Abstract

A debate over the impacts of invasive plants is fueled by varied terminology, ecosystem types, response metrics, and conflicting evidence across scales. We quantify the effects of invading Phalaris arundinacea (Pa) on species richness and abundance in Wisconsin sedge meadows dominated by Carex stricta (Cs). Among species area curves from four nested plots (0.25, 1, 4, and 16 m(2)) in seven sites, Pa stands averaged 48 % fewer species than the adjacent Cs. Species richness was reduced by about half regardless of plot size. Total richness for Pa stands was 50 species compared to 83 for Cs, and Pa stands had consistently lower additive percent cover of subordinate species at all scales. However, resident species responded differentially to Pa invasion. Only one species, Calamagrostis canadensis (a tall clonal grass), had similar importance values (using cover and frequency) in Cs and Pa stands at 0.25 and 16-m(2) scales. Other species declined with Pa invasion, mostly in frequency. Despite losing half of their resident species, sites did not differ significantly in their native exotic richness relationship (NERR). NERR confounds the effects of invasiveness with species' origins, and it under-represents impacts of dominant invaders in wetlands. Debates should be fewer if studies specify if the invader is exotic or native, type of invaded ecosystem, the metric used to assess responses, and the spatial scale.