Invasiveness Ranking System for Non-Native Plants of Alaska

DiscussionThe existing weed risk assessment systems that we evaluated have useful approaches to predicting invasion anddamage in particular contexts. The systems are largely driven by specific values of the authors (e.g., impacts toa specific area or to agriculture), many of which are not universal, and most require more information than istypically available (Williams and Newfield 2002), which is a severe drawback to a state such as Alaska that is onlybeginning to face large-scale introductions. For most non-native species, little to no research has been conductedin the state and the paucity of information curtails confidence in ranks. We emphasize the need for more researchon invasive species ecology, distribution, and trends in Alaska.The system presented here differs from the existing systems by including a climate filtering stage, handling specieswith a range of known information, and focusing on impacts to natural systems in Alaska. Despite thosedifferences, the Alaska system produced ranks that were very similar to the Southwest system. We borrowedmany components of the “innate ability to become a pest (i.e., biology)” and “ease of control sections” from theSouthwest system, but did not include a similar “level of impact” section and the systems differed in the relativeweight of the remaining sections.Scores for the four sections of the Alaska system are generally strongly positively related to one another (i.e., specieswith large ecosystem impacts also have a more invasive biology, broad distributions, and are more difficultto control). However, the feasibility of control section was not as strongly correlated with the other sections andthis is likely due to this section having only three questions, leading to greater variation in scores than the othersections. A number of species scored high values in the biological characteristics and distribution sections, butrelatively low scores in the ecology section. These species represent a group of ruderal specialists (e.g., Persicarialapathifolium, Poa annua, and Plantago major) that are good dispersers, highly dependent on soil disturbance, butpoor competitors (see Baker 1974). While invasions and their impacts are very difficult to predict, our systemstresses the importance of whether the species is invasive in other areas. This trait is most strongly correlated withthe probability of invasion into new areas (Forcella and Wood 1984, Rejmanek and Richardson 1996, Rejmanek2001).Species that are regarded as particularly threatening to Alaskan ecosystems such as Melilotus alba, Polygonum cuspidatum,and Vicia cracca consistently ranked high in the Alaska system among reviewers, while more ruderal speciessuch as Chenopodium album and Matricaria discoidea were consistently ranked with low scores, as expected.The Australian system, on the other hand, ranked species that are not considered problematic in Alaska with veryhigh scores. This system gives more weight than the others to agricultural pests and to species linked to specificecological concerns in Australia. Systems such as the Australian, Southwest, and Alaskan that produced continuousnumeric invasiveness ranks resulted in less variation among assessors.The distribution of scores in the Alaska system currently ranges from 27 (Lepidium densiflorum, the least invasivespecies) to 90 (Myriophyllum spicatum, the most invasive species, which is not yet recorded for Alaska). The averageis 58. While different users will have different concepts of what constitutes various levels of invasiveness (e.g.,what is “highly invasive” vs. “moderately invasive” may differ among management agencies), we divided the ranksinto six blocks in Appendix A. We consider species with scores ≥80 as “Extremely Invasive” and species withscores 70–79 as “Highly Invasive;” both of these groups are composed of species estimated to be very threateningto Alaska. Species with scores of 60–69 as “Moderately Invasive” and scores of 50–59 represent “ModestlyInvasive” species; both of these groups still pose significant risks to ecosystems. Species with scores of 40–49are “Weakly Invasive”, and