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Accueil du site → Doctorat → États-Unis → 2012 → Ecology of bison, elk, and vegetation in an arid ecosystem

Colorado State University (2012)

Ecology of bison, elk, and vegetation in an arid ecosystem

Schoenecker, Kathryn Alyce

Titre : Ecology of bison, elk, and vegetation in an arid ecosystem

Auteur : Schoenecker, Kathryn Alyce

Université de soutenance : Colorado State University

Grade : Doctor of Philosophy (PhD) 2012

Résumé partiel
Herbivory has profound effects on vegetation production and structure in many different plant communities. The influence of herbivory on plants and ultimately ecosystem processes is shaped by the types of plants consumed, the intensity of herbivory, the evolutionary history of grazing, and the availability of water and nutrients to plants. The effect of ungulate herbivores on vegetation is of great interest to ecologists, land managers and agriculturalists. In addition, the Department of Interior recently established a Bison Conservation Initiative to provide for the conservation and restoration of North American plains- and wood bison, which includes establishing new populations and expanding existing populations. The San Luis Valley, Colorado, is being considered as a potential location for a bison conservation herd. Resource managers need to know the vegetation impacts of adding a second large ungulate to a system that already has elk. In Chapter 1 I conducted a landscape-scale observational study comparing areas with bison and elk grazing to areas with just elk grazing. I studied 6 vegetation communities to evaluate differences between ungulate strata, elk-bison versus elk-only, in herbaceous and woody vegetation production, and ungulate utilization. I found few differences in herbaceous production or utilization between the two ungulate strata. Herbaceous production was lower in elk-bison than elk-only wet meadows and cottonwood stands, but no production differences were found in mesic meadow, greasewood, or rabbitbrush communities. Willow communities were not comparable in soil substrate and species composition, so I could not compare production between elk-bison and elk-only willow communities. Average winter percent offtake in wet meadows was higher with both ungulates but not summer utilization. These meadows are highly resilient, not water-limited, and the most able to sustain grazing pressure of all the vegetation types I evaluated. Mesic meadows had higher summer and winter utilization in meadows with both ungulates compared to meadows with just elk. In woody communities, there was no change in cottonwood sapling density at elk-bison grazed areas over the 4-year study period, but sapling density decreased from 2005 to 2009 in elk-only cottonwood communities. I found higher browsing levels of cottonwood saplings in areas with both ungulates, but no differences between ungulate strata in willow utilization. I found higher summer than winter browsing in both cottonwood and willow communities suggesting ungulates are utilizing woody species in the summer for shade in the hot, arid climate of the San Luis Valley and browsing while present, as opposed to relying on winter browse as their primary food source as they do in temperate systems. My finding that annual willow utilization was slightly lower in some years in sites with both elk and bison browsing than sites with only elk, suggests bison may spatially exclude elk from willow stands when bison are present. There is evidence in the literature for competitive interactions between these two ungulates. Areas with both elk and bison mostly did not incur greater levels of utilization than areas with just elk, suggesting that spatially ungulates are segregating on the landscape or the additional forage needed by a second large herbivore is coming from mesic meadows or other communities. Greasewood percent offtake was higher in areas with two ungulate grazers than one, but was not statistically significant likely due to low sample size. I propose that forage utilization by a second large ungulate is being absorbed in mesic meadow and greasewood communities


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Page publiée le 14 avril 2013, mise à jour le 22 septembre 2017