Native plant communities that are well established generally exist as climax communities, which are composed of species that have adapted to and thrive in the conditions of that ecosystem.  Plants in these ecosystems have evolved to operate with tightly-cycled or constrained resources.  If these communities are disturbed by perturbation or an abundant exotic seed source, a shift in species may occur as more aggressive exotics invade these communities by successfully out-competing natives for limited resources.  Once exotics have established, native species may be pushed out because they cannot tolerate a further constraint to the already limited resource.  The collection of tightly cycled soil nutrients in grassland ecosystems often defines the scope and scale of exotic species invasion.  Three studies were used to demonstrate how native and non-native grassland species respond to excess resources in disturbed sites.

Results/Conclusions

Soil K and NH₄⁺ supply rates were significantly lower in planted desert wheatgrass barriers compared to native controls, where medusahead successfully established.  Both desert wheatgrass and medusahead are exotics that begin production before native species, and there was minimal invasion of medusahead into desert wheatgrass barriers, suggesting both exotics are able to reduce soil bioavailable K and NH₄⁺ supplies before native species are active.  In a perturbed northern prairie ecosystem crested wheatgrass had more root production and thereby a greater potential to compete for underground resources when compared to native grassland.  Crested wheatgrass is also an early, fast growing species and is able to access nutrient supplies released after a disturbance and occupy soil space before native species become active.  A study in a similar ecoregion found the flush of soil nutrients with perturbation was a significant factor in restoring the native grassland community.  NO₃ supply rates in plots treated with glyphosate were significantly higher in May leading to the reestablishment of rhizotomous grasses, fast growing annual forbs, and also increased seed production in upland communities.  These studies demonstrate that constrained resources in native ecosystems are often better exploited by invasive species, and that excess nutrients result in increased production regardless of species.  It is commonly the case that in climax plant communities the soil nutrient supply will be a major constraint.  Plant Root Simulator (PRS™)-probes can be an effective tool for assessing the soil processes and mechanisms which may cause plant invasion.  Understanding the processes and mechanisms for invasion could help to manage native lands and reduce the potential negative effects of exotics.