When exotic species invade a region, it becomes important to assess their invasiveness in adjacent uninvaded regions to determine if weed prevention measures are needed. Leafy spurge and knapweed species are absent from the vast majority of eastern Montana, but the region is surrounded by regions heavily invaded by these species. To assess invasiveness of leafy spurge and Russian and spotted knapweed in common eastern Montana grassland sites, I introduced these species to three sites as seeds (120 live seeds plot⁻¹) and seedlings (6 plot⁻¹). I assessed how common grazing regimes influenced invasiveness by imposing cattle, sheep, mixed grazing (i.e., cattle plus sheep), and grazing exclusion treatments for 7 yr. Invader survival did not appear to differ greatly among sheep, cattle, and mixed grazing treatments, but excluding grazing lowered probabilities that plots maintained invaders for the entire study period at two of three sites. At these same sites, grazing exclusion increased growth rates of those invaders that did survive, at least in the case of leafy spurge. Regardless of grazing treatment or site, however, large proportions of plots did not maintain invaders through the end of the study period. At one heavy clay site, only one small leafy spurge plant persisted through the end of the study. In the seventh study year, the plots with the most leafy spurge and Russian knapweed produced 222 and 112 stems, respectively, and the stems remained mostly confined to the 2- by 2-m plots. These findings suggest that, barring intense disturbance, leafy spurge and spotted and Russian knapweed might be incapable of invading some grasslands of eastern Montana, particularly upland sites with high clay content. Any upland sites in the region these species are capable of invading will likely be invaded only very slowly.

Nomenclature: Leafy spurge, Euphorbia esula L., Russian knapweed, Acroptilon repens (L.) DC., Spotted knapweed, Centaurea stoebe L.

Management Implications: Important management decisions hinge on how capable invasive plants are of spreading from invaded to uninvaded regions. I evaluated the invasiveness of leafy spurge and spotted and Russian knapweed in eastern Montana, a largely weed-free region surrounded by regions supporting large infestations of these invaders. Despite being introduced from large numbers of seeds and seedlings, the invaders failed to survive the 6-yr study period at one of three study sites, suggesting that the species cannot invade the many areas of eastern Montana that resemble that study site, particularly upland sites with high clay content. Extinction rates were high at the other two sites as well, but invaders did persist and grow modestly in some plots at these sites. Six yr after introduction, all invaders remained mostly confined to the 2- by 2-m plots where they were planted. This suggests that any invasions by the focal species in eastern Montana will likely proceed slowly, and careful scouting of any given uninvaded area once every several years should be sufficient to detect and efficiently eradicate newly colonizing populations. Finally, the study had grazing exclusion, sheep, cattle, and mixed grazing (cattle plus sheep) treatments. Compared to the grazing treatments, grazing exclusion decreased the number of plots that maintained invaders for the entire study period. Conversely, grazing exclusion increased invader biomass production in those plots that did maintain invaders for the entire study period, at least in the case of leafy spurge. Collectively, these results suggest grazing exclusion alone is unlikely to prevent invasions indefinitely. These results are consistent with research showing sheep grazing can be used to constrain spotted knapweed and leafy spurge abundances.