Invasiveness Ranking System for Non-Native Plants of Alaska

The scoring from each system is very different, including both numerical and categorical ranks of differentscales. To compare effective scales and variation within and among the systems and to gauge how robust the fivesystems, we standardized invasiveness scores by dividing scores of each species by that of Polygonum cuspidatum,a species perceived to be one of the most invasive to natural habitats in Alaska by the authors1. A discrete numericalsystem was created for nominal categorical systems (e.g., “not invasive” = 0, “low invasiveness” = 1, “mediuminvasiveness” = 2, etc.). Additionally we graphically compared levels of variation among the scores and assessorsfor three species commonly believed to represent different levels of invasiveness: high (Polygonum cuspidatum),intermediate (Sorbus aucuparia), and low (Matricaria discoidea). To allow comparisons among the species westandardized scores relative to the maximum score possible for each system.Two of us (M.L. Carlson and I.V. Lapina) produced a draft ranking system that was evaluated in a similar mannerto the four existing ones. Scores were given in response to a series of questions and used to calculate subcategory(i.e., ecology, biology, distribution, and control) and final scores. The results section discusses the format and justificationsfor the questions. We then modified the draft Alaska system with input from all authors. Additionally,we included a climate screening procedure that is described in the results section.Similarities among the five systems in the ranks of the 12 test species were compared using hierarchical clusteranalysis in SPSS Base 9.0. All invasiveness scores were standardized to the potential maximum for each systemand squared Euclidean distances were used to generate the distance matrix for the cluster analysis.To determine which sections (ecology, biology, distribution, and control) in the Alaska system had greater explanatorypower, we explored the relationship of each of four section scores to overall invasiveness. We producedscores for all sections and overall invasiveness based on a consensus of the authors for all species. Spearman rankcorrelation coefficients were produced for all sections and overall invasiveness. Additionally, we removed thesectional component to invasiveness for each section comparison to avoid autocorrelation for this analysis (e.g.,overall invasiveness–ecology score was used in comparing to the ecology score and overall invasiveness–biologyscore was used in comparing to the biology score). R2 values were calculated for each sectional score relativeto the corrected invasiveness. Not all questions in the sectional scores were answered and these scores were removedfrom the analysis.Following the construction of the invasiveness ranking system for Alaska, we proceeded to rank 95 species presentin Alaska and 18 potential future invaders. The species were chosen to encompass all perceived degrees ofinvasiveness and distributions in Alaska, including the species considered to be the most invasive already presentin the state and the most threatening potential future invaders (the ranks are presented in Appendix B). Taxa thatare believed to have been absent from Alaska prior to European contact are considered “non-native.” We rank afew species that may have been present in the region for centuries in small populations (e.g., Phalaris arundinaceaat hot springs sites in interior Alaska), but have recently expanded their ranges and abundances dramatically andare now most likely combinations of Eurasian and North American genotypes. We included notes on nativity forall species with questionable origins.Short species biographies, initial scores, and documentation were produced based on literature reviews. For verysimilar congeneric species (e.g., Rumex crispus, R. longifolius, and R. obtusifolius), a single score was given to thegroup of species. Scores and documentation were then added or altered by the coauthors. Upon completion ofthe 113 scores the committee reevaluated each species relative to the others to ensure consistency in scoring,identify potential mistakes, and include new observations or documentation.1 Polygonum xbohemicum and P. sachalinense are more widespread in southeastern Alaska and appear to be more robust and ecologicallythreatening, but their documentation in the literature is weaker than P. cuspidatum. We expect that much of the literature has not distinguishedamong species and lumped the three taxa under P. cuspidatum (see Zika and Jacobson, 2003).5