Impacts of an aggressive riparian invader on community structure and ecosystem functioning in stream food webs

P>1. Bioassessment in running waters has focused primarily on the impacts of organic pollution on community structure. Other stressors (e.g. invasive species) and impacts on ecosystem processes have been largely ignored in many riverine biomonitoring schemes, despite being required increasingly by environmental legislation. 2. Exotic riparian plants can exert potentially powerful stresses by altering both autochthonous and allochthonous trophic pathways. We examined the impact of Rhododendron ponticum on community structure and three key ecosystem processes (decomposition, primary production, and herbivory) in nine streams bordered by three characteristic vegetation types (deciduous woodland, pasture, or Rhododendron). 3. Community structure and ecosystem process rates differed among vegetation types, with autochthonous pathways being relatively more important in the pasture streams than in the woodland reference streams. Overall ecosystem functioning, however, was compromised in the invaded streams because both allochthonous and autochthonous inputs were impaired. Rhododendron's poor quality litter and densely shaded canopy suppressed decomposition rates and algal production, and the availability of resources to consumer assemblages. 4. Synthesis and applications. Combining measures of invertebrate abundance, rates of litter decomposition and algal production in future bioassessments of stream ecosystem functioning can help to make better informed management decisions and to develop more focused priorities for mediating the negative effects of riparian invasions. We provide a series of specific recommendations for dealing with invasive riparian plants in general, and Rhododendron in particular, in order to minimize their impacts on stream ecosystems. For instance, where the invader produces poor quality litter the canopy should be kept as open as possible over the stream channel to reduce impacts on algal production, thereby retaining alternative food chains that can be exploited by generalist consumers in the absence of viable detrital resources.