Can grazing by elk and bison stimulate herbaceous plant productivity in semi-arid ecosystems?

Authors: SCHOENECKER, K - Us Geological Survey (USGS); ZEIGENFUSS, L - Colorado State University; Augustine, David

Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/13/2021
Publication Date: 4/9/2022
Citation: Schoenecker, K.A., Zeigenfuss, L.C., Augustine, D.J. 2022. Can grazing by elk and bison stimulate herbaceous plant productivity in semi-arid ecosystems? Ecology. 13. Article e4025. https://doi.org/10.1002/ecs2.4025.
DOI: https://doi.org/10.1002/ecs2.4025

Interpretive Summary: A central goal of rangeland management is to understand where and when grazing animals may negatively affect the long-term sustainability of plant growth. In this study, we measured both the intensity of grazing and the degree to which such grazing negatively affected plant growth in a cold desert ecosystem, under conditions where grazing occurred by elk only and by a combination of elk and bison. We found the with elk only, they consumed ~13 - 33% of the plant growth during the growing season, whereas elk+bison grazed 45 - 46% of plant growth during the growing season. In both areas, grazing did not negatively affect the productivity of the vegetation in either of two years of measurements. Furthermore, for riparian plant communities in the elk-only area, grazing actually increased the productivity of the vegetation, likely due to removal of senescent plant material by elk combined with fertilization by dung and urine. Overall, our results show that with average rainfall, the level of grazing imposed by elk and bison in this rangeland ecosystem is sustainable.

Technical Abstract: Plant communities in rangeland ecosystems vary widely in the degree to which they can compensate for losses to large mammalian herbivores. Many ecosystem-level factors have been proposed to affect this compensatory capacity, including timing and intensity of grazing, and availability of soil moisture and nutrients. Arid ecosystems are particularly challenging to predict because of their high degree of temporal variability in moisture inputs. We used a replicated herbivore exclusion experiment to evaluate herbaceous plant responses to grazing by large ungulates to test current theory on grazing optimization and identify potential constraints on plant compensation in a temperate, arid ecosystem of western North America. We measured nitrogen (N) yield and herbaceous production in three plant communities of a high elevation cold desert ecosystem: meadows, willow-associated herbaceous communities, and riparian herbaceous communities. We implemented grazing exclusion treatments from 2005 to 2008 in areas with elk plus bison and areas with only elk. Grazing by large ungulates increased herbaceous production and N yield in herbaceous riparian, demonstrating strong plant-herbivore feedbacks. In willow communities, herbaceous plants displayed equal compensation in response to grazing in total aboveground production and N yield. Our results support the idea that plant compensation is contingent on soil moisture availability, wherein the most productive sites (that received substantial moisture inputs from subsurface flow) exhibited overcompensation. Although the herbaceous riparian communities we studied are isolated patches of productive herbaceous vegetation in an otherwise shrub-dominated and minimally productive semi-arid landscape, grazing by a combination of bison and elk removed only 44-53% of ANPP during the growing season, and 25–38% of production overwinter. Consumption by ungulates at a given site was a positive linear function of herbaceous production, similar to reported patterns from other temperate and tropical grazing ecosystems. The slope of this relationship was affected by the analytical method used to calculate ANPP and consumption rates, but regardless of method was lower or similar to reported slopes for other intensively grazed systems (Yellowstone, Serengeti, Laikipia) that have sustained high ungulate densities for decades to centuries. Given that all vegetation communities exhibited equal or overcompensation in terms of total herbaceous ANPP in both years (with ~average rainfall), elk and bison population levels at the time of our study did not appear to occur at densities leading to degradation of herbaceous communities.