1 Biology Department, Concordia University, Montreal, Quebec, Canada.
2 Laboratorio de Ictiología, Instituto Nacional de Limnología (UNL-CONICET), Santa Fe, Argentina.
3 Facultad de Humanidades y Ciencias, Departamento de Ciencias Naturales, Universidad Nacional del Litoral, Ciudad Universitaria, Santa Fe, Argentina.
4 Departamento de Ecología y Gestión Ambiental, Centro Universitario Regional del Este, Universidad de la República, Maldonado, Uruguay.

Modularity and nestedness have been observed recurrently across different ecological networks, including food webs and occurrence networks. These patterns emerge from species-level processes, where interactions and occurrences are determined by niche-based and/or abundance-based mechanisms. Abundance-based processes promote nested networks with gradients in the number of links determined by species abundances. Niche-based processes can promote modular structures due to differential spatial filters or trait matching in discontinuous gradients of predators and prey traits or nestedness due to gradients in the strength of environmental filters or trait limitation for consumption. Here, we explore the mechanisms driving species-level interactions and the resulting network structure in both food webs and occurrence networks of piscivorous fishes from the Paraná River. Our study focused on 16 species of piscivorous fish. We constructed occurrence networks (149 communities, 3010 observations) and food webs (113 prey species, 1271 trophic interactions). Using null models, we assessed modularity and nestedness in both types of networks, as well as the existence of significant deviations in the trait composition, functional diversity and community-weighted mean among modules. Moreover, we assessed the relationship between species abundance and degree to identify the potential role of abundance-based processes. Occurrence networks and food webs exhibited a modular structure, with no evidence of nestedness. In both networks, niche-based mechanisms played an important role. Each module showed a distinct representation of habitat types in occurrence networks and prey types in food webs. A significant relationship was also observed between predator abundance and the number of interactions or occurrences, suggesting that abundance-based mechanisms also contribute to network organization. Here, we are getting ahead in understanding the mechanisms driving ecological organization in piscivorous fishes from the Middle Paraná River. Although food webs and occurrence networks represent distinct dimensions, our results reveal a consistent pattern: both are shaped by a combination of abundance- and niche-based processes. This convergence highlights shared principles of network assembly across contexts. By disentangling the contributions of these mechanisms, our findings advance ecological theory and highlight that protecting functional diversity and resource heterogeneity is essential for preserving the structure of ecological networks.