landscapes.