Impacts of herbivore grazing on woody encroachments in the savanna of an East African protected area

Abstract

Heavy ungulate grazing is related to increased woody encroachment in savannas. This study examined how varying ungulate grazing densities affect woody encroachment in Nech Sar National Park, Ethiopia. Woody density was surveyed in 103 plots along five transects in 2012 and 2022. Generalized linear mixed models were used to test relationships between ungulate densities, cattle corrals, and woody density. Woody density increased with the presence of cattle corrals, while wild herbivores showed contrasting effects: zebra reduced woody density, but this suppressive effect was reversed when cattle activity was present (corrals x zebra; β = 0.10, p < 0.05) in 2022. Woody density increased from about 600 woody species ha⁻¹ to over 2500 woody species ha⁻¹ at the high densities of corrals and zebra. Dichrostachys cinerea density was negatively related to zebra in both years, while Senegalia mellifera increased with zebra and with the zebra x corral interaction in 2012. In 2022, kudu and its interaction with livestock were negatively related to woody density (β =  −1.73, β =  −0.98, p < 0.05; respectively). Livestock had strong effects near corrals, while wild ungulate impacts were mediated by livestock activity. Herbivore grazing is a key factor in woody encroachment but it cannot fully explain encroachment. Thus, woody encroachment is likely due to the combined effects of bioclimatic factors, low browsing pressure, and reduced fire, alongside grazing in this study system and beyond.