Lack of Food Competition May Explain the Diversity of African Savanna Herbivores

Andrzej Kubik / shutterstock.com
Janet Fang 03 Jun 2015, 14:43

Zebras, impalas, elephants, and a handful of other large African herbivores have surprisingly different diets, according to a new analysis of plant DNA in animal dung. With each species enjoying their own favorite plants, the reduced competition over time may explain how so many wondrous species have managed to coexist. The findings, published in Proceedings of the National Academy of Sciences this week, suggest that diverse vegetation may be key in maintaining the continent’s rich variety of animals. 

Large plant-eating mammals in the semi-arid African savannas fall along a continuum, from grass-eating grazers to non-grass-eating browsers who dine from trees and shrubs. There are lots of different plants, and there’s a huge diversity of plant eaters, but exactly how so many herbivores manage to live near each other on such a limited range of resource types has been a mystery. 

To investigate, Princeton’s Tyler Kartzinel and colleagues analyzed the diets of an assemblage of animals at the Mpala Research Centre in Kenya. Buffalo, Grevy’s zebras, plains zebras, and domestic Boran cattle make up the grazers, while elephants, impalas, and small, 5-kilogram antelopes called dik-diks are the non-grazers. These 7 make up 99 percent of the large mammalian herbivores in this ecosystem. 

The team sequenced plant DNA from 292 fecal samples collected from June to July in 2013. They then turned to a recently developed technique called DNA metabarcoding: Gene sequences found in the fecal samples were matched to known plant DNA from a reference library. This allowed them to measure the breadth of the animals’ diet, the composition, and any overlap. 

“When I talk to non-ecologists, they are stunned to learn that we have never really had a clear picture of what all of these charismatic large mammals actually eat in nature,” Kartzinel explains. “What we show is that the dietary differences among species are far greater than everyone has assumed.”

The compositions of their diets were similar within species but strongly divergent across species. Dietary overlap was the greatest between species that were similar in size and closely related. The two types of zebras, for example, ate 45 different plants altogether, but 15 of those were found far more frequently in one zebra than the other. “This may not sound like much at first, but it’s a third of all of the food types that we detected in the diets of either species,” Kartzinel tells Smithsonian. Similarly, at least 12 grasses and 4 trees were found far more often in buffalo poo than in cow poo. “You might all choose the same main course, but when it comes to side dishes and condiments, you have hundreds of options,” he adds, speaking to National Geographic. “It’s unlikely that you’ll all end up with the same meal.”

By relieving some food competition, this so-called “dietary niche partitioning” made it possible for so many ecologically similar animals to coexist – and it likely contributed to the origin of this biodiversity.

Images: Andrzej Kubik (top), John Carnemolla / shutterstock.com (middle)

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