Natural History, May 2017
I was weaned on the concept of an ecological balance in nature, built on an intricate web of relationships among plants, herbivores, predators, and other life forms, where the fate of one can have a domino effect on the others.
Last fall, I was near the equator in the Moremi Game Reserve on Botswana’s Okavango Delta, one of the most pristine wildernesses on earth, containing perhaps a representative set of Pleistocene fauna. Maintenance of this vast wilderness is a national mission: no hunting, logging, herding, or industrial agriculture are allowed. I arrived at the beginning of the rainy season, and some of the trees were starting to break bud. However, I found myself in what looked like a tree-killing zone. Large trees had been toppled—some freshly felled, others moldering back into the ground. If I had witnessed this scene at home, I would have guessed the cause was a weather event—wind shear, hurricane, ice storm—or perhaps clear-cut logging. In this virgin wilderness there was something else surprising—something violating another of my cherished ecological concepts. Throughout the devastated landscape were large patches of low trees of nearly a single species: the mopane tree, Colophospermum mopane. Likewise, many, if not most, of the large destroyed trees were also mopane.
Tree species diversity is notoriously high in the tropics and declines steadily moving away from the equator (for a number of possible reasons). This diversity maintains conditions that allow multiple animal species to survive, as one species depends for its survival on the others. Here, in fauna-rich Botswana, there should be many species of trees. Some areas had a density of trees, yet these mopane woodlands cover vast expanses of northern parts of the Republic of South Africa, Botswana, Malawi, Zambia, Zimbabwe, Namibia, Angola, and the Republic of the Congo. Why do these areas have such low tree-diversity forests when Africa, especially southern Africa, has extraordinarily high plant and animal species diversity? Why were there so few tree species in Botswana’s Okavango Delta, a seasonally hot area with plenty of water readily available close to the ground surface?
Variations in the forms of the mopane tree may be a clue. Mopanes were growing in patches of three main forms. The trees were huge in semi-open forest, along with other tall trees such as acacias, fever trees, rain trees, and leadwood trees. There, they reached roughly thirty meters in height. In other patches, they were only two to three meters tall and spread uniformly in all directions like huge unkempt grape arbors. In their third form, they were near ten to fifteen meters tall, and also spread evenly and widely. Such varied forms and distribution are not indicative of a species that fits into a well-established ever-constant niche created by or adapted to others, but rather of one responding possibly in a variety of ways to episodic predation pressures or episodic weather events.
A large species of emperor moth, Gonimbrasia belina, lays its egg clusters primarily on mopane leaves, which are the prime diet of the larvae in all their stages of development. The trees, however, easily survive damage from “mopane worms,” as they are commonly known. Something large and more destructive was laying waste to the trees. Elephants and long-term drought were the obvious suspects.
During my visit, elephants were beginning to return to the bush, feeding on freshly leafed-out mopane. They had been concentrated for months during the dry season near the waters of the Khwai River. We saw forty-nine elephants on one day’s travel. During the height of the wet season, we could expect to see six to eight hundred per day, according to our guide, depending, no doubt, on geographic cycles of drought.
As it turns out, elephants, as well as mopane worms, goats, cattle, kudu, and impala, all relish mopane leaves because of their high nitrogen content—typical also to other members of the pea family—and, possibly, because of the water content. Elephants, however, can reach into the tops of young trees and browse the lower branches of even large trees. Their browsing, depending on when it occurs, can sculpt the trees’ form. In the areas where mopane trees were uniformly two to three meters tall, each tree had become multi-stemmed, as it regrew in several directions from the bottom, presumably after the tops and branches had been stripped off. In patches where the trees were tall, they had no branches within six meters of the ground, because those branches had been within reach of the elephants. When these trees were young, they had escaped elephant browsing, most likely because elephants were then absent. Once these trees had achieved sufficient height, despite elephants clearing lower branches, they had concentrated their growth at their tops. Some became formed like massive umbrellas. Intermediate-sized trees, whose upper branches were out of reach, had been knocked down by elephants. Weighing three to six tons each, elephants found few trees to be a challenge. The largest trees remained standing a much longer time, and were left as the elephants’ last pick, some eventually being debarked up to several meters by tusks, rather than getting pushed over. Debarked trees eventually weaken, die, and remain standing as dry skeletons. But mopane trees have a unique physiology that adds another dimension to this scenario.
Elephants, or a prolonged drought, could potentially eliminate all trees in an area. Afterward, there would have been regrowth for the mopane as formerly decimated areas were recolonized. When elephants returned, they could have favored certain species and eventually eliminated them locally. As we approached the town of Maun—the gateway to the Okavango Delta—almost all the small acacia trees were heavily browsed, and many had been nearly destroyed. But in the town, where there are few or no elephants, large acacia trees were still intact. If people were to be suddenly gone, elephants would move in, attracted by the acacia, their favorite browse, and they would remove the trees’ lower branches. The trees’ growth would then shift to the tops, and an open woodland of tall umbrella-like trees would result. Many would then get knocked down, and the forest would be destroyed. Another would replace it, provided the tree browsers were absent for a sufficient time. The point in time when the browsers returned would dictate the shape of the trees.
The mopane has also been under the strong selective pressure of elephant grazing. The two have likely had a history of millions of years of coevolution. However, this tree’s apparent weakness of being fodder for elephants may have turned into an advantage, much as savanna grasses have effectively evolved a dependency on browsers such as wildebeests. Most tree species that are knocked down soon die, but young intermediate-sized mopane trees that are pushed over by elephants, and browsed upon, still retain some root in the ground. These trees, unlike most others, not only can revive, but also can put down many more roots along the whole length of their fallen trunks. New branches sprout from the still-living tree trunk, and they grow upward to become new vertical trunks, and hence several trees sprout from what had been one. Similarly, where elephants had browsed younger mopane trees, these trees had generated new branches. Seed dispersal occurs from eating the fruit, and thus elephants—by their bruising treatment of this tree—help propagate it. In the first week of December, when I was visiting the area, I tracked new twigs that grew a foot long. Trees still had six to seven more months of growing time. With such growth, it is no wonder that these trees survive, and thrive. The trees’ varied forms in relatively uniform patches attest to periodic absences of elephants in the past, which allowed for sustained growth to reach different stages before massive browsing recurred.
Mopane trees are not the only beneficiaries of cohabitating with elephants. When elephants clear ground by knocking over or killing many trees but leaving the largest specimens, they create a parklike, albeit untidy, environment. Sunlight reaches the ground and grasses sprout. Hippos, traveling in and out of nearby rivers to browse on the grass, create channels for spreading water. At the beginning of the wet season, I saw shoots of grass, herbs, and tree seedlings emerging from the ground. Impala were there in herds, with their fresh and frisky fawns. Wild dogs and leopards were sure to follow. All of the grazers of the grassy plain and the biota in the bush and in woodlands may owe their debt of existence to elephants. I had not appreciated the factor of time and ecological succession until seeing this. Elephants, because they are large and powerful and because they travel huge distances, blur the ecological boundaries of time and place, which define much of the African landscape, in the long run.