Are invasive riparian plants associated with reduced biotic condition of fauna in western US streams?

Yes. Records on the presence or absence of 12 invasive riparian plant taxa and observations on macroinvertebrate and vertebrate communities within streams were collected at over 1000 stream reaches. The sampled reaches were selected on a probability basis to represent the population of perennial streams in the Western United States. Observations on faunal communities were represented as indices of biotic integrity. We account for the effects of  seven factors shown to have an effect on these biotic indices – water chemistry, (N, P, and conductivity), anthropogenic riparian disturbance, excess sediment, natural fish cover, and the number of dams in the upstream watershed – and then consider the remaining variability in these biotic indices. We use an ecological niche model, GARP, to partition the locations where each plant taxon is reported as absent into locations where it could reasonably be present and places where it is unlikely to occur, based on climate and topographic variables. We then evaluate the unexplained variation in biotic indices in light of the observed presence and absence of each invasive plants; restricting records of absence to places where each taxon could reasonably occur. Our conservative analysis shows that three plant taxa -- Phalaris arundincea (Reed canarygrass), Rubus armeniacus (Himalayan Blackberry), and Tamarix spp (Salt Cedar) – are associated with a statistically significant and substantial reduction in indices of biotic condition in portions of the study region.