Masai Mara. Credit: Christopher Michel CC BY 2.0
Masai Mara. Credit: Christopher Michel CC BY 2.0

How Do African Grasslands Support So Many Plant-Eaters?

Across the savannahs of Africa, millions of stomachs are busy converting plant tissue into animal flesh. The continent’s leaves and grasses are under constant assault from impala, wildebeest, buffalo, zebra, gazelles, and giraffes. Even acacia trees get bulldozed by elephants. There can be up to 25 species of these large plant-eaters in a given place, and many of them gather in gargantuan herds. How do they co-exist?

“It’s not obvious why competition for food doesn’t whittle the number of species down to just a few dominant competitors,” says Tyler Kartzinel from Princeton University. The prevailing idea says that different species have different food preferences. Grazers like zebra and wildebeest eat grass and little else. Browsers like dik-diks and giraffes nibble on leaves and shrubs—collectively called “browse”. Some animals, like elephants and impala, go for both.

Within each category, animals partition themselves in space. Zebras eat the tallest grasses; wildebeest munch the shorter ones. Dik-diks browse on the lowest leaves; impala take the mid-level; and giraffes pluck the loftiest foliage. But despite these nuances, “there’s still been this coarse distinction between grass and other plants,” says Kartzinel, “as if you partition those two resources finely enough, and suddenly there’s enough space in the savannah for dozens of herbivores.”

This picture is too simple. By using DNA to actually identify the plants that these animals eat—something no one had done before—Kartzinel has shown that their preferences go much deeper than just grass versus browse.

For example, the Grevy’s zebra and plains zebra—two species that live in the same places and consume almost nothing but grass—eat varying amounts of different species of grass. To them, a grassland isn’t just one uniform banquet. It’s a patchwork landscape full of different foods, with some bits that appeal to one species and others that delight another. “The appropriate question is not, ‘Does it eat grass?’ but rather ‘Which grasses does it eat?’,” says Kartzinel.

“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,” he adds. There are good reasons for that. These animals move over long distances, and they are hard (and dangerous) to observe up-close. They often eat small plants, in the middle of the night, under cover of thick bush. “Many of these plants are also exceedingly hard to identify to the species level, even for an expert botanical taxonomist with specimen in hand — meaning that it’s literally impossible to do while looking through binoculars at an animal feeding.”

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Dik-dik. By Henry Palm. CC BY 2.0

To solve these problems, he and his team, led by Princeton’s Rob Pringle, turned to poo. Driving around Kenya, they tracked seven plant-eaters: elephants, plains and Grevy’s zebra, domestic cows, buffalo, and Guenther’s dik-diks. They waited for the animals to defecate, before rushing over to (carefully) collect their dung. Back in the lab, they extracted DNA from the samples, sequenced it, and used those sequences to identify the specific plants that the beasts had eaten.

This approach, called DNA metabarcoding, confirmed the traditional divide between grazers and browsers, but also revealed that species which eat exactly the same amounts of either category still have very different diets. Cows and buffalo are closely related grazers, but they graze on different food. Even the zebras ate different amounts of 15 plant species, 14 of which are grasses. The Grevy’s also supplements its diet with small legumes that its plains cousin ignores. “That was the big surprise,” says Kartzinel. “We can finally see these very cryptic differences that these animals have.”

He compares the herbivores to a family at a buffet: “You might all choose the same main course, but when it comes to side dishes and condiments, you have hundreds of options. It’s unlikely that you’ll all end up with the same meal.”

This discovery helps to explain how the savannah supports so many plant-eaters. It offers them a buffet of riches, and each species eats only part of the full menu. The reasons for these preferences are unclear. Maybe the plains zebra just likes the taste of a particular plant. Perhaps the Grevy’s craves nutrients that only some species can provide. Perhaps dik-diks have unique gut microbes that can detoxify the poisons in plants that its competitors can’t touch. The only way of working out which of these possibilities is true is to work out exactly what these animals are eating—which is where DNA metabarcoding comes in.

The team also hopes that the technique will help to reduce conflicts between farmers and Africa’s wild herds. “If people think that livestock and wildlife compete fiercely for food, they will eliminate wildlife from rangelands,” says Kartzinel. If they can show that such competition doesn’t actually exist, that pressure might abate.

Reference: Kartzinel, Chen, Coverdale, Erickson, Kress, Kuzmina, Rubsenstein, Wang & Pringle. 2015. DNA metabarcoding illuminates dietary niche partitioning by African large herbivores. PNAS http://dx.doi.org/10.1073/pnas.1503283112