Implications of riparian willow invasion to instream community structure and function: a synthesis using causal criteria analysis

Invasive riparian plants are a significant threat to riverine environments and are thought to alter the structure and function of stream ecosystems. Salix spp. are a genus of highly invasive northern hemisphere trees and shrubs that have invaded substantial areas of southern hemisphere riparian corridors. We set out to review the existing peer reviewed literature surrounding the impacts of Salix spp. infestation to streams by rigorously testing a suite of cause–effect hypotheses using a causal criteria analysis. Our analysis found evidence in the literature that infestation by exotic Salix spp. can cause a decrease in incidental illumination and benthic periphyton density, increased rates of allochthonous litter leaching and decomposition and changes to secondary consumer assemblage and trophic organisation. The review also highlighted a number of aspects of Salix spp. invasion for which there are significant knowledge gaps in the literature. Our results emphasise the importance site specificity, seasonal variation, physical properties of supplanted vegetation, stream size and magnitude of infestation when predicting putative cause–effect relationships between Salix spp. invasion and stream structure and function. We show that, by possessing incongruent biological and physical characteristics to native plants, invasive terrestrial trees have the capacity to influence adjacent aquatic ecosystems.