Nonnative Species Limitation of Floristic Quality Scores in Wetland Mitigation Performance and Multimetric Health assessment

Nonnative species have minimum ecological conservatism that invariably limits floristic quality scores. This means nonnative species could prevent sites from reaching predefined floristic quality targets in restoration and ecological health applications. Using large datasets from a jurisdictional wetland determination program and the U.S. National Wetland Condition Assessment, we evaluated nonnative species limitation of floristic quality estimates in deciding restoration efficacy and multimetric health. Specifically, floristic quality indexed with and without nonnatives was tested as a stand-alone measure of wetland compensatory mitigation performance, and in a multimetric context of large-scale wetland condition monitoring. In the stand-alone application, nonnative species prevented over 50% of sites from reaching the original high, medium, and low floristic quality targets, and these failure rates remained high (11.7–19.2%) after recalibrating the targets to account for nonnative species. Sites falling below standards contained significantly higher percentages of nonnative species and scored closer to the performance targets. Rates of multimetric condition shift were much lower (4.4% overall), attributable to fewer nonnative records per site (x- = 4.2 species) and to weakening of single-metric influences. Where shifts among health ratings did occur, sites contained about 8–15% nonnative species and scores were already near the rating boundaries. In both examples outcomes clearly depended on percentages of nonnative species and the nearness of initial scoring estimates to predefined targets or adjacent health categories. Nonnative species acting on floristic quality scores may limit wetland mitigation performance and shift ecological health ratings, but with likely muted effect in multimetric situations.