Words and photographs by Ida K. Rigby, for Let’s Talk Plants! August 2024.
The Okavango Delta - Drought, Wildfire and Climate Change
Before turning to the problems that climate change is bringing to the Delta, we’ll discuss two more trees found there. In late winter, the knob-thorn tree, Senegal nigrescens, formerly Acacia nigrescens, is covered with sweet smelling, creamy white flowers.
Its trunk is covered with knobs out of which claw-like, downward turning thorns emerge. The thorns are most obvious on the younger trees, as protection from browsing animals. The name comes from the Greek “akakia” for spike or barb and the Latin “acacia” for thorny. Nigrescense, meaning becoming black, refers to the pods, which blacken as they ripen. Senegalia refers to Senegal. The knob-thorn tree can grow to fifty feet and prefers areas near water and flood plains.
Its wood is hard and termite resistant, so it is used for fences, mine shaft supports and fighting clubs. It makes excellent, very hot burning charcoal. Villagers erect knob-thorn tree wood poles next to their homes as lightning rods. More luxury uses are for parquet flooring and railroad sleepers. Because the bark has a high tannin content it is good for tanning leather. The bark also makes good twine. Powdered knobs are used to cure eye infections. Giraffes, monkeys and baboons love the flowers. Elephants, giraffes, kudu and impala eat the leaves and pods. Elephants feed on the mature bark, which fights tooth decay and heals abbesses. Browsing giraffes prune the trees into their characteristic rounded shapes. It is believed that giraffes aid in pollination as they spread the pollen from tree to tree (sabisabi.com).
The sycomore (or sycamore) fig, Ficus sycomorus, …
… is a white barked tree that grows along river banks with its feet in the water, next water holes, in areas with a high water table, swamps, open woodland, termite mounds and savanna, (treesa.org). Its white bark and large green leaves set it off from other trees on the termite islands of the Delta. Its roots form buttresses at the base of the tree. Its name comes from the Greek “syka” for fig and “mores” for mulberry.
The sycomore fig is fertilized by a female wasp. Because flowers are inside the “fig,” females must access the interior through tiny holes made by resident male wasps. She lays an egg in each flower. The flower produces a gall, which then nourishes the larvae. The male wasps hatch first, then fertilize the females. The males die and the pregnant females exit through the holes made by the males and carry pollen and fertilized eggs to another fig. The cycle begins again. The trees produce at least two crops a year and are productive for up to 100 years. Birds, baboons, monkeys and bats wait to eat the figs until they ripen to a red color and then spread the seeds. Birds, such as African darters, nest in them.
Crocodiles wait at the base of the trees for young birds to fall out of the nests and into the water. Squirrels, mice, snakes and other reptiles live in the tree’s hollows. Because it houses so many residents, it’s called the Mother Tree. In open areas, bush pigs and warthogs gather the fallen figs. Due to each kind of rhino’s different lip structure, black rhinos eat the leaves and white rhinos eat the bark and leaves. Along rivers fish eat the fallen leaves and fruit.
Humans eat the nutritious leaves and consume the figs stewed, raw or dried. They make fishing fences out of the branches. Indigenous people use the non-toxic latex sap and bark extracts for chest illnesses and diarrhea and apply the bark powder to burns. The bark and latex are also used to treat ringworm. Stools, beehives, drums, canoes and carvings are made out of the soft wood.
In addition to the threats posed by climate change and geological upheaval, is the introduction of the water hyacinth, Eichbornia crassipes. We did not visit any of the infested areas in the Delta, so these photos are from a river in Zambia in 2015.
Jonathan Drori discusses the water hyacinth in his book Around the World in 80 Plants (p. 82-3) in the section on South America because it’s native to the Amazon basin. Drori reports that it was exported worldwide as an ornamental and soon escaped to become a nightmare weed. Since it has no natural predators outside the Amazon and quickly regrows from small fragments, it soon clogs rivers and lakes.
“It blankets paddy fields, starves lakes of oxygen and other life, harbors mosquitoes and conceals dangerous hippopotamuses and crocodiles. Many of Africa’s lakes have been badly affected for decades. In 2019 . . . Kenya’s Lake Victoria’s coastline was covered in water hyacinth . . . a blanket so thick that boats became marooned in a vast green sea.” According to Drori, the most promising treatment has been the introduction of Brazilian Neochetina weevils, which evolved alongside the water hyacinth. It takes a few years, however, for them to get established. In the meantime, tons of water hyacinths are being dredged from infested waters. Some of the debris can be woven into baskets and large amounts are fermented into biogas that powers cooking stoves, thus relieving the pressure on firewood." (Drori, 82.)
Climate change is severely affecting the area through drought and fire. Historically the rains in the Angolan highlands, which feed the Okavango River, have always varied, but now there is sustained drought. To compound the problem of decreased water supply, in 2017 a 6.2 magnitude earthquake in Botswana tilted the land northward so the Okavango River does not spread into a delta as far south as previously. The tilting of the land and the sustained drought has created permanent grassy plains where once there were wetlands and islands.
I visited the Delta each August from 2014 through 2019; even over the course of that short time the effects of global warming became starkly visible. During the first trips we often forded rivers.
By 2019 we no longer did. Granted, the photos could indicate merely annual variations in the amount of water in the Delta, but in fact guides noted that there was now a trend, not just annual fluctuations. The “island” on which the lodge on my first trip to the Delta was situated …
…was barely an island by 2019. On earlier trips we had passed the lodge where I stayed in 2019 in motorboats leaving a large wake. Where there had been eight feet of water surrounding it there were only inches of water, lots of mosquitoes and a dry flood plain.
We had always approached our camps in the heart of the Delta via motorboat. Each year the boats became smaller with flatter and flatter bottoms; motorboats were replaced by boats that had to be poled until in 2019 we arrived overland by 4x4 vehicles. Our hostess reported that she had encountered a lion in the main area of the lodge a few weeks before as she started her day at 4:00 a.m. Lions can swim from island to island but are not usually so inclined. She thought to herself, “there are no lions on this island,” forgetting that this year it was not an island.
In 2019 the water level was so low that we used flat bottomed boats that had no reverse gear. We therefore did not visit the hippo ponds from which we might have to make a quick exit if a dominant bull decided we had invaded his territory. In past years, the flood plains became grassy open spaces as the waters receded and sank into the deep Kalahari sands. In 2014 we only could see the red lechwe from a great distance, if at all. Each year we could approach them more closely until they were next to the sandy roads.
By 2019 these areas have become permanent grasslands grazed by bachelor herds of red lechwe.
African mole rats moved into the new dry plains.
Early one morning in 2019 we followed a bloody-faced lioness as she returned to her pride. She crossed what had become permanently dry areas with vestigial islands until she reunited with her and her sister’s offspring who ran through the flood plains turned permanent grasslands to greet her.
Many areas that we had experienced in prior years as wetlands were at best muddy fields filled with dry grasses.
In 2019 familiar bridges no longer spanned rivers.
The new permanent grasslands are a boon for farsighted predators like the cheetah mom and her daughter, who just missed catching a warthog while we watched.
The former islands with their palms and sages are now perfect hiding places for lions.
We also had previously rare sightings such as hippos walking on dry land during the day.
One of the most devastating effects of climate change is of course wildfire. The bush has historically been fire prone, and villagers burn their fields in late winter to revitalize the soil, but the current situation is different. Wildfires that had been infrequent occurrences when we almost had to evacuate camp in 2016 …
…created blazing sunsets almost daily. In 2019 there was always smoke on the horizon.
We flew through some choking clouds of smoke on our bush flights.
In 2019 southern Africa also experienced unprecedented levels of migration by thirsty wildlife. On one drive in the drought-stricken Delta we saw a miles long, single file herd of thousands of African buffalo creating a dust cloud that looked like smoke as they searched for water.
We were used to seeing breeding herds of elephants enjoying green palm leaves.
In 2019 we came on an area where there had been a prescribed burn where a herd that had migrated from a great distance in search of fresh food was feeding on all they could find, burned palms.
Later we came upon a pride of lionesses stalking prey along a burned out plain.
The bleakest and most devastated area was where a pack of African wild dogs we were watching began chasing an impala who had imprudently darted out from its hiding place on a burned-out palm island.
In what would seem to the uninformed eye like a random chase is actually a very carefully choreographed plan led by the alpha female. Their strategy is based on a diamond pattern and relays; the pack did what it always does, outlasted the antelope. The puppy brigade arrived and joined in.
The pups were old enough to join the hunt, so the dogs that cared for them brought them from the den to learn pack tactics. The chase ended next to a long bridge that usually crossed a wide river. We parked on the bridge; below us was what should have been the river, but it was only a burned out, ash-filled plain.
This is a grim concluding image of the Okavango Delta, but it only mirrors what we read about happening worldwide as changes in climate patterns accelerate.
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