Invasiveness Ranking System for Non-Native Plants of Alaska

Biological Characteristics and Dispersal Ability:The Biological Characteristics section has nine questions worth two or three points each. The questions focus oncharacteristics associated with the potential ability of a species to spread, establish, and flourish in natural areas orotherwise associated with invasiveness. Table 3 shows this subsection of the ranking system with justifications forquestions.Distribution:One factor widely recognized for predicting which species may become invasive is a history of invasion in otherregions and the distribution of those invasions (Forcella and Wood 1984, Rejmanek 1995, 1996, 1999, 2001).This section is composed of five questions valued between four and six points. This subsection of the rankingsystem with justifications for questions is presented in table 4.Feasibility of Control:The smallest section of this ranking system, with three questions for 10 points total, is devoted to control feasibility.Species that are difficult to control pose a greater threat than those that can be easily eradicated, thereforeconsidered more invasive. We do not weight this section heavily (10 percent of the total rank), however, we considerpreventative measures and early eradication to be more urgent in Alaska than large-scale control efforts. Thequestions and their justifications are shown in table 5.Overall Score:Scores and the number of points possible are tabulated for each section. If insufficient information is present torespond to a question it is scored as “unknown” and the maximum potential points for that question are removedto obtain “points possible.” The final invasiveness score is given as the relative maximum score, which is equal tothe sum of scores from each section divided by the total possible. This has the advantage of evaluating specieson information known rather than artificially depressing the scores of poorly understood species. To illustrate,a hypothetical species that receives maximum scores for all questions except that it has unknown ecosystem impacts(i.e., Question 1.1 with 10 possible points = “unknown”) would score 90 points out of 90 possible, and itsRelative Maximum Score would be 100. The lowest (least invasive) score possible is zero.Table 2. Excerpt of the ecological section of the Alaska invasiveness ranking system1. Ecological Impacts1.1. Impact on Natural Ecosystem Processes.A. No perceivable impact on ecosystem processes. 0B. Has the potential to influences ecosystem processes to a minor degree (e.g., has a perceivable but mild3influence on soil nutrient availability).C. Has the potential to cause significant alteration of ecosystem processes (e.g., increases sedimentation rates 7along streams or coastlines, reduces open water that are important to waterfowl).D. Likely to cause major, possibly irreversible, alteration or disruption of ecosystem processes (e.g., the species 10alters geomorphology; hydrology; or affects fire frequency, altering community composition; species fixessubstantial levels of nitrogen in the soil making soil unlikely to support certain native plants or more likelyto favor non-native species.).U. Unknown11