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An Assessment of Exotic Plant Species of Rocky Mountain National Park


National Park Service policy requires that all exotic species which pose a threat to natural areas or human health be managed. In agreement with park policy, this project was designed to examine the current and potential ecological impacts of known exotic plant species in RMNP. There were three general goals for this project. The first goal was to estimate the potential effects of non-native plant species on native plant communities in RMNP. Second, the known exotic plants were ranked based on their potential impacts on native plant species and communities. Finally, a management approach for corrective action was developed to help control exotic species in RMNP. A ranking system was modified from the Handbook for Ranking Exotic Plants for Management and Control (Hiebert and Stubbendieck 1993) to help accomplish these goals.

The Handbook for Ranking Exotic Plants for Management and Control was developed to provide land managers with a tool to effectively evaluate the potential impacts of known exotic plant species. The advantage to using this approach is that managers can objectively evaluate different management strategies based on information obtained from literature and field surveys. This approach encourages managers to consider the full range of the potential impacts for their management decisions (Hiebert and Stubbendieck 1993). The benefits of managing specific exotic plants can be weighed against the potential costs of different management actions. The ranking system provides a sound justification for management plans, and can also provide justification for future program authorization and funding (Hiebert and Stubbendieck 1993).

The ranking system uses numerical ratings in an outline format to evaluate the current and potential ecological impacts and distributions of species in the areas of concern. The ranking system also evaluates different control options for a given species. Information for the ranking system can be obtained from both literature reviews and field surveys. Once a list of known exotic plant species has been obtained for a given area, each individual species can then be ranked relative to the other species. Species which pose an immediate threat to natural areas can then be targeted for control efforts, while species which have small potential impacts are given a lower priority for management.

The Handbook for Ranking Exotic Plants for Management and Control incorporates information obtained from a literature review and field surveys. As a result, its usefulness is somewhat restricted to species that have currently been studied in the field. However, for many species extensive surveys would be costly and unnecessary because of their low potential ecological impacts. This posed a particular problem for RMNP because field survey information was unavailable for many of the known exotic plants. Because of limited resources and time, collection of additional field survey information for all species was an unrealistic option.

To provide RMNP with useful information without additional field surveys, the Handbook for Ranking Exotic Plants for Management and Control was modified so that information from a literature review could be used to estimate the potential ecological impacts of all known exotic plants. The Ranking System for RMNP (Appendix 11) is divided into two general areas: an initial Screening Assessment section and a Final Assessment section. Information for the initial screening assessment can be obtained from literature reviews. If the species is identified as being a potential problem, then additional information and field surveys are conducted to provide a final assessment. Additional information on control methods and management considerations are also obtained for species which are identified as potential problems.

Figure 2 provides an overview of the ranking process. Initially, a list of known exotic plants is developed for an area. Once this list is complete, a general literature review is conducted for all species. After information has been collected for all species, an initial ranking is then performed using the Screening Assessment section. The screening assessment stage is designed to merely screen out those species which are not considered a potential ecological problem. For example, species which have an overall low potential ecological impact and low potential ecological distribution are unlikely to become a threat to natural areas.

GIF-General outline of weed ranking process
Figure 2. This figure provides a general outline of the ranking process. The initial screening assessment uses information from a literature review to rank all know exotic plants. Additional field surveys and literature reviews are then conducted for species which are identified as potential ecological threats.

Species which have a low potential impact and low ecological distribution can be screened out at this point. Species which pose a threat to native communities and/or have a high potential distribution are left on the list of species of concern. Additional information from literature and field surveys can then be collected for the remaining species of concern. This final assessment stage is designed to review information on different control options and on the overall feasibility of controlling the species of concern. Once the final assessment stage is completed, a management strategy can be developed for all species of concern.

The following example, using four exotic species found in RMNP, illustrates the ranking process. Canada thistle, dandelion, diffuse knapweed. and Russian thistle are four exotic species that are found in RMNP. A literature review was conducted for these species to determine their potential ecological impacts. The results of an initial literature review and screening assessment are illustrated in Figure 3. All four species were found to have a high overall potential distribution. However, dandelion and Russian thistle have relatively low potential ecological impacts. In contrast, both Canada thistle and diffuse knapweed have a high potential ecological impact. A final assessment is necessary to evaluate management options for Canada thistle and diffuse knapweed.

Figure 3. Results of the Screening Assessment for Canada thistle, dandelion, diffuse knapweed, and Russian thistle.

GIF-Screening assessment for Canada thistle, dandelion, diffuse knapweed, and Russian thistle.

Common Name
Potential Distribution
(15 total)
Potential Impact
(40 total)
Centaurea diffusa Diffuse knapweed
Cirsium arvense Canada thistle
Taraxacum officinale Dandelion
Salsola iberica Russian thistle

The results for the final assessment for Canada thistle and diffuse knapweed are presented in Figure 4. Canada thistle currently has a high overall distribution and is a difficult weed to control. As a result, attempts to eradicate Canada thistle in RMNP may be very difficult. Diffuse knapweed is also a very difficult weed to control. However, diffuse knapweed currently has a much smaller distribution in RMNP. Diffuse knapweed has a much higher likelihood of being effectively controlled. As a result, efforts to control diffuse knapweed while populations are small should be given priority over attempts to control Canada thistle.

GIF-Final Assessment for Canada thistle, dandelion, diffuse knapwee, and Russian thistle.
Figure 4. Results of the Final Assessment for Canada thistle, dandelion, diffuse knapweed, and Russian thistle. As these results illustrate, management efforts should focus on controlling diffuse knapweed because it currently is not widely distributed in RMNP.

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