The American bison has been re-introduced throughout North America since its near-extirpation in the 1800s. However, most herds are small, isolated, and no longer play a significant ecological role in the grassland ecosystem. In 2001, the nonprofit American Prairie Reserve (APR) was established to create a 3.5 million acre reserve in the Northern Great Plains of Montana with a restored herd of plains bison (Bison bison bison) large enough to fulfill its historic ecological role on the landscape. To date, the reserve has grown to over 400,000 acres with a bison herd of over 800 individuals. Previous research in this region has shown that upland vegetation has responded with higher species richness and compositional heterogeneity with continuous year-round bison grazing than with seasonal rest-rotation cattle grazing. However, it is unknown how riparian vegetation may respond to bison grazing compared to cattle grazing, especially when bison have been shown to spend less time near water than cattle. Our study’s objective is to compare the response of the riparian vegetation community to continuous year-round bison grazing versus seasonally rotational cattle grazing. We are especially interested in comparing the richness and diversity of species categorized as native versus exotic, hydrophytes versus upland, ruderal versus competitive or stress-tolerant, and woody versus non-woody. We use transect-based vegetation surveys in riparian zones to assess plant species richness, diversity, and structure in adjacent bison and cattle sites, with 2 cattle treatment sites and 5 bison treatment sites of varying times since reintroduction.
Results/Conclusions
Preliminary results from our first field season suggest that riparian vegetation communities have responded similarly to grazing by both species. However, exotic plant species richness was significantly higher in the cattle treatments than in the bison treatments (p = 0.015, F = 7.123). In general, riparian communities were speciose with 75 encountered plant species from 18 families. Once completed with a second field season, our study will add a riparian component to our understanding of the ecological role of bison in this region. Thus far, it seems that year-round bison grazing is not more impactful to the vegetation community than rest-rotation cattle grazing, and this finding has implications for the political controversy of using public lands for bison grazing in the Northern Great Plains.