Direct and indirect effects of livestock grazing intensity on a grassland food web

Background/Question/Methods

Livestock graze the majority of North American grasslands and livestock production is an important contributor to many rural economies. Developing sustainable grazing practices for public and private grasslands depends on a comprehensive understanding of how livestock grazing intensity affects biodiversity and ecological function of grasslands. However, sustainable livestock production on the country’s rangelands is difficult to identify because few studies have examined the economic and ecological impacts of livestock grazing together. Also most of the ecological literature is focused on the effects of livestock on soils and plants with little information on native fauna. We combined landscape-scale experimental manipulations of cattle stocking rates and structural equation modeling to partition direct and indirect effects of grazing intensity on grassland soils, flora and fauna (ground-nesting songbirds and invertebrates) and on livestock performance. The study area is The Nature Conservancy’s Zumwalt Prairie Preserve in northeastern OR which is located in one of the last large remnants of the Pacific Northwest Bunchgrass prairie.

Results/Conclusions

Increasing cattle stocking rate resulted in direct increases to soil compaction and cattle performance, but decreased vegetation structure. Invertebrate taxa showed differential responses to grazing intensity. The abundance of several invertebrate taxa identified as important to nesting passerines declined with increased stocking rate, but these effects were indirect and were mediated through changes in vegetation structure. Results were also suggestive of a direct negative effect of increasing stocking rate on native pollinators, but patterns were not statistically significant. Although univariate analyses suggested high stocking rates negatively impacted avian abundance of some species, model selection procedures suggested that grassland passerines should not be included in the final model, implying effects of stocking rate may not be a primary factor structuring grassland passerine abundance or productivity. The lack of differences in plant species composition among stocking rates suggest short-term effects of grazing intensity are influenced by altered physical properties like vegetation and soil structure, but that those affects may be largely restricted to basal trophic levels when evaluated in the context of the grassland food web. These results, combined with previous results yielded from the same experiment, suggest low-to-moderate stocking rates as applied in this experiment may be a more suitable grazing practice for sustaining grassland biodiversity and function than high stocking rates.