14 January 2013

Botswana: An Urgency to Understand the Okavango Delta

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As a native of Botswana, Gaolathe Tsheboeng is familiar with his research subject - plants in watery places.

He was born in the village of Moletemane, near the "great, gray-green, greasy Limpopo River," as Rudyard Kipling once called it. As a RISE student, he has spent the past two years working on another watery region of Botswana, the Okavango Delta, which covers much of the northern corner of the country.

He has studied the response of the huge expanses of grasses and sedges to varying water levels in the Delta. These plants provide most of the food for both the wildlife of the region and the cattle of native herders, whose ancestors have inhabited the Okavango since the end of the Stone Age several millennia ago.

The annual "pulse" of floodwater, rushing southward out of the highlands of Angola, has gradually created this hot, flat environment (its elevation varies only about six feet over some 100 miles), which today is rich in organic matter and teeming with life.

The yearly floods nourish a unique network of productive ecosystems: languid lagoons, reed- and papyrus-filled swamps, spongy grasslands, and tree communities of the riverbanks and elevated woodlands. Although any census of life forms in this vast wetland can only be approximate, the Okavango Research Institute (ORI), near Maun, estimates that the Delta supports some 1,300 species of plants, 70 species of fishes, 30 species of amphibians, 65 species of reptiles, 445 species of birds and 120 species of mammals.

Among the mammals are the "Big 5" - lion, elephant, buffalo, leopard, and rhinoceros - which, along with spectacular birdlife, attract tourists world-wide.

The source of the Delta's fertility is the floods themselves. Each time they come, they release the nutrients of decayed vegetation from the previous year, more than tripling the concentration available to growing plants.

A steady, year-round flow of water would not provide this service, confining Delta flow to permanent channels and swamps that are low in dissolved organic carbon and support little biodiversity. "Seasonal flood plains become productive grazing areas only after the rains," says Michael Murray-Hudson, a research scholar at ORI who supervises Gaolathe's work.

"The dissolved nutrients fuel microbial populations and biological productivity at every level, up to the herbivores and predators."

In addition, the floodwaters support many thousands of humans, including hunters, fishers, harvesters, and farmers.

The farmers are keenly attuned to the timing of the floods, planting their crops at the very end of the flood season so as to reap the nutrient bounty of the retreating waters. This technique, called molapo (flood recession) farming, has also been practiced for thousands of years along the Nile.

Yet the pulsing of the floodwaters is neither regular nor reliable, and its reach can vary by hundreds of miles. During the wet years, the Thamalakane River may fill to the banks, spill over into the dusty town of Maun, and push far to the southeast where it covers the vast salt pans and attracts huge flocks of flamingoes and other water birds. During dry years, stream flow in the rivers and smaller channels may practically cease, threatening cattle and wildlife alike.

Like most other water systems, the behavior of the Okavango is expected to vary more widely with climate change. For this reason, many scientists at the ORI, including Gaolathe, are learning as much about the hydrology of the region as they can before such changes are obvious. Gaolathe began his studies on grasses and sedges two years ago, and has completed the requirements for his MSc degree at ORI, part of the University of Botswana.

He has now begun his PhD studies, shifting his emphasis to woody plants, the other dominant kind of plant community in the Delta. He hopes to learn how the floodplain tree communities and the nutrients that support them change in response to flood variation.

As he did for the grasses and sedges, he will examine the response to recent extreme conditions, as well as setting up his own test plots. For nearly a decade after the turn of the millennium, the floods were weak throughout the Delta and water levels sank. Suddenly, in 2009, the picture changed abruptly as heavy floods arrived.

By looking through the data gathered over the past decade, Gaolathe has found that these abundant pulses encourage the spread of water-loving grasses and sedges, which in turn supports larger numbers of grazing animals. These plant species (and their nutrients) are most abundant near waterways, expanding outward during years of heavy floods.

If the flooding is too intense, however, the water-loving papyrus becomes dominant along the waterways, crowding out the smaller grasses and sedges. During dry years, the papyrus and then the grasses and sedges retreat, reducing the available stand of food for grazers. Trees and bushes quickly encroach on the wetlands, reducing the grazing areas and offering fewer nutrients.

On the basis of his early studies, Gaolathe proposes that the Delta is best suited to a flood pattern that alternates between wet and dry. If either is too prolonged or severe, plants and animals do less well. Without full flooding, the edible grasses would not expand, and woody species would move in to take their place. Without some drying, the inedible papyrus would overtop the grasses.

This question has gained urgency in recent years as the countries bordering the Okavango region - namely Botswana, Namibia, Angola, Zambia, and Zimbabwe - jockey for the right to Okavango water.

A major source of debate is the Okavango River itself, which flows out of Angola, where it is known as the Cubango. It also crosses a sliver of land known as the Caprivi Strip, which protrudes about 280 miles from Namibia to the Zambezi River. Caprivi was created for the convenience of a land swap in 1890 between England and Germany, in which Germany gave up its claim to Zanzibar for Caprivi.

It did this to gain access to the Indian Ocean via the Zambezi River, which turned out to be impossible. Today, Namibia lays claim to the strip (as does Botswana), and its water.

A continual worry for ORI scientists is the potential interruption of the Okavango's flow through dams or removal, which would likely alter the life-giving pulses of the flood waters and jeopardize the Delta ecosystem. A body known as OKACOM, the Okavango River Basin Water Commission, is trying to provide information and mediate scientific discussions among Botswana, Namibia, and Angola, and the ORI scientists are working hard to spread more understanding of the Okavango system.

For Gaolathe, the next job is to create a vegetation map for the whole Delta and a classification and tracking system for its tree species. He hopes to learn how these and other plants replace themselves after they die, and understand the environmental forces that determine their distribution.

He has begun to set out study plots, running from the river outward, for the entire Delta. He has the good fortune to be supported by the Future Okavango Project, a five-year program sponsored by the German government, along with another student from Botswana supervised by Dr Mike Murray-Hudson. He is not quite looking over his shoulder as he works, but he understands well the urgency of a research question that is both complex and politically charged.

Read a previous blog post about Gaolathe Tsheboeng and his research here (2010).

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