The Plot Thickens: Effects of Large Herbivore Exclusion on Understory Community Composition in an East African Savanna

Abstract

In an East African savanna, large mammalian herbivores (LMH) exert top-down pressures on plant communities which, along with abiotic effects such as rainfall, shape ecosystem structure and function. Given the range of both rainfall levels and LMH sizes in the ecosystem, and their variation over time due to environmental changes, it is important to understand how they impact composition of the understory plant community of the savanna. After 14 years of herbivore exclusion in the Ungulate Herbivory Under Rainfall Uncertainty (UHURU) experiment at Mpala Research Centre in Laikipia, Kenya, I analyzed biodiversity and composition of the understory community. The removal of mesoherbivores had the largest effect on plant abundance but removing mega- and small LMH increased evenness. Species richness changed at different sites, which had historically been at different rainfall levels, although it was not impacted by herbivore exclusion, except very slightly in the site with highest rainfall. Removing LMH dampened the positive relationship between species richness and vegetation abundance. 50 species changed in abundance between treatments, with eight species as the most abundant across the experiment. Palatable species were more likely to be found in plots excluding meso- and megaherbivores. There was no difference in the shape of the species–area curve between herbivore exclusion treatments. Further study should include impacts of livestock on understory composition and diversity under varying abiotic pressures, especially as drought conditions become more severe.