Ranking Nonindigenous Weed Species by Their Potential to Invade the United States

Abstract

Because of the large number of potentially invasive species, and the time required to complete weed risk assessments (WRAs) with the use of the current, mandated system in the United States, species need to be prioritized for assessment and possible listing as Federal Noxious Weeds. Our objective was to rank the potential invasiveness of weedy or pest plant species not yet naturalized in the United States. We created a new model of invasiveness (hereafter the U.S. weed-ranking model) based on scoring factors within four elements: (1) invasiveness potential, or likelihood to exhibit invasive behavior; (2) geographic potential, or habitat suitability; (3) damage potential, or likely impact; and (4) entry potential, or likelihood to be introduced. The ranking score was the product of the four elements. We scored 250 species satisfactorily, from a list of 700 +. We analyzed model sensitivity to scoring factors, and compared results to those from a WRA model for Hawaii. For species not in cultivation in the United States, the top 25 species included a mix of annuals, perennials, sedges, shrubs, and trees. Most had exhibited invasive behavior in at least several other countries. Because of greater entry potential scores, the highest-scoring species were weeds in cultivation. Twenty-nine such species, out of 44 total, had scores greater than the highest scoring species not in cultivation. In comparison to the Hawaii WRA model, correlation and regression analyses indicated that the U.S. weed-ranking model produced similar, but not exact, results. The ranking model differs from other WRAs in the inclusion of entry potential and the use of a multiplicative approach, which better suited our objectives and United States regulations. Two highly ranked species have recently been listed as Federal Noxious Weeds, and we expect most top-tier species to be similarly assessed.

A review of risk prioritisation schemes of pathogens, pests and weeds: principles and practices

“…The mean number of questions was 22 and the median was 17. The Australian WRA model involves a relatively large number of questions so as to reduce the need for subsequent evaluation (Parker et al 2007). Generally, it can be argued that the higher is the number of questions, the more precise is the outcome of the model, and the more difference there will be in the total scores of the organisms.…”

“…For instance, if the probability of entry is zero, then the total score is zero as well. In an additive model it is possible that the total score is still relatively high, although for instance habitat suitability might be very low (Parker et al 2007, Cox 2009). Makowski and Mittinty (2010) simulated the outcome of several scoring systems and showed that multiplication-based systems performed better than sum-based systems.…”

“…Model sensitivity tested by changing the input values by a given percentage and calculating the impact on the final results (Parker et al 2007). …”

“…In the reviewed studies, when mentioned, it took 1 or 2 days to find the required information and evaluate a species. In a thorough risk assessment an evaluation may take months or years, whereas the current methods used in the United States take about 2-8 weeks per species (Parker et al 2007). Further, prioritisation models evaluate the organisms using the same set of questions.…”

“…Among the 78 models, there were two that included entry, establishment, impacts, control feasibility, input and output uncertainty and allowed weighting, these being the U.S. Weed Ranking Model (Parker et al 2007) and the Smallwood and Salmon Rating System (Smallwood and Salmon 1992). A further three models included all but one of the above elements (Australian WRA by Pheloung 1995 and its derivatives; AWRAM model by Champion and Clayton 2000, and the Chinese Weed Risk Assessment by Ou et al 2008).…”