Thresholds are in the eye of the beholder: Plants and wildlife respond differently to short-term cattle corrals

Authors: VEBLEN, KARI - Utah State University; Porensky, Lauren

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2019
Publication Date: 12/3/2019
Citation: Veblen, K.E., Porensky, L.M. 2019. Thresholds are in the eye of the beholder: Plants and wildlife respond differently to short-term cattle corrals. Ecological Applications. 29(8):e01982. https://doi.org/10.1002/eap.1982.
DOI: https://doi.org/10.1002/eap.1982

Interpretive Summary: Factors such as drought, wildfire and grazing can drive rangeland ecosystems over ecological thresholds, causing dramatic changes to plant and animal communities. Most work on this topic has focused on shifts in vegetation, and little research has explored whether thresholds for vegetation match thresholds for animals or other parts of the ecosystem. In sub-Saharan African, mobile overnight livestock corrals (“bomas”) can be used by managers to transform areas dominated by bare ground into productive ecosystem hotspots (“glades”) that are attractive to wild herbivores. We investigated thresholds for plants and animals, asking how long bomas must be occupied by cattle to undergo conversion to glade hotspot vegetation and initiate wildlife preference. We tested five durations of boma occupation: zero, four, seven, fourteen and twenty-eight days. Each treatment was replicated five times, and we assessed vegetation as well as animal dung for three years following boma abandonment. Vegetation in 7-, 14-, and 28-day boma duration treatments transitioned to glade-like plant communities, whereas the shortest 4-day treatment had not converted to a glade plant community by Year 3. Wildlife responses lagged behind vegetation responses, with threshold transitions occurring only in the longest duration (14- and 28-day) treatments. Our results show that the duration of mobile boma occupation required to create a glade differs by response variable. Although shorter-occupied bomas might be effective for reducing bare ground, they may not attract enough wild herbivores to create self-sustaining glades. Understanding threshold dynamics is challenging, but will provide a more complete representation of ecosystem function and greater opportunity for successful ecosystem management.

Technical Abstract: Rangelands are governed by threshold dynamics, and factors such as drought, wildfire and herbivory can drive change across thresholds and between ecological states. Most work on this topic has focused on shifts in vegetation, and little research has explored how thresholds and tipping points may differ across response variables. In sub-Saharan African, mobile overnight livestock corrals (“bomas”) can be used by managers to precipitate ecological transitions from areas dominated by bare ground to productive ecosystem hotspots (“glades”) that are attractive to wild herbivores. We investigated thresholds for plants and animals, asking how long bomas must be occupied by cattle to undergo conversion to glade hotspot vegetation and initiate wildlife preference. We tested five durations of boma occupation: zero, four, seven, fourteen and twenty-eight days. Each treatment was replicated five times, and we assessed vegetation as well as animal dung (as a proxy of use) at multiple time points over three years following boma abandonment. Vegetation in 7-, 14-, and 28-day boma duration treatments appeared to undergo a complete transition to glade-like plant communities, whereas the shortest 4-day treatment had not converted to a glade plant community by Year 3. Wildlife responses appeared to lag behind vegetation responses, with threshold transitions occurring only in the longest duration (14- and 28-day) treatments. Our results show that the duration of mobile boma occupation required to create a glade differs by response variable. Although shorter-occupied bomas might be effective for reducing bare ground, they may not attract enough wild herbivores to create feedbacks that maintain elevated nutrient levels and glade vegetation. Understanding threshold dynamics associated not only with vegetation responses but with a broader suite of response variables is challenging, but will provide a more complete representation of ecosystem function and greater opportunity for more successful ecosystem management.