Species and environmental characteristics point to flow regulation and drought as drivers of riparian plant invasion

Aim: Many factors facilitate biological invasions, making it difficult to determine their relative importance, especially when relying on survey data that include confounded variables. Incorporating information about species characteristics can improve inferences drawn from species-environment relationships, which can inform management. We seek to understand why the abundance of exotic, and not native, terrestrial plants is higher in riparian wetlands most impacted by flow regulation. Location: River Murray, SE Australia. Methods: We use variance components analysis and hierarchical generalised linear models to examine whether the positive relationship between flow regulation and proportional cover of exotic plants is driven by altered hydrological regimes, wetland drying and drought, superior colonisation ability of exotic species following disturbance or human-increased propagule pressure. Results: Of the four hypotheses, hydrological modification (indicated by flood magnitude) most likely drives invasion. Flow regulation may inhibit native species adapted to the historical hydrological regime, facilitating exotic species with different environmental ranges. A symptom of environmental change, invasion may have been exacerbated by drought, although it is unclear why. There was no indication that human-increased propagule pressure or colonisation ability facilitated invasion. Exotic cover was unrelated to proximity to towns, recent flood frequency and cattle grazing intensity. Additionally, similar proportions of exotic and native species were used in cultivation and, despite a higher proportion of exotics being known weeds, weed status was unrelated to exotic species occupancy. Overall, colonisation ability was unrelated to species' origin or response to water depth and hydrological change. Although exotics had higher specific leaf area and shorter longevity (indicative of higher colonisation ability), they had heavier (not lighter) seeds and did not differ in height from natives. Main conclusions: Using environmental flows to reinstate mid-range floods and augmenting the propagule supply of native species with characteristics suitable for modified conditions may help limit invasion in these wetlands.