IPAW Working List of the
Invasive Plants of Wisconsin

Results of the Survey - The Working List

There were 72 species for which IPAW received 10 or more surveys, and that had mean impact scores greater than 2.25.

Of these 72 species, 5 species were removed from the list because their disturbance scores averaged less than 5.0. Low disturbance scores suggested that these species may be primarily weedy rather than invasive (Bromus tectorum, Downy chess; Carduus nutans, Musk thistle; Cirsium vulgare, Bull thistle; Lactuca serriola, Prickly lettuce; and Silene latifolia, White campion). One additional species, Lonicera japonica, was eliminated from the list. Nineteen respondents reported observing Lonicera japonica, Japanese honeysuckle, in Wisconsin. However, because there was reason to suspect the field identification of this species, it was excluded from the working list pending more data or verification. The "IPAW Working List of the Invasive Plants of Wisconsin", includes 66 species.

Of the 66 species catalogued on the "IPAW Working List of the Invasive Plants of Wisconsin", only two species (Crepis tectorum, Hawksbeard; and Leonurus cardiaca, Motherwort) are not listed as invasive plants on the Wisconsin DNR website, and all 66 species are catalogued as invasive in the draft list contained in Czarapata (In press). Sixteen of the 66 species listed are not on the Wisconsin State Herbarium database list of "Ecologically Invasive" plants, however, all but three of these 16 species are described as "potentially invasive" in the Herbarium's species description, and these three are described as "naturalized".

Since "Impact" was judged to be the survey variable that best defined the invasiveness of these non-native species in natural plant communities, it is helpful to sort the list by mean impact scores. Survey respondents described the top species on this list as having the highest negative impact on natural communities when they were present.

The species reported as having the highest "Impact" are not necessarily the most abundant naturalized or invasive species currently found in Wis consin. The variable "Impact" was defined in the survey as modification or replacement of native communities at sites where the species occurs. Average "Impact" scores do not correlate well with the current "Abundance" scores given the same species (see Correlations). Impact is highly correlated with competitive ability (Figure 1), rate of spread, and feasibility of control. As one would expect the number of observers that provided data on a species is well correlated with the current Abundance score given the species. The Abundance variable was defined on the survey to estimate the percent of vulnerable sites in which the species currently occurs. The distribution of mean Abundance scores (Figure 2) shows that for the majority of invasive species it was the judgment of survey respondents that the species currently occupies only a small fraction of the sites that might be vulnerable to its invasion. Relatively few species were thought to have achieved an invasion rate of even 50% of vulnerable sites. On the survey a score of 15 was given to species that currently occurred in >50% of vulnerable sites.

We received a total of 2993 observations on the plants listed in the survey. Several respondents had observed some of the species in more than one ecoregion (Figure 3), so there were a total of 4062 records of species in specific ecoregions (Table 4). The unadjusted counts of plant observations by ecoregions cannot be considered a good index of the relative level of the invasive plant problem in different ecoregions of Wisconsin, because the familiarity of observers with the various ecoregions was not uniform. For example, if all observers had by chance only been familiar with the Green Bay area then all reports of invasives would have been from the Northeast ecoregion. We compared the number of reports of all species in each ecoregion with the percent of our observers that were familiar with our seven ecoregions (Table 4). Reports by a survey respondent of any plant from an ecoregion were used as an indication that that observer had some familiarity with that ecoregion. There was a slight tendency for a higher percent of reports of non-native species from the southeast region (35.8%, Region SE) compared with the percent of respondents familiar with the southeast (29.2%); but in general the proportion of reports of species by ecoregions matched very closely the proportion of respondents familiar with the ecoregions (Table 4). The frequency of species reports in each ecoregion contained in this data set should, therefore, not be interpreted as an accurate representation of the density distribution of the species in Wisconsin, since the ecoregion reports in these data seem to be primarily an artifact of where respondents were from.

References