LAUSR

Our goal was to report the progress of Serrasalmus marginatus invasion process in the upper Paraná River floodplain by exploring its mechanisms and coexistence with its native congeneric Serrasalmus maculatus. We described their temporal abundance variations; nonnative species’ predominance in specific habitats; recruitment effectiveness for both species and its spatial variations; spatial organization pattern of native and nonnative species; and how their abundance is related to their co-occurrence. Abundance data encompassed 26 years after the invasion, while the proportion analyses between the species occurred on a monthly basis and in different environments, and the spatial organization pattern was assessed using C-score index. Results showed that the population of nonnative species had a fast increase along the early years followed by a decrease in the population of the native species. Recently, their proportion seems to be stabilizing, apparently resulting from a differentiation on the use of resources. The species are allowed by the spatial organization pattern to coexist in the floodplain, despite their variable co-occurrence in different environments. It is possible to state the persistence of native species in the floodplain at lower abundances in relation to the nonnative species as well as when occupying different habitats.