Blackfoot Drainage New Weeds Risk Assessment

Species Selection and Assessment of Weed Potential

The geographic region treated in this analysis was the 11-county area approximately centered on the Blackfoot River drainage (Missoula + Powell and their 9 surrounding counties  Mineral, Sanders, Lake, Flathead, Lewis and Clark, Jefferson, Deerlodge, Granite, and Ravalli). A list of plant species exotic to North America that have been recorded in the 11-county "infiltration area" (Wilson and Lee 1989) was generated from the INVADERS Database Release 6.4 (Rice 1997).  Distribution records were  available for 411 exotics in the project infiltration area.  This initial list was reduced by first excluding well-known widespread weeds (e.g., spotted knapweed, leafy spurge, Dalmatian toadflax, cheatgrass) and other common exotic plants known to be non-invasive or only minor ruderal weeds (e.g., Draba verna, Myosotis micrantha).  The reduced list contained 308 species.  For these species the following information was examined:

1. Presence on noxious weed lists:

2. Presence on other lists of pest/invasive plants: 3. Overall rate of spread in the five-state northwest region (WA, OR, ID, MT, WY), expressed as the average number of counties with records per decade (first year of record to 1996, derived from the INVADERS Database)

4. Number of occurrence records and number of counties within the 11-county infiltration area (INVADERS Database).

We looked for species with the following characteristics, but that were not presently listed as noxious weeds in Montana: 1) Reported frequently as a noxious weed or problem plant elsewhere, and 2) Spreading rapidly in the northwest region, yet recently introduced into North America, recently invading in Montana or otherwise not well known in comparison with species already declared noxious in Montana.  Thirty-two species were selected to comprise an Alert List for the Blackfoot River drainage. Spring annuals, weeds which complete their life cycle in a single growing season, are seldom declared noxious.  Most species which are designated noxious are perennials, while a limited number are biennials or winter annuals. However, we have included seven species on the Alert List that are primarily annuals because of their potential to alter the species composition of native grassland communities or impact agriculture.

For each of the 32 species on the Alert List, ratings of weed potential, which generally should be considered preliminary, were assigned for six broad environmental types. The small proportion of total land area comprising riparian and wetland environments in Montana increases the resource and ecological  value of these types far above the actual acreage. These environments are  particularly important to the Blackfoot area..

These risk ratings were determined by examination of the specimen comments field for INVADERS distribution records (i.e. associated vegetation, environmental factors, land use, extent/severity of weed infestations, etc.), available scientific literature, and previous work on cover/habitat type susceptibility (Rice and Rider 1995, Rice 1997) and climatic correlations (Rice et al. 1997) of weeds in the northwestern United States.


Hobbs, R. J. and S. E. Humphries.  1995.  An integrated approach to the ecology and management of plant invasions.  Conserv. Biol. 9:761-770.

Holm, L.,  J. V. Pancho, J. P. Herberger, and D. L. Plucknett. 1979.  A Geographical Atlas of World Weeds. John Wiley, New York.

Parsons, W. T. and E. G. Cuthbertson.  1992.  Noxious Weeds of Australia.  Inkata Press, Melbourne.

Randall, J. M. and J. Marinelli (eds).  1996.  Invasive Plants.  Weeds of the Global Garden.  Brooklyn Botanic Garden, Brooklyn, New York.

Rice, P. M.  1997.  INVADERS Database Release 6.4.  Division of Biological Sciences, The University of Montana, Missoula.

Rice, P.M and C. Toney. 1997. Susceptibility of Northern Region Habitat Types to Invasion by Five Noxious Weed Species - First Approximations.

Rice, P. M.,  J. C. Toney, R. Sacco and D. Cooksey.  1997.  Climatic correlations and predicted potential ranges of species in the Montana Weed Seed Free Forage Program: first approximations.  Final Report, Coop. Agric. Pest Survey, Dept. Plant & Soil Sci., Montana State Univ., Bozeman.  156 pp.

Rice, P.M. and J. Rider. 1995. Landscape Ecology: Noxious weed invasion analysis. Interior Columbia Basin Ecosystem Management Project. Contract Order Number 43-0E00-5-5264. Contract Officer Jim Olivarez, USFS Northern Regional Office, Missoula, MT.  (susceptibility matrix available for download on the ICBEMP Databases page - reference the "Noxious Weeds Database")

Whitson, T. D. (ed.), L. C. Burrill, S. A. Dewey, D. W. Cudney, B. E. Nelson, R. D. Lee, and R. Parker.  1991.  Weeds of the West.  The Western Society of Weed Science in cooperation with the Western United States Land Grant Universities Cooperative Extension Services (printer: Pioneer of Jackson Hole), Jackson, Wyoming.

Wilson, J.B. and W.G. Lee. 1989.  Infiltration invasion.  Functional Ecology 3:379-382.

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