Invasiveness Ranking System for Non-Native Plants of Alaska

ResultsThe four existing systems, Invasive Exotic Plant Species in Virginia (Heffernan et al. 2001), California Exotic PestPlant Council (Warner et al. 2003), the Australian AQIS Weed Risk Assessment System (Pheloung et al. 1999),and Southwest Exotic Species Ranking System (Hiebert and Stubbendieck 1993) all evaluate taxa in roughlysimilar categories of ecosystem alteration, community alteration, biological characteristics, and ease of control.The Virginia, California, and Southwest systems all require site specific data and are difficult to evaluate forbroader regions. In addition to relying on biogeographical information, the California system requires populationtrend data that are not available for Alaska. For all systems, many characters that are traditionally associated withinvasiveness, such as invasive elsewhere and breadth of native distribution are given little weight relative to lessrelevant characters. We recognized that none of the systems appear to handle unknown or ambiguous informationwell. For most systems missing information resulted in lower scores. Poorly known species should not beconsidered a priori to be less invasive. The scoring for the Australian and the California systems is not intuitive,requiring the use of look-up tables and scoring matrices. The outputs of most systems are categorical (i.e., high,medium, low invasiveness), resulting in an overly simplified rank and loss of potential information. No systemused climatic or known habitat information to screen species from consideration; however, the Australian systemdoes score species on suitability to Australian climates (2 points out of approximately 50 total points).Overall, the existing systems and newly derived Alaska system were positively correlated in the relative ranks ofthe 12 test species (not shown). Despite the Southwest system’s reliance on site data, its species ranks were verysimilar to those of the Alaska system (figure 2). The Virginia and California systems were more weakly clusteredwith the Alaska and Southwest systems. The most distantly related system to the others was Australian. The poorrelationship among the Australian system scores and those of the other systems stems from a greater emphasis onagricultural impacts and specific ecological concerns (e.g., “causes a fire hazard”) of the Australian system.Figure 2. Dendrogram based on hierarchical cluster analysis representing similaritiesamong five invasiveness ranking systems 12 non-native species ranksSWEPAKVTNCCALAQISRescaled Distance0 5 10 15 20 25Systems that ranked the same species most similarly are more closely joined (lower rescaled distance value). The cluster analysis used squaredEuclidean distances in the distance matrix. SWEP = Southwest Exotic Species Ranking System, AK = Alaska Invasiveness Ranking System,VTNC = Invasive Exotic Plant Species in Virginia, CAL = California Exotic Pest Plant Council, AQIS = Australia Weed Risk Assessment System.6