Simplification of the composition, diversity and structure of woody vegetation in a semi-arid African savanna reserve following the re-introduction of elephants

Abstract

Elephant populations at high density commonly transform their habitat, but a low density population would not be expected to have a marked impact. Re-introduction of elephants into the Venetia-Limpopo Nature Reserve (320km2) in the early 1990s established a low density population for the period of survey (0.16–0.33 individuals km−2). Accordingly, their impact on the composition and structure of the woody vegetation of three riparian and nine dryland vegetation types was measured between 1997 and 2010 using 148 permanent transects. Riparian habitat showed a greater change in composition and diversity, and also a greater decline of species richness, density of tall trees or total basal area, than dryland habitat. Change of dryland Commiphora Woodland was comparable to changes of riparian types. These conspicuous changes were a consequence primarily of severe use by elephants. Some species within these vegetation types declined markedly in abundance. Vegetation types dominated by Colophospermum mopane showed an increase in total basal area and relatively minor change in composition or structure, resulting mainly from the impact of moisture stress. Vegetation types that were severely impacted by elephants constituted <10% of reserve area; lightly impacted dryland C. mopane types constituted >70% of area. Some uncommon, selected dryland species were heavily impacted by elephants. A number of species may therefore be trending toward local extirpation. It was concluded that the coexistence of elephants and some plant species in this medium-sized, contained reserve was not possible.