Freshwater invasions have occurred worldwide with significant ecological, evolutionary, and economic impacts on recipient ecosystems. Researchers now have access to a combination of approaches to better identify the potential ecological impacts on recipient communities. We used an integrative approach based on morphological traits, stomach contents, and stable isotope ratios (13C and 15N) to examine multiple dimensions of the niche of native Texas cyprinodontids, Red River pupfish (
Cyprinodon rubrofluviatilis) and plains killifish ( Fundulus zebrinus), and their sympatric invasive congeners sheepshead minnow ( C. variegates) and gulf killifish ( F. grandis). We analyzed specimens from the Brazos River and Red River basins in Texas collected during multiple surveys. Because of their phylogenetic relatedness, we predicted high overlap in the feeding habits and habitat-use between native and invasive congeners. A principal component analysis performed on 24 morphological traits associated with habitat-use and feeding ecology suggested some overlap on trait space occupied by the two Cyprinodonspecies, but no overlap between the two Fundulusspecies. High overlap on dietary and isotopic niche space was observed between the native and invasive Cyprinodonand the Fundulusspecies. Values of 13C and 15N ratios revealed that the native and invasive Cyprinodonspecies occupied virtually identical isotopic niches, while two Fundulusspecies showed only slight differences. While species differed in their trait space, similarities in the feeding ecology between cyprinodontid congeners could lead to negative ecological interactions related to food resources. Our findings highlight the needs for continued monitoring of invasive cyprinodontids to prevent their spread into upstream reaches of the Brazos River to ensure conservation of native congeners.