The fruits that dangle from Solanum campylacanthum, an indigenous East African shrub, taste awful—like unripened tomato, stinging the tongue with acridity. Locals give them an aptly uninviting name—apples of Sodom—because they are poisonous to people and cattle. S. campylacanthum grows quickly, lately overtaking Kenya’s pasturelands. Getting rid of it is expensive, inefficient work.
Luckily, native herbivores eat the shrubs. Though few herders might welcome the thought of elephants, impalas, or dik-diks (adorable though they are) trampling across fields towards their sheep, these wildlife effectively control S. campylacanthum’s spread, as Robert Pringle of Princeton University and his colleagues report in a new paper in the Proceedings of the Royal Society B.
The toxic chemical in S. campylacanthum is benign to these creatures. Pernicious compounds are common in bushes and shrubs, so “browsers” of branched plants have evolved to withstand eating the things “grazers” of grasses (which usually don’t contain such chemicals) cannot. What would spell famine for the domesticated ungulate brings a feast for many an untamed beast, as captured by camera footage from the researchers’ study:
To understand the effects different species had on S. campylacanthum growth, the scientists fenced off separate areas of their study site in the Mpala Research Centre in central Kenya. Different types of barriers excluded particular animals. For five years, they recorded the S. campylacanthum consumption patterns in each one. They used field measurements and a model to demonstrate each animal’s influence on plant proliferation.
Elephants often uprooted whole plants and contained S. campylacanthum growth best. Impalas less so: Though a doe might devour as many as 18 fruits within minutes, the undamaged seeds they spread though their droppings could plausibly offset what was eaten. (It was much less likely that enough seeds digested by elephants could germinate to replace the plants they had destroyed.) Dik-diks curbed S. campylacanthum with constant nibbling. Rodents and insects pitch in with some consumption, but not to any effectual degree.
The native fauna limiting S. campylacanthum are a mark of the stabilizing mechanisms inherent in a mixed ecosystem. Diversity is key: Each plant-eating critter played a slightly different part in managing the noxious weed.
But this discovery came about because the balance of that ecosystem is threatened. Overgrazing in Kenya’s fields has encouraged S. campylacanthum’s encroachment. Changing climate and increased farming activities have diminished access to well-watered lands for the country’s seven million pastoralists, leaving their livestock (numbering in the tens of millions, though precise counts are hard to come by) with smaller patches to roam. Pastures are chomped through with greater intensity, easing the way for invasive, inedible shrubs to take their place. “You end up with pastures choked with plants that are unusable,” Pringle told me, where sheep and cattle can no longer feed.
Even more troubling is the destruction of the wildlife that could check this imbalance, especially the killing of elephants. The price of ivory has soared in recent years, buoyed by demand from Asia (a pound of it goes for $1,500 today). Of the half million elephants left in Africa, some 35,000 are estimated to be slaughtered for tusks in a year. Kenya enacted a strict new anti-poaching measure in January, with heightened punishment for law-breakers, but that will do little to stem foreign appetites. A recent census shows that the country’s elephant population has declined over the past three years, from 12,573 to around 11,000—the outcome of continued poaching, habitat loss, and drought. Projections from the Center for International Forestry Research of the continent’s elephant population for the next 25 years are grim.
“We know that elephants can change landscapes and ecosystems,” says Truman Young, an ecologist at the University of California-Davis. He notes Sodom apple is not the most destructive invasive flora in East Africa: Non-native species such as prickly pear and lantana—a poisonous intruder from Central America—plague the region, too. “If there were more elephants, it would certainly help,” says Truman, for they are known to consume these exotic plants. The elephants’ decline leaves East Africa’s habitats all the more vulnerable.
There is some good news. In the past few years, local people have affirmed the mutual benefit between themselves and elephants that Pringle and his collaborators observe, and they are trying radical new protection efforts. Networks of community conservancies have sprung up, mostly with support from pastoralists. In 2012, a national group was formed. It has been a “revolution, unlike anything I’ve seen before,” as Robin Reid, a conservationist and author of Savannas of Our Birth, puts it. Even as elephants continue to be illegally bagged in Kenya (more than 200 fell victim to poaching last year), conservancies have reported increasing elephant populations.
Moreover, such preservation efforts can aid economic stability—a 2010 study of three community conservancies found that 84 percent of pastoral households living within a conservancy reported either stable or increased incomes, compared to 44 percent of those in non-conserved areas. Part of the improvement is due to better grazing security—which healthy stocks of wildlife help boost, as the new research on S. campylacanthum shows.
Conservation is more successful when locals benefit directly from these endeavors. Elephants and other wildlife are a boon for controlling the explosive expansion of an invasive shrub threatening the livelihoods of many Kenyans and herders throughout East Africa. Understanding what other roles are filled by the continent’s many species is crucial. So is protecting them.
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