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The iconic wildlife of the African savanna—zebras, gazelles, and other grazers—has for decades been under pressure from some unnatural rivals. Ranchers’ cattle compete with local wildlife for food and water, and they starve much of the soil of nutrients.
Cattle ranching, which has been intensifying across the savanna, can lead to overgrazing, eroding and impoverishing the soil and helping shrubs invade the grasslands. The damaged ecosystem provides less food for wildlife and cows alike.
To investigate the ecological effects of ranching—and the role of elephants in the ecosystem—scientists launched a long-term experiment in 1995 at the Mpala Research Centre in central Kenya.
There, herds of up to 120 cows graze in fenced areas, each about the size of four soccer fields. (The cattle still spend nights in corrals outside the experimental plots.) Some of these grazing areas exclude all wildlife. Others have tall electrified fences that let in gazelles, zebras, and similar-size grazers, but not elephants. And still others included cattle, medium-size wildlife, and elephants.
“Creating those kinds of combinations [of cattle and wildlife] is really, really difficult to do,” says Mark Ritchie, an ecologist at Syracuse University who was not involved in the research.
Judith Sitters, an ecologist at the Free University Brussels, visited Mpala in 2015. She analyzed soil and vegetation samples from each of the areas—plus a plot that had no cattle—to see the impact of livestock and wildlife on nutrient levels. She was surprised to find nearly twice as much carbon in the soil in grazing areas that included elephants, compared with those without them.
Soil nitrogen was also much higher when elephants were present, providing additional nutrients for plant growth, she and colleagues reported last month in Nature Sustainability. Those levels were similar to measurements for plots that had no cattle at all. “We didn’t expect that there would be such a positive impact from these elephants,” Sitters says.
More soil nutrients had a knock-on benefit. Sitters and colleagues found that a common grass (Brachiaria lachnantha) contained about 50% more nitrogen in the grazing areas with elephants than the areas without them, making that grass more nutritious.
One reason is that elephants don’t remove many nutrients from the soil because they defecate in the plots where they graze. That’s true of the other wild grazers, but elephants have another important habit: They sometimes knock over trees to eat leaves and branches. That vegetation decomposes, adding nutrients to the soil.
The findings add to evidence that wildlife can coexist sustainably with livestock. “If you have elephants, then it’s no problem to also have cattle, as long as you don’t overstock them,” Sitters says.