National Status: U.S. ESA and COSEWIC

NatureServe Invasive Species Impact Rank (I-Rank)

Which non-native plant species pose the most serious threats to native species and ecosystems? To answer this question, NatureServe, in collaboration with The Nature Conservancy and the U.S. National Park Service, developed a protocol for assessing and categorizing non-native plants according to their impacts on native biodiversity. The protocol is designed to focus specifically on the effects non-native plant species have on native plants, animals, and ecosystems and to make the assessment process objective, transparent, and systematic. It is intended to be applied to non-native plants as they occur over a large area, such as a nation, ecoregion, province, or state.

The protocol is used to assess non-native plant species individually for a specified region (e.g., the 50 U.S. states) and to assign each an Invasive Species Impact Rank (I-Rank) of High, Medium, Low, or Insignificant to categorize its negative impact on natural biodiversity within that region. The protocol includes 20 questions grouped into four sections: Ecological Impact, Current Distribution and Abundance, Trend in Distribution and Abundance, and Management Difficulty. The answers to the questions within each section are used to calculate a subrank for that section, and an overall I-Rank is then calculated from these subranks. Text comments and citations to information sources are required to document the answers selected. If information (as evidenced by protocol questions answered) is sufficient to eliminate at least two of the four possible I-Ranks, but insufficient to narrow the I-Rank to a single value, a range I-Rank (e.g. High/Medium) is assigned. An I-Rank of Unknown is assigned when an attempt has been made to assess the species, but it was not possible to eliminate at least two of the four possible I-Ranks.

NatureServe is currently using this protocol to assess the impact of all non-native vascular plants that have established outside of cultivation in the United States. The 50 U.S. states comprise the assessment region, such that I-Ranks displayed on NatureServe Explorer reflect each species’ impact on United States natural biodiversity as a whole. At this time, the dataset is not yet comprehensive (all non-native vascular plants established in the U.S. have not yet been assessed). The species assessed thus far were not selected randomly.


Copyright 2014
State and provincial distribution data sources. The Heritage Conservation Status Rank System and Definitions. Name sources for plant, animal, and ecological communty records in NatureServe.