We anticipate that issue 102:3 will be online this week. Consider it an Easter treat without the calories!

The Editor’s Choice paper from this issue is “Restoration of a megaherbivore: landscape-level impacts of white rhinoceros in Kruger National Park, South Africa” by Cromsigt & te Beest.

The Swedish University of Agricultural Sciences published a press release on this paper earlier this year.

Editor’s Choice 102:3

Large-bodied herbivores – megaherbivores – have been lost from many ecosystems worldwide.  A growing appreciation of the ecological roles that they play has led to reintroductions of megaherbivores into some ecosystems, for example of muskox and wisent into a “Pleistocene Park” in Siberia, or even calls for surrogates to be introduced into others, such as elephants as replacements for long-extinct marsupial megaherbivores in northern Australia.

So far there is little evidence to show what the consequences for ecosystems could be that result from “rewilding” through the reintroduction of megaherbivores.  A new study by Cromsigt and te Beest provides insights into their effects.  In Kruger National Park, South Africa, white rhinoceroses were hunted to extinction in 1896.  They were reintroduced to the Park between 1961 and 1972, most of them to the south-western corner of the Park, and much of the Park remains unoccupied by white rhinoceroses.  Thus there are zones in the Park with either no rhinos, sparse rhino populations, and comparatively dense rhino populations.  Cromsigt and te Beest studied the impacts of the rhinos on the savanna grasslands of the Park.  They capitalised upon the “natural” experiment that the reintroductions presented, focussing on zones with sparse and dense rhino populations.

Photo credit: Jan Graf

Rhinos aren’t the only megaherbivores present in Kruger National Park: there are also African elephants and hippopotami (the latter don’t venture far from rivers).  There are also a large range of other grazers, including buffalo, wildebeest, zebras and various antelopes.  Yet the effects of the rhinos could be clearly distinguished.  Where their numbers were greatest, short grasses were more prevalent and there were many more distinct grazing lawns that are maintained by the rhinos.  These effects were consistent across more fertile soils derived from basalts and less fertile soils derived from granite.

Cromsigt and te Beest conclude that the rhinos are important drivers of grassland heterogeneity in these savannas.  An intriguing possibility supported by observational data in this study is that this could also be driven, at least on the infertile granitic soils, by an interaction between rhinos and termites.  Grazing lawns on these soils were usually close to termite mounds, which the authors think may be linked to locally enhanced nutrient availability.  This begs further study about whether there may be positive feedbacks between termites, rhinos, and the distinct plant communities that develop in the grazing lawns.

Time for such studies however may be running out.  Cromsigt and te Beest note that poaching pressure on white rhinos, should it continue at current rates, would result in a second extinction of these animals in Kruger National Park within 20 years, and would end of this “rewilding” experiment.  It is a grim prospect that the opportunities presented to determine the ecosystem effects of a rhino population, reintroduced barely thirty years ago, could instead, in future years, give way to a chance to document trophic downgrading after these megaherbivores have gone.

Peter Bellingham
Associate Editor, Journal of Ecology