Complex interactions among invasive fishes and flow barriers combine to maintain an endangered native species.

The Breede River in the Western Cape Province is one of South Africa's major freshwater systems and of high ecological and socio-economic value. Similar to other freshwater systems worldwide, it is threatened by environmental degradation and multiple invasive species. Understanding the biotic and abiotic processes that facilitate or hinder establishment and impact of co-occurring biological invasions is a critical knowledge gap that prevents proactive management efforts. This study combined data from an extensive monitoring survey with indices of human impact to explore whether biodiversity patterns indicate signals of environmental filtering and invasion meltdown using freshwater fish diversity and abiotic factors along the entire length of the Breede River. Native fish species occupancy was uneven while invasive non-native fishes were widely distributed and represented over 40% of the overall species richness. Fish diversity was negatively correlated with the relative frequency of a translocated predator, the sharptooth catfish (Clarias gariepinus) but positively correlated with the frequencies of invasive black basses (Micropterus nigricans, M. dolomieu). Flow barriers, high abundance of competitive catfish, and increased water turbidity suppressed black bass predation and influenced fish-community dynamics, allowing an endangered native species, the Berg-Breede whitefish (Cheilobarbus capensis), to persist at extremely low frequencies. Thereby, we show empirically that the ecosystem engineering behaviours of the turbidity-adapted sharptooth catfish suppress the high predatory impact of visual predators, black basses. Our results show that the Breede River is heavily impacted by species invasions, but that these species have antagonistic interactions which moderate ecological impact.