Invasiveness

Protocols for the Assessment of Plant Invasiveness

In November 2003, a conference titled "Invasive Plants in Natural and Managed Systems: Linking Science and Management" was held in conjunction with the 7th International Conference on Ecology and Management of Alien Plant Invasions hosted by the Ecological Society of America.  Highlights from the meeting were reported in the Michigan Invasive Plant Council Newsletter (Volume 1, Issue 5, 2004).  The following highlights are of particular relevance to the assessment of plant invasiveness:

A)    Weed lists within the United States have been generated by governmental units for regulatory and advisory purposes (federal and state agencies), by professional groups (Exotic Plant Pest Councils) and by private organizations (botany clubs, gardening groups).  A total of 113 lists are known at the state or higher level.  The vast majority has been produced by government agencies.  Only 12% of these lists require any sort of documentation for placing a species on the list.  It was stressed that the authors of lists should document reasons for including a species on a list using multiple criteria and expert assessment.

B)    Plant Assessment- the Virginia Dept. of Conservation and Resources started plant assessment work in 1992 with a list of over 100 "invasive" plants with no ranking.  Its intended use was for advisory purposes.  They were challenged by the American Seed Trade Association officials on 11 species.  Lesson learned after considerable legal exchanges was that official disclaimers need to be put on non-regulatory lists and that plants need to be ranked according to a science based system.

Further information supporting the development of a comprehensive and scientific based  assessment of plant invasiveness was emphasized at the St. Louis and Chicago symposia.  The St. Louis Principles specifically encourage the use of available assessment tools, resources and voluntary codes of conduct and stress the fundamental value of broad-based collaboration. Recommendations evolving from the Chicago Meeting stated that: 1) Regional groups must be allowed to develop their own responses to regional invasive plant problems, including guidance on or lists of invasive plant species, since plants may exhibit invasive characteristics in one region and not in another; 2) Efforts to place invasive plant species on official lists must include research, where needed, to ensure that "listed" plants do, in fact, have invasive characteristics. Lists that use anecdotal information to determine the "invasive" potential of a species are not preferred by the nursery industry, since this highly important approach could cause some plants to be deemed "invasive" without sufficient basis; and, 3) Well understood criteria for listing a plant as "invasive" must be developed prior to completing invasive plant species lists. Industry representatives emphasized the distinction between processes for identifying and ranking invasive plants, and processes for selecting and applying non-regulatory or regulatory management measures.  Responsible evaluation and assessment of plant invasiveness is essential to fostering collaboration among the diverse audiences interested and involved in the invasive plant issue.  

Discussions on plant invasiveness assessment can be divided into two categories: 1) Risk assessment based on predictive models for first time introductions; and 2) Assessment systems for plants already present in a given region.

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