Detecting and measuring impacts of a spreading invasive predator across a wetland landscape

Impacts of invasions may be assumed to be spatially uniform, but few published studies have tested whether impacts from the initial location of an invasion are transferrable. We address this gap using the Asian swamp eel ( Monopterus albus/javanensis ), a predatory fish whose establishment in one smaller drainage (Taylor Slough) of southern Everglades National Park (Florida, USA) was linked to subsequent declines of native fishes and crayfishes. The swamp eel has since spread into other wetland habitats and regions of the Everglades and we analysed more than 28 years of standardised monitoring data (1996–2025) from two long-hydroperiod (slough) systems (Shark River Slough and Water Conservation Area 3B [WCA-3B]) plus a 20-year dataset from two short-hydroperiod marl prairies to assess evidence for impacts. Using region- and species-specific baseline hydrologic models from pre-establishment periods, we predicted prey densities in the post-establishment period and tested whether deviations exceeded natural variability. Some species were consistently sensitive across regions, but impact patterns varied amongst regions and species. In WCA-3B, reductions of crayfish and several small fishes were strong and similar to those in Taylor Slough, whereas impacts in Shark River Slough were variable and weaker. In marl prairies, Everglades crayfish declined more strongly following swamp eel establishment in the invaded prairie than in a reference prairie, but crayfish populations persisted even after swamp eel invasion. We hypothesise that relatively lower swamp eel abundances and/or greater habitat structural complexity in Shark River Slough may have weakened impacts there, relative to other regions. Our results suggest that impacts of a spreading invasive predator on its prey are dynamic outcomes, mediated by ecological context with implications for predicting impacts and managing invasions across heterogeneous landscapes.