The iconic wildlife of the African savannazebras, gazelles, and other grazershas 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. But a new study suggests wildlife and cattle can coexistif elephants remain to help distribute nutrients into the soil, via their poop and their habit of knocking over trees.
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. A significant cause is that cattle herds dont give back to the land in the form of fertilizer. Instead, their dung and urine accumulate in small corrals where they spend their nights sequestered from lions and other predators. In general, when livestock numbers in the savanna increase, wildlife numbers go down.
To investigate the ecological effects of ranchingand the role of elephants in the ecosystemscientists 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 areasplus a plot that had no cattleto 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 didnt 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. That should benefit both cows and wild herbivores, Sitters says, but its especially good for smaller wildlife, such as gazelles. All of this emphasizes the key role of elephants in the functioning of an ecosystem, Sitters says. Ritchie calls their impact pretty surprising and striking.
One reason is that elephants dont remove many nutrients from the soil because they defecate in the plots where they graze. Thats 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. Sitters estimates that such vegetation is enough to account for about 19% of the extra nutrients. (The team wasnt able to account for all the sources of extra carbon and nitrogen.) Elephants also chow down on grasses that cows prefer, reducing the amount of grassand nutrientsthat cattle in the mixed plots could remove.
The findings add to evidence that wildlife can coexist sustainably with livestock. If you have elephants, then its no problem to also have cattle, as long as you dont overstock them, Sitters says. The study cant say whether elephants can compensate for damage caused by larger herds. The number of elephants in Kenya has been increasing, but populations remain threatened by poaching in other countries. Elephants sometime annoy people, especially when they raid crops. So the finding that the giant animals help keep soil fertile and the land productive might improve their reputation, Ritchie says.
sciencemag.org, 1 April 2020