Assessment systems for Plants Established in a Given Region

Most of the plant invasiveness assessment systems being employed today are evaluating plants already present in a given region. These systems vary in their overall intent and evaluate: biological traits; ecological impact; current distribution and abundance; trend in distribution; management ease/difficulty; and value of the species. Some require scientifically credible documentation; others are based on observations and anecdotal information. Some provide recommendations for use or control associated with their assessment and others simply classify a plant's invasiveness by a scale from highly invasive to insignificant. Most outcomes are non-regulatory and considered advisory. It is important that assessments and subsequent lists include documentation and be based on a clearly understood criteria and a science based system. Assessments must also consider "regionality" and require research where necessary to determine whether a plant is in fact "invasive" according to the accepted definition in Executive Order 13112.

Here are a few points to consider when reviewing the criteria associated with an assessment system. 

    1) Does the system account for individual characteristic among species and their subgroups such as varieties, cultivars, forma, etc? Most assessments evaluate plants at the species level and include all subgroups in the results. They do not consider differences in growth or reproductive traits of the subgroups that may be deemed non-invasive. This is especially troublesome when species are identified on regulatory lists and no provisions for acceptable cultivars or other subgroups are made. In most instances, blanket statements about species results from the "assessor's" (individual or organization) lack of familiarity with the plants in question and the biology associated with subgroup of the species in question. There are numerous examples where reproductive and growth habits differ among species and their cultivar selections.

    2) Does or is the plant in question likely to cause environmental or economic harm or harm to human health? Whether it "does or is likely to" requires documentation. Presence does not constitute harm. Much research is needed to identify and substantial not only whether harm is in fact being caused but to the extent of the harm. Most of the concerns with credible assessment lie in the lack of substantiated documentation of harm being caused.

    3) Does the system account for regionality, not only in the broad sense such as the Midwest, Northeast, West, but within a more defined geographic or political boundary such as a state? For example, Michigan is classified according to four eco-regions. Variations in plant behaviors can and does occur across and within these classified "regions". The same is true for other geographic or ecological regions. Information gathered nationally may aid in providing information but the true determination whether plant behavior is deemed invasive must be made within the boundaries of the given region. Regional differences in plant behavior are common place and a factor strongly considered at the Chicago symposium.  

    4) Is a plant's invasiveness based on an ability to disperse over spatial gaps (a distance from the original place of introduction) by natural means? Wind, water, and/or wildlife are natural means of dispersal and move plants away from the site of introduction. This is an important factor in determining whether a plant is truly a problem. Seeds and vegetative fragments with an ability to root can be a problem when dispersed beyond the place of introduction. However, a plant may have the ability to root from a fragment but if it does not fragment easily then disperse over spatial gaps is not a potential problem. In addition, many species have been identified as being invasive simply because they spread. There are some species where this is a problem, such as with Kudzu in the south, however, the extent or degree of spread can be misinterpreted within many assessment systems. The classic example is the misinterpretation and inaccurate classification of Vinca minor by some assessment systems.

    5) Does the system identify the difference between presence and invasion when evaluating distribution? This is a concern when evaluating plants on "public" lands. Many public land holdings such as forests, natural areas, and parks were originally homesteads, agricultural farms, plantations, or involved in other non-natural uses. Plants may be remnants of prior uses and not due to invasive behavior. The extent of their presence may be directly related to the time in which they were planted.

    6) Does the system identify available control measures and whether there are or have been any efforts to control the plants? Control measures include mechanical means, chemical applications, and in some cases, fire. Control method may be selective or non-selective. There are cases such as with Purple Loosestrife where control methods may suppress but do not eradicate the plants in its entirety. However, on the other hand, there are species where persistence of control can lead to eradication of the plant in a given area. In assessing control, it is importance to note whether control of a given species is a priority and if any attempts at control were made.

     7) Does the system consider the value of the plant? Plant introductions are intentional and unintentional. Unintentional introduction is also referred to as hitchhiking. Hitchhikers for the most part are weeds species with no intended value.  Intentional introductions enter with a specific purpose and targeted use. An example is Autumn Olive; its introduction was intended for wildlife conservation as food and shelter with plantings targeted at wildlife areas. There are differences of opinion within Departments of Natural Resources Wildlife divisions as to whether the benefits of Autumn Olive for wildlife populations outweigh its suggested invasiveness. Earlier versions of invasiveness assessment systems were developed by the natural resource community and did not consider value of a species in the equation leading to classifying or listing of a plant. Subsequent generations of assessment protocols evaluate value in relation to production and sales at major retail outlets.  However, value goes beyond whether a plant is produced in a given region and purchased from national retail chains. Plants are introduced for a number of different reasons, all of which contributes to a plants value for food, shelter, aesthetics, recreation, and conservation purposes such as soil and water erosion and wildlife habitat (food and shelter). Value is especially important when selecting and applying non-regulatory or regulatory management measures.  

     8) What are the outcomes of the evaluation and how are they reported? Most assessment systems result in categorized lists. Some rank or rate the degree of species invasiveness; some have accompanying use recommendations. Some will publish the documentation leading to a recommendation; some lists are reported without documentation. Keep in mind that outcomes and their use are only as good as the information used to generate the information.

There are several plant invasiveness assessment systems currently being employed; some of which are cited below. Each addresses the above criteria in varying degrees. Reviewing these systems and their resultant evaluations will provide an overview of the process of plant invasiveness evaluation.

Michigan Plant Invasiveness Assessment System. 2004 Schutzki, R. E., D. Pearsall, A. Cleveland, J. Schultz, K. Herman, D. MacKenzie, S. MacDonald, T. Wood, and T. Myers. Michigan Invasive Plant Council, Lansing, MI

IFAS Assessment of Non-Native Plants in Florida's Natural Areas. 2005
Fox, A.M., D.R. Gordon, J.A. Dusky, L. Tyson, and R.K. Stocker. Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Document SS-AGR-79

The Evaluation of Non-Native Plant Species for Invasiveness in Massachusetts. 2005 Massachusetts Invasive Plant Advisory Group

An Invasive Species Assessment Protocol: Evaluating Non-Native Plants for Their Impact on Biodiversity.  Version 1. 2004 Morse, L.E., J.M. Randall, N. Benton, R. Hiebert, and S. Lu. NatureServe, Arlington, Virginia

Criteria for Categorizing Invasive Non-native Plants that Threaten Wildlands.  2003. Warner, P. J., C.C. Bossard, M.L. Brooks, J.M. DiTomaso, J.A. Hall, A.M. Howard, D.W. Johnson, J.M. Randall, C.L. Roye, M.M. Ryan, and A.E. Stanton. California Exotic Pest Plant Council and Southwest Vegetation Management Association.

Invasive Plant Species Assessment Working Group- Indiana (IPSAWG). 2002. Draft Assessment of Invasive Plants in Indiana's Natural Areas.

NatureServe Summary of Results - Invasive Species Impact Ranks for the United States

Summary of Results - Invasive Species Impact Ranks for the United States XLS
Evaluations as of January 10, 2005 (further assessments in progress)
Invasive Species Impact Ranks (I-Ranks) developed by NatureServe using "An Invasive Species Assessment Protocol: Evaluating Non-Native Plants for Their Impact on Biodiversity." (Morse, et. al., NatureServe, 2004). See
Download Below

NatureServe Species Data Form Scoresheet.

NatureServe Species Data Form Scoresheet. XLS
Download Below

An Invasive Species Assessment Protocol :Evaluating Non-Native Plants

Evaluating Non-Native Plants
For Their Impact On Biodiversity

Invasive Species Assessment Protocol : Examples of Completed U.S. Assessments.

Download the attached PDF for the Examples of Completed U.S. Assessments

Chinese Tallow-tree (Triadica sebifera)

Kudzu (Pueraria montana)

Tansy Ragwort (Senecio jacobaea)

Common Lilac (Syringa vulgaris)

No Common Name (Acanthospermum xanthioides)

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