Risk Assessments for Invasive Plants: A Midwestern U.S. Comparison

Several states in the Midwestern United States are using risk assessment to determine the invasiveness of introduced plant species, and each assessment process is different. This may lead to differences in results for the same species between states, creating concern about credibility by those using the assessments. In this study, risk assessments for six Midwestern states were compared, examining format, content, and assessment committee membership. Case studies were conducted for four species for which at least five of the six states in the study completed a risk assessment; results were compared in the context of general differences in assessment content and those specific to each species. Furthermore, 14 species for which only four of the six states completed assessments were briefly examined for outcome differences only, and possible reasons for these inconsistencies. Overall, differences in assessments did not result in incompatible conclusions for the species compared, suggesting that unique assessments in each state can provide consistent and credible results. We propose that these Midwestern states share species resources with each other to further improve consistency between the assessments. Nomenclature: cutleaf teasel, Dipsacus laciniatus L.; Japanese barberry, Berberis thunbergii DC.; Amur honeysuckle, Lonicera maackii (Rupr.); Herder;Oriental bittersweet, Celastrus orbiculatus Thunb. Management Implications: Risk assessments are conducted to measure the invasiveness of introduced plant species. As defined in this paper, Type I assessments are primarily used for education and management and Type II assessments to guide policy. In Indiana, a Type I state, the completed assessments were shared with land managers to help prioritize control efforts and with the nursery and landscape industry to recommend which species should not be sold commercially. Educational brochures sharing the assessment results were widely disseminated from 1996 to 2014, and articles on the assessment results were published in the Indiana Nursery and Landscape Association's journal. Discussions with land managers indicate that risk assessments did change some management goals, although often site-specific factors weighed more heavily in choosing control goals than the species' rank (e.g., a medium-risk species present in small quantities may be controlled before a high-risk species present in large quantities). There are no data on the impacts of the risk assessments on commercial sales or spread of invasive plant species in Indiana. Wisconsin is a Type II state and used its assessments as a tool to recommend regulatory status for each species assessed. Information about the rule and the species to be included was shared widely with stakeholders and the general public before and after the rule went into effect. Although there had been extensive outreach on invasive plants for years, the media and organizations picked up the news of the new regulation and made it available to many more people. The Wisconsin Department of Natural Resources created a field guide to provide identification and control information on all of the regulated plants, and over 35,000 copies have been purchased. Many nurseries and landscapers were aware of the species to be regulated and stopped growing, ordering, or designing with them before the rule went into effect. The public began reporting garden center sales of regulated plants, leading to their removal from their stock. All of these efforts have led to greatly increased prevention of the spread of regulated species. Prohibited plants have become a priority for land managers, with efforts being made to contain and, where possible, eradicate these species at all known locations statewide. Risk assessments provide a solid foundation for education and for regulation, but do not guarantee reduction in invasive plant introduction and spread without significant follow-up.