How IPAW Formed the "Working List of Invasive Plants of Wisconsin"

The IPAW Science Committee was asked to create a draft, working list of invasive plants for review by the IPAW Board of Directors. There are already some formal or informal lists of invasive plants of Wisconsin and wider regions that include Wisconsin.

The Wisconsin DNR website ( currently listed 116 non-native plants as invasive or potentially invasive in Wisconsin. The Wisconsin State Herbarium database ( listed 67 vascular plants as "Ecologically Invasive" in Wisconsin, based on "Wisconsin DNR Status Information"; and the U.S. Forest Service maintains a list of the invasive plants of the Eastern Region of the United States.

While other catalogues of invasive plants exist, they are either not specific to Wisconsin, or were not developed with a formal process that involved the collection of a wide variety of personal observations from people concerned with invasive plants. IPAW considers development of a Wisconsin list to be an important function of the organization.

There are currently no broad studies of non-native species that provide the empirical measures of plant populations and their spread that would allow us to categorize plants as invasive. However, we all know from personal experience that there are many "non-indigenous species or strains that become established in natural plant communities and wild areas and replace native vegetation". The IPAW Science Committee felt that for the "IPAW Working List of the Invasive Plants of Wisconsin" to have credibility, it must be based on the observations and experience of many people who live and work across the state.

In early 2002 IPAW collaborated with The Great Lakes Indian Fish and Wildlife Commission (GLIFWC) to develop a survey to gather observations from people familiar with the impact and ecology of invasive plants. GLIFWC staff compiled the survey responses as part of a larger non-native plant database being developed with EPA-Great Lakes National Program Office funds (grant# GL00557201). The survey was adapted from the Alien Plant Ranking System (Hiebert and Stubbendieck 1993). The survey included a list of 311 non-native species that could possibly be considered to be invasive in some situations. This initial comprehensive list was formed by combining the U.S. Forest Service's list of invasive plants for the Eastern Region (USDA, NRCS. 2002., and the Wisconsin State Herbarium's ( list of ecologically invasive plants. Nomenclature for this list and for all species included in this report follows Gleason and Cronquist (1991), except for a few species not included in that manual, which follow the nomenclature in Kartesz (1994). Survey respondents were encouraged to add species that they had observed as invasive, but which were not included in the original list on the survey form.

A request for volunteers to complete the survey was distributed widely. The survey was distributed by email and regular mail using a list of known Wisconsin natural area and plant experts and all appropriate email lists available to IPAW. People receiving the email were requested to forward the survey to others they thought might be able to contribute. A call for volunteers to complete the survey was published in "Plants out of Place", IPAW's newsletter; and a call for volunteers, including the complete, downloadable survey was posted on both the IPAW and GLIFWC websites. Anyone having personal experience with any invasive plant was encouraged to submit his or her observations, and every completed survey received by IPAW and GLIFWC was accepted and included in a database of responses. After circulation of the survey, responses were accepted for almost one year before the results of the survey were tabulated and summarized.

People who volunteered to complete the survey were asked to answer questions only about those species with which they had personal experience. These respondents, therefore, constituted a large "panel of experts" on the invasive plants of the state. The survey asked the volunteers to record the ecoregion in which they had observed the plant, and to identify the habitats or communities in which they observed the species. It also asked observers to score species based on:

1) the level of disturbance required for a species to become established and spread,

2) the current abundance of the species in vulnerable sites,

3) the ecological impact of the species in sites where it currently occurs,

4) the competitive ability of the species,

5) the observed rate of spread of the species in the past 5 years, and

6) their observations concerning the feasibility of effective long-term control of the species.

IPAW and GLIFWC received 60 completed surveys. These 60 completed surveys provided 2993 observations on the listed plants; individual observers provided information on an average of nearly 50 species. Two surveys provided information on only a single species; the maximum number of species reported from a single volunteer was 161; and seven volunteers each provided information for over 100 species.

By definition, an Invasive Plant both invades native plant communities and impacts those native communities by displacing or replacing native vegetation. A plant that establishes and invades only in seriously disturbed areas (especially in disturbed ground) is defined as a "Weed" rather than an "Invasive Plant". Considering the six plant characteristic variables in the survey (Disturbance, Abundance, Impact, Competitive ability, Rate of Spread, and Feasibility of Control), the IPAW Science Committee determined that level of Impact, and level of Disturbance required for the plant to establish, were the most appropriate variables to use to sort the list to determine which species the survey respondents had clearly observed as invasive. In the survey responses the variables "Impact" and "Competitive Ability" were very highly correlated (r 2 = 0.863), and sorting the list by either of these variables would provide nearly identical results. Information on "Abundance", "Rate of Spread", and "Feasibility of Control" was considered important data to collect for the species, but these variables do not have as direct a bearing on the definition of whether or not a plant is invasive.

Three criteria were used to determine which species to place on this initial working list of the invasive plants of Wisconsin. The first criterion was that only species with a mean survey response greater than 2.25 for "Impact" (indicating some tendency to "invade and modify native communities") were placed on the list. Secondly, only species having a mean survey response for "Disturbance" of 5.0 or greater (indicting species found frequently in sites that have not been disturbed within the past 10 years) were included. The third criterion was the number of survey respondents that provided observations of the species. It was the intention of IPAW to begin the formation of a working list with those species for which we had gathered an adequate number of observations. Therefore, only species for which we have received 10 or more survey responses have been placed on this working list. A separate list of species having mean Impact and Disturbance scores high enough to suggest that they are invasive, but for which we only received between 3 and 9 reports, is provided as a list of species for which we need more information. These species are also included on the "IPAW Working List of the Potentially Invasive Plants for Wisconsin".

Using these criteria based on the average scores calculated from the survey, there were 112 species for which we had 10 or more observers; of these 112 species, there were 67 species that had a mean Impact greater than 2.25 and a mean Disturbance score of 5.0 or greater. In order to determine if this list of 67 species included those plants that other individuals and organizations have previously recognized as the invasive plants of the state, we compared this survey-generated list with the Wisconsin DNR list provided on their website (, with the Wisconsin State Herbarium database ( list of "Ecologically Invasive" plants, and with a list in a draft manuscript kindly provided by Elizabeth Czarapata (In press).


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