Little is known about the relationship between frequency of occurrence and the expected maximum distance a nonnative species might spread along a trail once introduced to an ecological system with high native species integrity. Understanding how colonization and invasive plant habitat degradation occur in largely intact ecosystems is challenging. Determining which nonnative species are most likely to spread might be possible, given a suite of environmental or trail conditions. Spread may be linked to a particular set of environmental conditions, or to type and level of trail use. A field study conducted on trails in Forest Service and State Park lands on the Kenai Peninsula, Alaska, was designed to determine frequency and spread distance of all nonnative vascular plant species per 100-m segments keyed to vegetation type, canopy cover class, aspect, trail use level, and trail use type. Although the maximum total number of nonnative species decreased with increased distances from trailheads, the average number of species remained nearly constant. Common dandelion, broadleaf plantain, and annual bluegrass exhibited consistent presence per canopy cover class or vegetation type. A nested subset analysis revealed a significant reduction in nonnative species presence beyond a 500-m distance from a trailhead and a moderately strong nestedness pattern. High-use trails exhibited the greatest numbers of nonnative species at the farthest distances from the trailhead and contained a greater number of less common nonnative species. Alaska and other northern biomes have relatively few widespread invasive problems, offering an opportunity to limit ecosystem degradation by invasion. Results suggest that control strategies might focus on high-use trails with open-canopy habitats to prevent spread.

It is essential that we improve our ability to predict which nonnative species will become invasive in order to prevent their introduction and spread. Past attempts to foresee invasions have met with limited success, but increased computing power, increased availability of information about exotic species, and comprehensive evaluations of invasion potential are improving our ability to predict which species are likely to invade most successfully. We used data from Colorado and other states to develop an effective means of predicting the spread of invasive plant species among states. Qualitative criteria were used to develop a numerical threat index, which rates potential invaders based on distribution and abundance with respect to climate, biological characteristics, and preferred habitats of the species. Out of a compiled list of 388 species, we identified six invasive nonnative plants that are highly likely to invade Colorado, 10 with medium invasive potential and five with low potential. Species found to be likely to invade Colorado included garlic mustard, smooth distaff thistle, and Syrian beancaper.