Invasive species are cited as a major threat to native community composition and biodiversity throughout the world. Some recent studies have focused on whether invasive species are the drivers or passengers of change in degraded systems. A strongly-interactive community is supposed to resist invasion by all but the most strong invasive competitors (`drivers') which can establish there without the aid of disturbance and actively reduce the abundance of natives. A weakly-interactive community, impaired by some anthropogenic disruption, is invasible by weaker exotics which are merely `passengers' of the habitat degradation that is more constraining to natives. Though competitive and disturbance-adapted species fit into this model, there is no correlate for species with superior toleration of stress. Systems with high degrees of natural abiotic stress are weakly-interactive and as a result may be invasible by an exotic `tolerator' in the same way that anthropogenically-weak systems are invasible by passengers. Dolomite prairie, differentiated from typic tallgrass prairie by its shallow soils, represents a relatively stressful system. A study of its plant community composition and relationships to environmental variables was done to get a better understanding of the natural abiotic drivers of composition. Compositional patterns were most closely associated with the soil depth gradient. Exotic Poa species were by far the most frequent invaders, a finding more typical of Great Lakes alvars than of typic tallgrass prairie of which the dolomite prairie is a subset. I hypothesized that Poa species dominated dolomite prairie via the tolerator model. In a series of removal treatments, I determined that Poa is neither a driver nor a tolerator, but a passenger of environmental degradation. My results suggest that historic anthropogenic degradation rather than specific competitive ability is a common explanation for exotic dominance on the landscape today. More empirical work needs to be conducted in other stressful (particularly relatively undisturbed) systems to further investigate the tolerator model.