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Vegetation type matters. Some are more at risk to invasion by alien species than others, and thus will require more intensive management if alien species are to be excluded. As in other studies (DeFerrari and Naiman 1994, Planty-Tabacchi et al. 1996, Stromberg et al. 1997, Stohlgren et al. 1998), riparian areas seem especially at risk. Disturbance, as we defined it in this study, was rarely associated with the most abundant alien species (e.g., E. esula, M. officinalis, and P. pratensis), which suggests that simply limiting construction of roads, trails, and other facilities will not protect vulnerable areas from invasion.

Differences in the frequencies of alien species between the two park units at TRNP suggest that prediction will always be uncertain. A monitoring plan that emphasizes the most vulnerable vegetation types and includes searches for all known potential invaders is critical for early detection as well as to determine which species are, in fact, increasing in number and range. Such strategic planning is an important first step in prioritizing use of limited resources for alien plant control.