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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.”

Dik-dik. By Henry Palm. CC BY 2.0
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

8 thoughts on “How Do African Grasslands Support So Many Plant-Eaters?

  1. So good to see these thoughts (about which animals eat what) being published. I grew up in bush and jungle in Burma, South-West Africa (Namibia) and old Northern Rhodesia (Zambia), and saw what has been written here. Of course I didn’t have botanical or zoological names for what I saw, but years later, via ‘Varsity education, I understood the taxonomy. The real thing that one can glean from this article, is that Mankind is so ignorant about what “good food” really means, even though we pride ourselves as being so intelligent and knowledgeable. This prideful ignorance is a major reason for why we have messed up our own diets.

  2. In the absence of the wild herbivores the grass lands are deteriorating into desert. The herd effect of wild herbivores can be mimicked by using holistic planning and high density grazing domestic herbivores to obtain the impact needed to remove grass and forbes, trample ungrazed plant material to develop a protective mulch over the soil, and hoof action to develop seed beds for new grass, the impact needs to be dense and short in duration, with an appropriately long rest and recovery period. See the “Savory Institute” for further information.

  3. Thanks for this, great stuff, please, keep us informed as research on this important management toll progresses. Regards, helena

  4. Peter
    “The real thing that one can glean from this article, is that Mankind is so ignorant about what “good food” really means, even though we pride ourselves as being so intelligent and knowledgeable. This prideful ignorance is a major reason for why we have messed up our own diets.”

    There’s no proof that our diets are “messed up”, humans historically and in recent times have thrived and lived to old ages on a wide variety of diets. Mankind boomed when we started farming, when we started drinking milk, etc. Being omnivorous is a huge advantage and taking advantage of it is not hubris.

  5. “If they can show that such competition doesn’t actually exist, that pressure might abate.”

    Or the farmers can have the more normal human response and refuse to credit anything that contradicts what they already Know. Still, hope springs eternal, like a tireless Dik-dik.

  6. The Savory Institute mentioned above has a vested interest in promoting cattle as a way to restore “depleted” desert lands. This despite the fact that deserts are natural and biologically diverse habitats in their own rights. And there is no or little historical occurrence of bison/elk herds in many of the regions of Nevada/Arizona/California/New Mexico that they suggest cattle do good.

    1. RE: This despite the fact that deserts are natural and biologically diverse habitats in their own rights.

      Yes, but not nearly as diverse as grasslands. And not all deserts are “natural” unless you believe that “natural” includes humans killing off all the fauna in a region. See Pleistocene Overkill.

      RE: And there is no or little historical occurrence of bison/elk herds in many of the regions of Nevada/Arizona/California/New Mexico that they suggest cattle do good.

      Not sure what your reference is for this. However, in all the regions cited above there is evidence that they were once grasslands, which required grazing/herding animals and predators to exist. In fact, they were loaded with mega fauna at one point.

      Nobody is saying that they’re going to restore the grasslands using the fauna that historically flourished there. That would require bringing back mega fauna in many regions. Instead, the goal is to use the resources we currently have at our disposal to recreate the vital grasslands in the best way possible.

  7. My experience was chiefly in Zimbabwe/Rhodesia where using cattle and goats regenerated the veld to the extent whereby the native game species were able to recover their diversity which had been badly effected by the Rinderpest outbreak, and never recovered due to veld deterioration and poaching. I did develop sustainable grazing and improved soil on old cotton fields in NC by grazing Tuli cattle on whatever was growing there, and stimulating grass and edible forbes and legumes through high density grazing, this is all natural timber ecosystem, but maintained a biodiverse grassland by managed grazing, attracting white tail deer, rabbits and an increase in birds. I only had five years there so don’t have the long term results I would have liked to have achieved.

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