The tall and bushy plant known as the Sodom apple has overrun vast swaths of East African savanna and pastureland, including parts of Kenya’s Amboseli National Park. New research suggests that certain animals, including elephants and impalas, could keep this invasive plant in check. (Photo courtesy of Rob Pringle)
AFRICAN WILDLIFE OFTEN RUN AFOUL of ranchers securing food and water resources for their animals, but the interests of fauna and farmer might finally be unified by the “Sodom apple,” a toxic invasive plant that has overrun vast swaths of East African savanna and pastureland.
Not a true apple, Solanum campylacanthum is a relative of the eggplant that smothers native grasses with its thorny stalks, while its striking yellow fruit provides a deadly temptation to sheep and cattle.
New research suggests, however, that certain wild animals, particularly elephants, could be a boon to human-raised livestock because of their voracious appetite for the Sodom apple. A fiveyear study led by Princeton University researchers found that elephants and impalas, among other wild animals, can not only safely gorge themselves on the plant, but also can efficiently regulate its otherwise explosive growth.
Just as the governments of nations such as Kenya prepare to pour millions into eradicating the plant, the findings present a method for controlling the Sodom apple that is cost-effective for humans and beneficial for the survival of African elephants, explained first author Robert Pringle, a Princeton assistant professor of ecology and evolutionary biology.
An elephant prepares to uproot a Solanum campylacanthum plant in the upland savanna of central Kenya. Although this woody shrub is toxic to many mammal species, large browsers such as elephants can eat it, and in so doing help to reduce its abundance.
“The Holy Grail in ecology is these win-win situations where we can preserve wildlife in a way that is beneficial to human livelihoods,” Pringle said “Elephants have a reputation as destructive, but they may be playing a role in keeping pastures grassy.”
The findings are important given the threats to elephants from poaching, Pringle said. “We need to understand to what extent these threatened animals have unique ecological functions.”
Elephants and impalas can withstand S. campylacanthum’s poison because they belong to a class of herbivores known as “browsers” that subsist on woody plants and shrubs, many species of which pack a toxic punch, Pringle said. On the other hand, “grazers” such as cows, sheep and zebras primarily eat grass, which is rarely poisonous. These animals easily succumb to the Sodom apple, which causes emphysema, pneumonia, bleeding ulcers, brain swelling and death.
An unexpected feast Pringle was roughly three years into a study about the effects of elephants on plant diversity when he noticed that the Sodom apple was conspicuously absent from some experiment sites. He and other researchers had set up 36 exclosures — which are designed to keep animals out rather than in — totaling nearly 89 acres (36 hectares) at the Mpala Research Centre in Kenya, a multi-institutional research preserve with which Princeton has been long involved.
There were four types of exclosure: one type open to all animals; another where only elephants were excluded; one in which elephants and impalas were excluded; and another off limits to all animals.
It was in the sites that excluded elephants and impala that the Sodom apple particularly flourished, Pringle said, which defied everything he knew about the plant.
“I had always thought that these fruits were horrible and toxic, but when I saw them in the experiment, I knew some animal was otherwise eating them. I just didn’t know which one,” Pringle said. “The question became, ‘Who’s eating the apple?”
Using the exclosures, Pringle and his coauthors documented the zest with which wild African browsers will eat S. campylacanthum. Pringle worked with Corina Tarnita, a Princeton mathematical biologist and assistant professor of ecology and evolutionary biology, as well as with collaborators from the University of Wyoming, University of Florida, University of California-Davis, Mpala Center and University of British Columbia.
The researchers specifically observed the foraging activity of elephants, impalas, smalldog- sized antelopes known as dik-diks, and rodents. Using cameras, they captured about 30,000 hours of foraging, and discovered that elephants and impalas were the primary eaters of the plants.
There is a catch to the elephants’ and impalas’ appetite for the Sodom apple: When fruit goes in one end, seeds come out the other. Though some seeds are destroyed during digestion, most reemerge and are potentially able to germinate.
Pringle and Tarnita developed a mathematical model to conduct a sort of cost-benefit analysis of how the Sodom apple’s ability to proliferate is affected by being eaten. The model weighed the “cost” to the plant of being partially consumed against the potential benefit of having healthy seeds scattered across the countryside in an animal’s droppings.
While elephants ate an enormous amount of Solanum seeds, they also often destroyed the entire plant, ripping it out of the ground and stuffing the whole bush into their mouths. The model showed that to offset the damage an elephant wreaks on a plant, 80 percent of the seeds the animal eats would have to emerge from it unscathed. On top of that, each seed would have to be 10-times more likely to take root than one that simply fell to the ground from its parent.
Impalas, on the other hand, can have a positive overall effect on the plants, the researchers found. Impalas ate the majority of the fruit consumed — one impala ate 18 fruit in just a few minutes. But they do not severely damage the parent plant while feeding and also spread a lot of seeds in their dung. Of the seeds eaten by an impala, only 60 percent would need to survive, and those seeds would have to be a mere three-times more likely to sprout than a seed that simply fell from its parent.
“A model allows you to explore a space you’re not fully able to reach experimentally,” said Tarnita, who uses math to understand the outcome of interactions between organisms. “This model helped us conclude that although it is theoretically possible for elephants to benefit the plant, that outcome is extremely unlikely.”
The study was published in the June 22, 2014, edition of the Proceedings of the Royal Society B. The work was supported by the National Science Foundation, the National Sciences and Engineering Research Council of Canada, the Sherwood Family Foundation, and the National Geographic Committee for Research and Exploration.
–By Morgan Kelly
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