Biological invasions are a major threat to freshwater fish communities with more than 500 introduced freshwater fish species established globally. Despite their widespread distribution, ecosystem-level impacts remain poorly quantified in many regions. The Mount Lofty Ranges (South Australia) provide an important system for assessing these impacts because they support several threatened native fish species alongside established populations of introduced brown trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss), and redfin perch (Perca fluviatilis).
We analysed fish community data from 1,338 sampling events, across 356 sites, collected between 2002 and 2025 to quantify the influence of introduced predators on native fish abundance, diversity and community structure. Trout presence was associated with significant declines in native fish abundance and diversity, whereas redfin perch primarily affected exotic fish abundance.
Network analyses identified brown trout as the dominant ecological driver, exerting strong predatory effects on four native species of conservation concern, while also indicating strong competitive interactions with redfin perch. Centrality analyses further demonstrated that brown trout and, to a lesser extent, redfin perch have fundamentally restructured fish community composition, trophic interactions, and energy flow across the Mount Lofty Ranges.
These findings demonstrate that invasive predators differentially reshape freshwater fish communities and suggest that management targeting redfin perch in isolation could unintentionally accelerate native fish declines by releasing competitive or predatory pressures from trout. Effective conservation of freshwater ecosystems will therefore require ecosystem-based management strategies that explicitly account for interactions among multiple invasive species.