Wildlife keeps savannah from becoming woodland

Published - December 19, 2013 12:25 am IST

The vast African savannah, known best for some of the most dramatic — and perhaps most photographed — wildlife migrations, owe their existence to not just frequent fires and scant rainfall. Three groups of wildlife — antelopes, giraffes and elephants – have kept these grasslands from turning into wooded Acacia forests, finds a ten year study of the ecosystem by researchers at the National Centre for Biological Sciences (NCBS).

Without these browsing herbivores, the savannah would have seven times the density of trees, says the research paper published in the latest edition of the Journal of Ecology . Smaller ungulates such as antelope prevent seedlings and saplings from establishing themselves by creating a ‘browsing trap’ similar to the way fire creates ‘fire traps’ that limit proliferation of certain plants.

Megaherbivores such as elephants and giraffe, on the other hand, “rapidly reduce tree cover by killing mature trees,” says the paper. The dominant shrubs in the savannah, which rely on spines as their primary defence mechanism, don’t quite stand a chance with “small browsers that are nimble enough to remove leaves between spines, and large browsers capable of consuming both leaves and spines.”

In the 10-year ecological experiment, herbivores were excluded in a semi-arid portion of the savannah in Kenya’s north-central Laikipia, where fires were also actively suppressed. Researchers observed a nearly a sevenfold increase in shrubs and trees and halving of tree mortality.

A diversity of browsing herbivores such as impala, dik dik, eland, greater kudu, giraffe and elephants ensure a year-round control on woody plants, the paper said, while a varied community of carnivores including lions, hyena, leopards, cheetahs ensures a predator-prey balance.

“We know that woody encroachments are a problem in grasslands — which also happen to be used as grazing land for domestic cattle across the world,” said co-author Jayashree Ratnam, senior fellow at the Ecosystems Group at NCBS. “The results of this study show that native herbivores can provide critical ecosystem services in maintaining these grazing lands in a healthy state.” Although herbivores are well recognized for their role in regulating woody cover in savannahs, “the extent to which browsing ungulates are capable of regulating woody populations in the absence of other disturbances such as fires [was] unclear,” says the research paper.

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