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 (http://dnr.wi.gov/topic/Invasives/)
currently listed 116 non-native plants as invasive or potentially invasive
in Wisconsin. The Wisconsin State Herbarium database (www.botany.wisc.edu/wisflora/)
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. http://plants.usda.gov),
and the Wisconsin State Herbarium's (www.botany.wisc.edu/wisflora)
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
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
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 (http://dnr.wi.gov/topic/Invasives/),
with the Wisconsin State Herbarium database (www.botany.wisc.edu/wisflora)
list of "Ecologically Invasive" plants, and with a list in a
draft manuscript kindly provided by Elizabeth Czarapata (In press).