Vujnovic et al. determined how different levels of disturbances, including grazing, affect the plant species composition and diversity in Alberta's Aspen Parkland. Species diversity was greatest at the intermediate disturbance level. Species diversity and richness were low at low levels of disturbance, due to the dominance of the late succession species in these plant communities. At high levels of disturbance, species diversity and richness were low, due to the inability of plants to survive the continual heavy disturbances. Richness and diversity of exotic species was greatest in plant communities with high levels of disturbance because of the increased availability of colonization sites. Exotics were less abundant in areas with low levels of disturbance due to limited space available for plant invasion in these late successional plant communities. These results suggest that disturbance levels can alter the exotic and native plant species diversity and richness in the Aspen Parkland of Alberta, following the predictions of the Intermediate Disturbance Hypothesis.
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