Biological consequences of invasion by reed canary grass (Phalaris arundinacea)

Although they are typically assumed to be negative, the consequences of plant invasions for native diversity or biological integrity are seldom broadly quantified (i.e., for multiple taxa or across large regions). We investigated the impacts associated with invasion of wetlands by reed canary grass (Phalaris arundinacea L.; RCG) on plants and several animal groups. In a local study, we compared plants, arthropods, and small mammals on treatment plots with reduced RCG dominance to those on highly invaded plots. We also conducted a companion study, where we measured RCG dominance and plants, arthropods, and birds in 82 randomly selected wetlands across Illinois (USA) to determine if our experimental results were consistent in communities across the region. Plant diversity, floristic quality, and diversity and abundance of Homopteran insects decreased with RCG dominance in all instances. Richness and abundance of all other arthropods decreased with increasing RCG in the local study, but no trend was detected in communities statewide. No relationship between total abundance or richness of small mammals (local) or birds (statewide) with RCG was detected. However, voles and shrews were more abundant, and mice less abundant, in RCG-dominated plots. These results support the hypothesis that there are negative effects for multiple taxa from RCG invasion. Because negative effects observed in the local study either corroborated, or were neutral with respect to results from statewide surveys, they suggest that native biodiversity and biological integrity are being dampened across wide areas of this invader's range.