Bison (Bos bison bison) As Ecosystem Engineers in the Aspen Parkland

Abstract

The Aspen Parkland is threatened by many factors including the loss of natural disturbance processes such as fire and herbivory. Bison, as the dominant large herbivore, are suspected to have played an important role in maintaining and structuring the plant community by preventing shrub encroachment into open grassland. In this study we examined the effect of bison on plant communities across the forest – grassland ecotone in Riding Mountain National Park. The impact of bison on rough-fescue grassland communities was examined using range exclosures. We found that over a single year bison reduced sward heights but did not alter the composition of grassland communities, with the possible exception of an important interaction with the invasive Poa pratensis. However, the long-term (>80 year) presence of bison was associated with greater species richness, reduced litter accumulation, and a distinct community composition compared to areas without bison. By comparing areas where bison had recently been introduced to areas of long-term presence and absence, we found that the influence of bison on the shrub community at the forest grassland edge is limited. Bison occurred at very low densities in areas with dense, tall shrub understories, and thus cannot be a major factor in limiting shrub growth and survival. Shrub stem mortality due to fire may be a necessary precursor to bison mitigating woody encroachment in the Aspen Parkland.