The role of megaherbivores in transforming ecosystem structure and function is a leading issue for the ecology of both regions in Africa retaining abundant elephants, rhinos and hippos and for other parts of the world that formerly hosted species with potentially similar impacts on vegetation. In African savannahs, fire and large herbivores act synergistically to alter woody vegetation. Riparian woodlands are hotspots of biodiversity which attract huge concentrations of water-dependent animals during the dry season. Moreover, fire is largely excluded. Here we report on the transformation of a riparian woodland caused by impacts from elephants (Loxodonta africana). We undertook a study of structural and compositional changes of woody vegetation within a 45 km strip flanking the Linyanti River in northern Botswana between an earlier survey in 1991/2 and 17 years later in 2007/8. This area is subject to extreme elephant concentrations amounting to over 12 animals per km2 during the dry season. We compared the proportional representation changes of tree (>2.5 m) and shrub or sapling (<2.5 m) stages and species and established their density. We also surveyed intensive elephant impact on trees and shrubs as well as the composition of the seedling layer.
Results/Conclusions
Tall (>2.5m) canopy tree density decreased by half between 1992 and 2008, representing an annual loss rate of 2.7% without replacement. Differential felling or debarking by elephants shifted the canopy tree composition to just a few resistant or unpalatable species. Except for one species, there was no compensatory regeneration of saplings, although seedlings of trees species were present. Instead, the overall density of understorey shrub-forming species increased 2.5 times. A single shrub species increased five-fold in density to constitute 50% of total woody plant density. This shrub encroachment wave was incipient in 1992 and by 2008 many of these plants had grown beyond 2.5 m in height, resulting in a transition of functional woodland state. Small plants of this shrub species <1 m in height had become sparse by 2008, suggesting that the rapid invasion had become curtailed by then. We propose that the encroachment of this shrub is due to its unpalatability by elephants, although we cannot rule out other potential drivers of increasing regional aridity or atmospheric CO2. Our study documents how differential megaherbivore impacts on woody vegetation can promote state and functional transitions of tall canopy woodlands towards dense shrublands.