Assessing the environmental impacts of invasive alien plants: a review of assessment approaches

Assessing the impacts of alien plant species is a major task in invasion science and vitally important for supporting invasion-related policies. Since 1993, a range of assessment approaches have been developed to support decisions on the introduction or management of alien species. Here we review the extent to which assessments (27 approaches) appraised the following: (i) different types of environmental impacts, (ii) context dependence of environmental impacts, (iii) prospects for successful management, and (iv) transparency of assessment methods and criteria, underlying values and terminology. While nearly all approaches covered environmental effects, changes in genetic diversity and the incorporation of relevant impact parameters were less likely to be included. Many approaches considered context dependence by incorporating information about the actual or potential range of alien species. However, only a few went further and identified which resources of conservation concern might be affected by specific alien plant species. Only some approaches acknowledged underlying values by distinguishing negative from positive impacts or by considering the conservation value of affected resources. Several approaches directly addressed the feasibility of management, whereas relevant factors such as availability of suitable management methods were rarely considered. Finally, underlying values were rarely disclosed, and definitions of value-laden or controversial terms were often lacking. We conclude that despite important progress in assessing the manifold facets of invasion impacts, opportunities remain for further developing impact assessment approaches. These changes can improve assessment results and their acceptance in invasion-related environmental policies.