Okavango: A Field Guide

By Lee Gutteridge and Tony Reumerman

After The South African Bushveld—A Field Guide from the Waterberg, this is the second comprehensive field guide to be published in southern Africa that covers a detailed cross-section of the most prominent animals, plants, birds, fish, insects and tracks and signs of a particular region. It includes a detailed natural history section for a greater understanding of the geology, habitats and ecology of the region. This book negates the need to carry more than one guide book when visiting the Okavango. Contents include: Geology, Habitats, Ecology, Mammals (& diseases), Birds, Reptiles, Amphibians, Scorpions, Spiders, Insects, Butterflies & Moths, Fish, Flowers, Trees, Grasses, Tracks & Signs, Fungi, Aquatic Plants and more.

Lee Gutteridge was born in Sheffield, England, in 1973. His family moved to South Africa in 1982. Since his first encounters with snakes in the mid-eighties he has been passionate about African wildlife. He is now a professional field guide with 19 years’ of bush experience. He is the principal trainer of the Entabeni Nature Guide Training School. Passionate about the learning and sharing of knowledge, his first book, the best-selling South African Bushveld—A Field Guide from the Waterberg, was published in 2008. He has also co-authored San Rock Art: A Field Guide (2011). He lives in the eastern Waterberg with his wife Sarah and his two children, Kellen and Savannah. Tony Reumerman was born and raised on the Highveld of South Africa and from an early age developed an interest in bird, insect and plant life. He was schooled at St. John Bosco College in Daleside where his interest in natural history unfolded, becoming an obsession during his years as a soldier and later as a microbiology student. Excursions into wildlife areas in southern Africa became so regular that he decided to pursue his passion and work as a field guide. He was to spend eight years guiding, managing and training other guides at Sabi Sabi Game Reserve in the Kruger region before moving, in 2000, to the Okavango where he joined Wilderness Safaris. He heads up the training team and has an avid interest in mammal behaviour, photography, botany and ornithology. Tony is based in Maun where he lives with his wife Andrea and son Aidan.

• Language: English
• Publisher: 30 Degrees South Publishers
• Release date: Apr 1, 2011
• ISBN: 9781928211167

Lee Gutteridge

Lee Gutteridge was born in Sheffield, England in 1973. Passionate about African wildlife, he is now a professional field guide with 18 years’ bush experience. He is currently the chairman of the Waterberg region of the Field Guides’ Association of Southern Africa (FGASA) and also the principal trainer of the Entabeni Nature Guide Training School. He books include the best-selling The Bushveld: A South African Field Guide, including the Kruger Lowveld (2008, 2nd edition 2012), Okavango Field Guide (co-authored, 2011) and Bushman Rock Art: An Interpretive Guide (co-authored, 2012).

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Introduction to the Okavango Delta

by Map Ives

Over the years, I have been asked many times about the Okavango Delta and ‘how it works’ by many people and different groups, and it has always struck me just how complex this supposedly simple question is. I say complex because, as with everything else on earth, one cannot possibly understand the workings of the modern Okavango Delta without uncovering the myriad layers of time that have passed since the formation of our earth and the ongoing sculpting of the continent of Africa. It also cannot be understood without incorporating the influences of climate: weather; the nature of the soil underpinning the Okavango; the position of the Okavango on the African continent; and the influences of fire, water and biological activity—all of which contribute to the Okavango’s uniqueness.

It is simplest to divide a description of the Okavango Delta into two sections: the first being an outline of the geomorphology, or the formation of the geological conditions that influenced and caused the Okavango Delta to be what it is today; the second, a description of the structure, hydrology and forces that drive the delta in its modern form.

Of ancient lands and wandering rivers

As with everything else on earth, the continent of Africa has changed much over the last 200 million years, or since it stood alone as a continent separated from the supercontinent Gondwana (or Gondwanaland)—for it was the break-up of this supercontinent that ultimately led to the conditions for the formation of the Okavango Delta. I shall start the story of the formation of the Okavango at this time, although there are basement rocks and sediments situated hundreds of metres below the Okavango that have their origins from a time before the break-up.

The supercontinent Gondwana existed between 600 and 200 million years ago and consisted of a combination of the continents of South America, Africa, Australia and Antarctica as well as the subcontinents of India and the remnant islands of New Guinea and, more recently, Madagascar. The break-up and dispersal of the continents had a profound effect on the current surface topography of the continents; the distribution of oceans; ocean and atmospheric circulation; global climate; and the biodiversity of our planet, including Africa and the Okavango Delta. Tectonic-plate movement and crustal adjustments of the earth have pulled continents apart and driven them together many times in the past and continue to do so now. It was this broad tectonic activity that created the conditions leading to the deposition of the Kalahari sands, which are so important to the functioning of the Okavango Delta today.

Between 180 and 80 million years ago, as South America and Antarctica separated from Africa as we know it today, the relatively catastrophic rifting and tearing had a dramatic effect on the perimeter of the continent that they left behind. The buckling and bending associated with continental rift has the effect of creating down-warping and associated upliftment along the fringes of both continents. A sort of ripple effect—not unlike the release felt when an elastic band is stretched and finally broken—leaves a down-warped zone along the coastal areas, along with an uplifted or highland belt a short distance inland from the coast and finally an associated inland down-warping. The illustration facing below is a rough depiction of the African continent cut across the 18° south latitude, which shows from left to right: a) the down-warping of the Atlantic Ocean belt; b) the upliftment of the western Namaqua highlands; c) the inland down-warping that has filled with Kalahari sediments; d) the Drakensberg escarpment; e) the KwaZulu-Natal coastal belt; and f) the Indian Ocean. It also shows the initial drainage from the highlands, from both endoreic (inflowing) and exoreic (outflowing) rivers, and the resultant sediments in the two oceans and the inland Kalahari basin.

Approximate configuration of the modern continents as they probably fitted together before the separation of Gondwanaland.

A cross-section of southern Africa, showing the features and sedimentation after the break-up of Gondwanaland.

The diagram below illustrates the initial situation where many of southern Africa’s great rivers flowed inland, depositing sediments into the down-warped centre of Africa, and their outflowing (exoreic) counterparts, which deposited sediments into offshore basins.

The southern African subcontinent, showing the uplifted escarpments around the fringe; the short, aggressive, outflowing rivers; and the inland-flowing rivers that deposited the first Kalahari sediments.

Between 80 and 5 million years ago, a myriad of slow-flowing rivers flowed from these highlands into the interior of Africa, while on the coastal side the rivers flowed with speed and aggression owing to the greater angle of flow towards the oceans. Over time, many of these coastal-flowing rivers carved their way through the escarpments and cut back into the interior of Africa. Many of these rivers succeeded in capturing rivers that flowed into the interior, increasing their flows to the sea. The diagram illustrates some of the larger and more prominent of these.

The outcome was that much of the raised Kalahari basin was filled by sediments deposited by rivers over a period of 75 million years. Of course, over 75 million years, as the continent of Africa drifted across the southern hemisphere of our globe, there would have been extensive periods of very wet conditions as well as dry episodes, probably associated with ice ages. During the wet periods, the interior basin would have been inundated with extensive water and the ‘first Kalahari sediments’ would have been laid down under lake conditions. During the dry periods in between, these sediments would have been reworked by strong winds and formed into the relatively featureless, flat environment that we see today. Of course, with the capture of many of the inflowing rivers by their outflowing counterparts, the interior became much drier, a phenomenon also influenced by ocean temperatures. The colder currents in the Atlantic Ocean produce less precipitation in the interior so the western sections of the southern African subcontinent are also drier than those to the east, which are influenced by the warmer Indian Ocean.

Between 5 and 2 million years ago a series of events occurred that had a profound effect on the entire African continent and effectively established the conditions for the formation of the modern Okavango. These events were all related to continued tectonic activity and caused the continued rifting of the African continent from the Gulf of Aden, through East Africa and out into the Indian Ocean off the Mozambican coast. This is the well-known Great East African Rift Valley, which is illustrated in basic terms overleaf. A lesser-known feature of this rift is that it has an ‘arm’ that runs south-west from northern Malawi, down the length of the Luangwa Valley in Zambia, along the Zambezi Valley and under the sediments mentioned earlier.

A simple map of Africa showing the approximate alignments of the East African Rift Valley system outlined in red and the twin graben faults, into which the Okavango flows, outlined in black with red filler.

Sedimentary and ichthyological evidence has shown that many of the central African rivers draining from Angola and Zambia today had originally flowed across the Kalahari as one single river, and on to the Indian Ocean via the Limpopo River valley. As part of the tectonic evolution of that region and the beginnings of the Okavango rifting, the earth’s crust began to lift gently between modern-day Harare in Zimbabwe and Gaborone in Botswana in what geologists call the Zimbabwe–Kalahari axis of uplift. At the same time, the earth to the north-west of this uplift began to slowly subside, effectively cutting across the drainage path of this great river. The result was that an inland lake began to form which was to exist at various levels and size for the next 3 million years. At times, this lake may have been completely dry, while at others the extent of the waters would have extended from central Botswana to the Caprivi area of Namibia—an area of 120,000km². This lake is commonly referred to as Lake Makgadikgadi or the Kalahari Superlake.

About 1.8 million years ago the rifting intensified and the inland-flowing Zambezi and its associated tributaries were captured by an east-flowing river to form the course of the modern Zambezi, thereby diverting the largest inflow from the inland lake, which would have begun to rapidly shrink and become very saline. Continued rifting and subsidence in a south-westerly direction has led to the capture of the Kwando River and the down-warping of the Okavango basin between twin faults beneath the Kalahari sand deposits. The inland lake would by now have virtually no inflow and by 100,000 years ago would have been a largely dry lakebed filled with very salty deposits. The lowest-lying parts of this superlake are what are now known as the Makgadikgadi Pans, a vast network of salt pans to the south-east of the Okavango Delta.

The two diagrams that follow are a rough depiction of the changes in drainage patterns for the central African rivers associated with this geological history. In the first diagram the drainage of the central African rivers, including the Cubango and the Cuito (which today flow into the Okavango); the Kwando; the upper Zambezi; the Kafue; the Luangwa; and the Deka is illustrated. These rivers flowed across the Kalahari sands into a single river we now call the Motloutse and on into the Limpopo and thence to the Indian Ocean. The size of this river can only be appreciated if one travels to the lower Limpopo River valley, where the present Limpopo is about a kilometre wide, but on inspection, the valley is about 28km wide in places.

The second diagram (overleaf) shows the post-lifting of the Zimbabwe–Kalahari axis, which was followed by the rifting of the twin graben faults of the Okavango and the rifting of the Linyanti. It also shows the approximate position of the Kalahari superlake and the modern drainage of the rivers into the lower Zambezi. Remember that this illustrates events that happened sequentially over several million years: a) the axis of uplift from Zimbabwe to Botswana; b) the formation of the great lake into which the mighty river flowed; c) the rifting of the earth’s crust, which created the conditions for the capture of the upper Zambezi and its tributaries by the lower Zambezi; d) the capture of the Kwando by the Linyanti Fault; and e) the subsiding of the graben faults to form the Okavango Delta.

A smaller-scale illustration showing the positions of the faulting in Botswana.

The continued faulting and down-warping between the south-west-striking faults had the effect of creating a graben fault or, in simple terms, a subsidence between two weakened areas of the earth’s crust. Into this 200-kilometre-wide fault has poured the waters of the Okavango River and along with it the sediments that have filled the depression. The sediments within the Okavango are mostly of Kalahari origin, having been deposited by the inflowing rivers mentioned earlier. However, they are in many cases slightly smaller-grained, as they have been reworked by water during their transportation into the graben fault. A closer view is provided in the map of Botswana, which illustrates the graben fault between the Kunyere and the Gomare-Linyanti faults.

The major fault systems in Botswana, showing the Gomare and Kunyere faults, between which the crust has subsided.

Remembering that the Okavango graben is a depression within the greater Kalahari, one can then get a picture of the structure as it is today. The faulting is in the original bedrock below the first Kalahari beds, but manifests itself as a down-warped area on the surface. This down-warping cut directly across the flow of the remaining two rivers, the Cubango and Cuito, which joined together to form the Okavango about 250km above the Gomare-Linyanti fault. Once again, water and sediment poured into the depression until it had filled up and levelled off to form a classic delta-shaped alluvial fan. Of course over the last 80,000 years the continent of Africa, as elsewhere, has experienced much wetter and much drier conditions than those being experienced today. Therefore it will come as no surprise to know that the delta was once considerably larger than the 15,000km² it is today. In fact, satellite imagery suggests that it may have occupied an area approximately three times the current size, evidence of which can clearly be seen from the air as one flies between Maun and the Linyanti, where the mosaic of higher and lower land depicts the past channels and islands of the area.

In the illustration I have attempted to depict diagrammatically the Okavango Delta fan in cross section, showing the approximate alignment of the graben faults and the resulting infill of reworked Kalahari sands. Of interest and no less importance is that the reworking of the sand by water has left the sand grains a rounded shape, which has important implications for groundwater movement and is one of the most important aspects of the daily functioning of the delta and its habitats.

A diagrammatic cross-section of the Okavango graben, or twin faults, showing the faulting sediment infill.

The modern Okavango—a miracle in space and time

Given its geo-history (or geomorphology) briefly presented here, the structure of the Okavango as we know it today is relatively easy to describe. However, it becomes important for the reader to understand how important the timing and positioning of the delta is to both the natural ecosystems and the human inhabitants of the region. Without being excessively scientific, the following facts outline the Okavango in its regional context and what currently occurs there:

•     The Okavango Delta is situated in north-west Botswana between about 18° and 21° south and 23° and 27° east.

•     It is an area of about 15,000km² consisting of rivers, flood plains, grasslands, sand masses and islands.

•     The average annual rainfall is about 500mm, all of which falls within the southern hemisphere’s summer months of October to April, with peaks during January and February. The delta itself falls within the geographic boundaries of the Kalahari thirstland, which has an annual evaporation rate five times that of the rainfall, or approximately 2,500mm a year. This means that there would be no surface water in the Kalahari region for up to seven months of the year were it not for the Okavango Delta—an oasis in so many ways.

•     The main catchment of the Okavango lies in the central highlands, or Planalto, of Angola about 900km north-west of the Okavango Delta. This catchment receives an average of between 1,200mm in the west to 1,800mm in the east, where the Zambezi River has its origins.

•     The catchment, or watershed line, runs west to east across Angola and effectively forms a dividing watershed in drainage between the rivers that flow south and south-east, and those flowing north into the Congo River basin.

•     Kalahari basin sands dominate the terrain through which the rivers flow before forming the delta proper in Botswana. Therefore, the sediments carried by the rivers are predominantly Kalahari sands already deposited by endoreic rivers millions of years before. These were reworked and moved about by wind during drier episodes.

•     The sands are largely quartz and are of low fertility, with low organic content.

The map below shows the rivers that originate along Angola’s highlands and those that drain south and south-east from the watershed. It can clearly be seen that these rivers once flowed south and were interrupted by river capture (the Zambezi and the Kunene); river capture by faulting (Luangwa, Kafue, Deka, Zambezi and Kwando); and lastly by graben rifting (the Cubango and the Cuito, forming the Okavango). It is no coincidence that colonial planners, particularly the Germans, devised a system of canals and railroads using these rivers to link the Atlantic and the Indian oceans to form a trading route.

The real miracle is in the timing of the annual flood, which starts with annual summer rains in the catchment and over the delta itself, for local rains can contribute between 5 and 35 per cent of the final input into the delta. In Angola and within the fan itself, the rains peak during January and February, although there can be considerable late rain in Angola in March. The amount of rain can vary enormously, as shown by records which have been kept since 1922 in Maun, Botswana. The highest recorded rainfall for an entire rainy season (October to April) is 1,213mm and the lowest 110mm.

Over the highlands of Angola the annual rainfall average is about 1,200mm, although this can also vary and has been known to reach 2,000mm, and has been as low as 240mm. The first rains in the catchment during October will begin to reach the top of the panhandle at Mohembo in Botswana during January of the following year, so the water takes about eight to nine weeks from source to Botswana. The main pulse of floodwaters enters Botswana around April, but the rains in Angola, as elsewhere, do vary and the waters can arrive early or late. These waters begin to spread throughout the main alluvial fan during May, June and July, with the Okavango Delta at its fullest during August. From then on, evaporation or evapotranspiration starts to exceed input, as the rains have ceased in Angola and over Botswana itself. This will reduce the area under inundation in the Okavango and so begins the so-called ‘dry season’ of September, October and November. At such latitudes, these months can be extremely hot, with food and water scarce. The Okavango Delta, along with the Linyanti swamp and the Zambezi River, become the foci of large numbers of animals, from elephants to tiny birds.

When the first rains arrive over the delta during late October, or more usually during November, the delta is at its driest and is invigorated by these rains. Combined with higher temperatures and longer daylight hours, the grasses, shrubs and trees grow vigorously, leading to the summer months of plenty. December, January and February are when the larger mammals spread out over an area far wider than the delta, as food and water are plentiful over a large swathe of northern Botswana, Zimbabwe, Zambia, Namibia and Angola—a vast area earmarked for international conservation efforts known as the Kavango-Zambezi Transfrontier Conservation Area (TFCA).

This is a simplistic depiction of the workings of the Okavango Delta. The detailed hydrology and sedimentology, along with specialized vegetative growth and mammal movements, combine to create a system of such dynamism that it would take a book of its own to describe all its subtleties. What is of fundamental importance, however, is that this unique alluvial fan, stranded as it is in the Kalahari thirstland, supports one of the world’s great biodiversity lists and is home to truly wild animals. It is worthy of our protection at all levels.

Ecology and habitats

Okavango fact sheet

* Other species and predators are unknown. These figures do not include the Linyanti Swamp to the north, where elephant populations are many times larger than in the Okavango.

Components of the Okavango ecosystem

Following the description and workings of the Okavango by Map Ives in the introduction, this chapter looks at the resultant habitat formation.

To really appreciate the particular and unique variety of Okavango habitats one has to take cognizance of one key fact: the topographic relief along the length of the delta is nominal. The variation in gradient is only approximately 60m along its full 250-kilometre length, unique for a river of this magnitude. The result is a widespread, lazy, fickle and meandering sequence of rivers, channels and oxbow lakes, and long, meandering islands, sandbanks and flood plains. Almost all of the water dams up against a double fault line and only exits by evaporation. The surrounding marginal regions are wooded, forested or scrubby depending on the soil quality, water flow and availability.

To be scientifically correct, the Okavango Delta is not a delta, but rather an alluvial fan.

The Okavango can be divided into three main components: the so-called ‘panhandle’, the permanent swamp and the seasonal swamp.

The panhandle is the riverine region that feeds the swamp and is essentially the Okavango River meandering through dense, homogeneous stands of papyrus, channelled and directed by two parallel faults towards the permanent swamp.

The permanent swamp is fed by the panhandle and here the Okavango River splits into three main channels: the Nqogha, Jao-Boro and Thaoge. This component of the Okavango is typified by papyrus-fringed channels, small, forest-rimmed, round islands, lagoons and thick stands of Wild date palms.

The seasonal swamp, fed by the permanent swamp, is a variable and seasonal area of extensive, shallow, seasonal flood plains. These generally only hold water between May and September as the seasonal flood arrives. As the water recedes and evaporates, large open grasslands eventually appear. It is within this component that the large herds of herbivores are found. Here, most large islands are conspicuously long and meandering, formed over many years by channels that have been clogged with sediment and dead organic matter owing to the gentle gradient.

During years of extremely high rainfall water may leak out of the system via the main fault line, the Thamalakane, close to Maun, and eventually flow down towards Lake Ngami or the Mababe depression. With regular, consecutive high rainfall years, the water may flow down the Boteti River towards the Makgadikgadi pans.

The following animals have a distinct effect on the Okavango’s ecosystem.

Hippopotamuses

Hippopotamuses open up aquatic waterways, helping water flow down the gentle gradient. In their absence, the Okavango would choke up and divert.

Elephants

Elephants are responsible for converting woodland to savannah and making dead trees available for other life forms, especially termites.

Termites

By creating mounds, termites are responsible for the formation of all the small islands in the delta. Without these islands there would be no place for trees to grow. They also provide a substantial amount of food for all forms of animals during the early summer months when millions of flying termites emerge from their mounds only to be devoured by birds, frogs, mammals, fish and other invertebrates.

Primates and birds

These deposit seeds in their droppings on termite mounds, starting new tree growth and transforming termite mounds into life-giving islands.

Bees

Bees pollinate the flowers of trees, providing fruit for primates and birds and the further creation of new islands.

Besides these animals, fire also has a role to play in the ecosystem: grasses can survive its effects, but shrubby, woody components cannot. Consequently, fire has the effect of clearing savannahs and seasonal flood plains from woody saplings, thus ensuring the sustainability of the grassland.

Habitats of the Okavango ecosystem

The result of this unique geomorphology, seasonal flooding, semi-arid climate and fire, together with the influence of the key ecological species, are the following habitats:

•     Savannah

•     Woodland

•     Riverine forest

•     Islands

•     Open water

•     Permanent swamp

•     Seasonal flood plains

Savannah

Savannah mainly occurs on the adjacent perimeter of the delta and is essentially grassland, dotted with low scrub and occasional trees. Savannah and woodland are quite similar, with transitional forms. Patches of woodland occasionally occur in savannah. Soils are mainly alluvial, but may be interspersed with or dominated by clayey deposits. Due to the availability of sunlight, grasses are abundant. This habitat can be grouped into three main types, depending on the dominant tree cover and soil type:

Mopane scrub

This is mainly found in the north and eastern adjacent areas, but also on certain larger islands in the delta. Mopane scrub is typically dominated by low, stunted or damaged multi-stemmed mopane trees (Colophospermum mopane) on clayey soils, providing a prime habitat for breeding herds of elephant.

Kalahari sandveld

This habitat is found throughout the adjacent areas, especially in the north, and is the main component of the Sandveld tongue, which is a large semi-desert region penetrating the delta in the south. Kalahari sandveld is dominated by Silver terminalia (Terminalia sericea) and Kalahari apple-leaf (Philenoptera nelsii) or both, on deep, sandy Kalahari soils. In some areas, this habitat is characterized by the presence of Camel-thorn trees (Acacia eriloba). It is prime habitat for desert animal species, such as steenbok, ostrich and Bat-eared fox, and springhare and giraffe when Camel-thorn trees are present.

Umbrella thorn savannah

This occurs mostly on fertile, less well drained soils. It is dominated by low, young, scrubby Umbrella thorn trees (Acacia tortilis), but may be transitional and as well wooded, as in the savannah image. It is typical of extinct papyrus beds, such as in the north-western regions of Mombo. Grasses and trees are highly palatable and attract large volumes of herbivores, particularly impala, giraffe, zebra and wildebeest.

Woodland

Mixed woodland, mainly Acacia species. Note the close spacing of trees, typical of woodlands.

Mopane woodland and associated seasonal pans on grey clay soil.

Woodland occurs throughout the Okavango, but predominantly in the north and east. This habitat is characterized by large trees that are closely spaced, but whose crowns don’t touch, and reduced grass cover. Trees are mainly deciduous species, creating a dry, barren landscape during the late winter months when the leaves fall. This habitat can be grouped into three main types, depending on the dominant trees and soil type:

Mopane woodland

This type of woodland is mainly in the north and eastern adjacent areas of the delta, especially north of Vumbura and east of Mombo and the rest of the eastern Moremi. It is dominated almost exclusively by large, mature Mopane trees (Colophospermum mopane). Occasionally, Russet bushwillow (Combretum hereroense), Shepherd’s tree (Boscia albitrunca) and Mozambique Shepherd’s tree (Boscia mossambicensis) are present. A broad variety of grasses occur, which attract grazers during the summer months. The soils are predominantly grey clay and poorly drained, resulting in large, water-filled pans during the wet summer months. These pans are important water providers during the drier months, as they may hold water for long periods before the first summer rains. Animals often ‘vanish’ into the mopane while water, graze and browse are still plentiful and emerge in large numbers once these resources are depleted during September and October. African elephants are attracted to mopane more than any other tree species and, therefore, have a major effect on this habitat, its water and neighbouring habitats.

Zambezi teak woodland

This habitat is typified by large homogeneous stands of Zambezi teak (Baikiaea plurijuga). Although predominantly occurring in the Chobe, some distance from the Okavango, large stands do exist in the northern Okavango in deep Kalahari sands. The trees are not favoured by elephants, which results in a uniform, almost untouched, habitat compared with mopane woodland, which is often seriously damaged by elephants. Wild seringa (Burkea africana) also occurs in this habitat, as well as Copalwood (Guibourtia coleosperma). Where water is easily available this habitat provides a haven for Sable and Roan antelope.

Mixed woodland

This habitat, as the name implies, consists of a mixture of species, mainly Knob-thorn (Acacia nigrescens), Rain tree (Philenoptera violacea), Leadwood (Combretum imberbe), Umbrella thorn (Acacia tortilis) and Sausage trees (Kigelia africana). It generally occurs in the eco tone between riverine forest and the dry mopane and Kalahari apple-leaf regions of the northern Okavango. This habitat is very productive and hosts numerous herbivores, especially giraffe, elephant, wildebeest, buffalo, zebra and impala. The soils are alluvial, with occasional clay deposits, which are slightly sodic in nature. The palatable Bushveld signal grass (Urochloa mosambicensis) dominates on these soils in summer, attracting and holding large populations of grazers.

Riverine forest

Riverine forest is characterized by a mixture of trees, with the canopies touching or interlocking. Grasses are almost absent due to the lack of light and are replaced by creepers and herbaceous plants. This habitat, as the name suggests, is associated with riverbanks where sufficient water is available, yet not in excess. It also occurs on the margins of large islands. Trees are numerous, closely spaced and tall because of the availability of water, and continuously compete for light. Riverine forest is preferred by the Chacma baboon, leopard, bushbuck, Peters’ epauletted fruit bat, Green-pigeon and Paradise flycatcher.

Riverine forest on the edge of a seasonal channel. Jackalberry trees (Diospyros mespiliformis), African mangosteen (Garcinia livingstonei) and figs (F. thonningii and sycomorus) make up a large component of the touching canopy.

The most common trees in this habitat are African mangosteen (Garcinea livingstonei), and Jackalberry (Diospyros mespiliformis), Sycamore fig (Ficus sycomorous), Bird plum (Berchemia discolor) and Sausage tree (Kigelia africana). The riverside margins are often fringed with Wild date palm (Phoenix reclinata), Bluebush (Diospyros lycoides) or Large fever berry (Croton megalobotrys). The drier side is often fringed with Rain tree (Philenoptera violacea), Leadwood (Combretum imberbe), Marula (Sclerocarya birrea) and Knob-thorn (Acacia nigrescens). Most large mammals are absent from this habitat because of the lack of available browse and graze, although the margins may be well populated by impala, giraffe, kudu, buffalo and elephant. Most of the camps in the Okavango are situated in this habitat, given the availability of shade and water.

Islands

Although various shapes are present, the islands in the Okavango tend to be circular, long and sinuous or very large, dry expanses.

Round island formation in process; each individual termite mound is colonized by a tree, allowing the tree to survive annual flooding by being elevated. The island grows in size, from more seeds deposited by animal droppings, as well as fallen branches and leaves from the tree.

Small circular islands

Small circular islands are formed from termite mounds that eventually increase in size from seeds being deposited by bird and baboon droppings, which result in plants growing from the seeds. These islands are often fringed with Wild date palm (Phoenix reclinata), with larger circular islands having an inner secondary forest mainly consisting of Jackalberry (Diospyros mespiliformis), Knob-thorn (Acacia nigrescens) and Real fan palms (Hyphaenae petersiana). The islands often have a barren inner zone, which is characterized by dead trees, and a white ground surface. This white substance, called trona, mainly consists of sodium bicarbonate that has been aspirated over many years by the trees, with vegetation and evaporation eventually reaching toxic levels and killing off the vegetation. The only plants growing on these highly alkaline soils are Spike grass (Sporoblus spicatus) and further away from the toxic centre, palatable Couch grass (Cynodon dactylon). Lechwe enjoy these islands for refuge from water and predators, often resting for long hours in direct sunlight on the fringes of the bare, open patches. The Wild date palm fringes may provide refuge for hippopotamus and buffalo bulls and should be avoided on foot.

Numerous small round islands, covered in Wild date palms (Phoenix reclinata) typical of the permanently flooded regions.

Long sinuous islands

Long sinuous islands are common and can be up to a few kilometres in length. They are the result of channels failing, owing to low flow rates and increased sedimentation. The blocked channel becomes higher than the surrounding flood plains and the water seeps out, eventually diverting the flow and totally drying up. Finally, the island is colonized by plants and trees from fruit seeds dispersed by baboons, birds and elephants. These islands often have a mixture of riverine forest and woodland communities and provide a habitat and haven for many large mammal species, especially elephant, buffalo, lion, leopard, zebra, giraffe, kudu, impala, waterbuck, Chacma baboon and hippo.

Long sinuous islands, also called long meandering islands, occur in the seasonal swamps and are the final result of channels that have failed. These can be several kilometres long, providing ideal habitats for many species of animals.

The outer edge of a long sinuous island; note all the animal pathways between the seasonal flood plain and the island.

An older island showing distinct vegetation zoning due to increasing salinity from the outer rim towards the almost toxic centre. Wild date palms (Phoenix reclinata) are growing on the outer rim, and Real fan palms (Hyphaene petersiana) towards the centre on more salty soils. Note the smaller, newer islands scattered around this island.

Islands showing a white toxic centre (sodium carbonate) with no tree growth. Trees are only growing on the outer rim due to reduced salinity.

Large islands

Large islands, such as Chief’s Island, are mostly formed as a result of tectonic uplifting. These islands are generally dry in nature and resemble the adjacent wooded and savannah regions of the Okavango, with riverine forest, mopane woodland and savannah habitats. They are generally heavily populated by the same large mammal species that occur on long sinuous islands because of the varied habitats and easy access to water and browse and graze.

Open water

Open water is abundant in the Okavango and provides a large habitat for aquatic insects, fish and amphibians and their respective predators—in particular, waterbirds and crocodiles. There are many hippopotamus in the Okavango and Linyanti swamps; they have a key role in keeping slow-moving channels open and flowing.

Rivers

Rivers are deep; slow-moving, with partially solid banks. The Okavango River feeds the Okavango Delta; the Kwando River feeds the Linyanti Swamp. The rivers have low mammal densities owing to the lack of associated graze and browse. Hippopotamuses are the exception: they occur in large numbers and will travel vast distances at night in search of graze. The banks are fringed with Woodland waterberry (Syzygium guineense), Papyrus and Common reed (Phragmites australis).

Channels

Channels are numerous in the Okavango. They differ from rivers in that they flow deepest and strongest in their upper reaches, dying out further down their route as they flow into seasonal swamps or dry Kalahari sandveld. They are mainly directed by vegetation and rarely have solid banks, like rivers. The predominant vegetation is Papyrus, Common reed and a variety of sedges. These channels are utilized by hippopotamus to access grazing areas at night; by elephant bulls to access islands; and by crocodiles in search of fish and mammalian prey.

Oxbow lakes and lagoons

These are small- to medium-sized permanent lakes within the delta. Lakes and lagoons are the result of cut-off meanderings on main channels and are generally fringed with papyrus. They provide habitat for fish, hippos, otters and crocodiles. They are often covered in floating water lilies.

A large oxbow lagoon in the northen Okavango in the Maunachira channel.

Pans

These are temporary, seasonal water bodies. Two main types exist in the Okavango—those that are the result of water accumulating in a depression after a flood plain has receded and those that have been filled by rainwater in poorly drained or clayey soil. The water is generally turbid and muddy, providing a preferred habitat for many frog and bird species.

A hippo pool in a seasonal channel which will form a pan as the channel dries up. Fish will accumulate in this depression, attracting many species of birds.

Permanent swamp

The permanent swamp habitat (photo below right) consists of areas that are permanently covered in water, with submerged plants, surface plants and rooted plants, such as papyrus. Vast and fairly inaccessible regions of the Okavango are made up of this type of habitat. Below the thick stands of vegetation is a substrate of peat, silt or occasionally sand. This habitat is restricted to the upper reaches of the main Okavango River and the larger channels.

Rooted emergent plants cover most of the permanent swamp and are the most abundant plant type. The four main, almost dominant, species are Papyrus (Cyperus papyrus), Common reed (Phragmites australis), Wire-leaved daba grass (Miscanthus junceus) and Bulrush (Typha capensis). On the fringes of the smaller channels, Swamp fern (Thelypteris interrupta) grows in stands with Papyrus. Woodland waterberry (Syzygium guinieense), Waterberry (Syzygium cordatum) and Water fig (Ficus verriculosa) form dense structures for waterbirds to nest on. These may eventually develop into islands as bird droppings provide extra plant nutrients.

Submerged plants are inconspicuous but visible through the clear waters. They provide structure and micro-habitats for aquatic insects, fish, crustaceans, snails and amphibians. Bladderwort (Utricularia spp.), with its long, many-branched fine stems, is common, as are Saw weed (Najas horridus) and the White ottelia (Ottelia muricata), with its attractive white flowers. Floating-leaved plants prefer deeper waters on the fringes of lagoons and main channels. These plants are mainly Blue waterlily (Nymphaea nouchali) and in the quieter and more turbid lagoons, Water chestnut (Trapa natans).

The permanent swamp has very few large mammal inhabitants, but provides an ideal habitat for the rare Sitatunga, crocodiles, Water monitors and large populations of hippopotamus.

Typical backwater vegetation in the permanent swamps, consisting of Papyrus (Cyperus papyrus), Blue waterlilies (Nymphaea nouchali) and Lotus waterlilies (Nymphaea lotus).

Seasonal flood plains

The seasonal flood plains cover most of the lower reaches of the Okavango. These flood plains, unlike the permanent swamps, vary considerably in water depth during the year and from year to year. Every year, the flood plains go through a similar cycle of flooding, receding and partial or permanent drying up. There is also an associated arrival and exodus of bulk-grazers.

With the onset of winter and the arrival of the annual floods from Angola, the seasonal flood plains are inundated with water—not deep, but sufficient to submerge old grass stems from the previous season. With this, spawning fish and predatory catfish arrive to feed on other smaller species. Birds also arrive, feeding on the edges of the newly flooded grasslands, preying on fish, frogs and insects as many inhabitants are driven out of their holes by the approaching floodwaters.

At the end of winter and the onset of spring, the shallow flood plains begin to recede through evaporation and the reduction in the flood surge. During this drying-up period grazers emerge from the adjacent savannahs and woodlands to feed on the new grasses. This coincides with the onset of the driest time of the year, when rain-filled pans in the woodlands begin drying up. Elephants are forced to leave their favoured mopane and acacia woodlands due to a lack of water and migrate daily, back and forth between their food and water sources. As the flood plains recede, temporary hippo pools and depressions trap fish and become thick with fish as they dry out even further. These ‘fish traps’ attract huge numbers of birds, from terns to skimmers, herons and storks.

Mombo, Chief’s Island. Buffalo move down to a seasonal flood plain from an adjacent woodland to graze and drink.

A typcal seasonal flood plain in the western Okavango in the Jao concession with an abundance of Real fan palms (Hyphaene petersiana).

The Kunyere fault line at the end of the Okavango Delta. Note how the water has spread along the length of the fault.

Fires in the seasonal flood plains during February, caused by lightning strikes.

Just before the first summer rains in October the flood plains have dried out and are vulnerable to fires, caused by lightning strikes with the first rains, and occasionally by humans. With the first summer rains, usually in November, the flood plains receive a second dose of water, but not enough to inundate them—only enough to fill up some of the depressions and dry pans. The newly sprouted grasses continue to grow until the beginning of winter when the temperature drops and the new floods arrive.

This consecutive cycle of events with each year’s floods is vital to the continued existence of this highly sensitive ecosystem and makes the Okavango Delta what it is. Any form of water-flow alteration or diversion upstream will have detrimental effects on this ecological phenomenon and cause a decline in the biodiversity of this unique wilderness.

Dynamics of the Okavango ecosystem

Remarkably, the waters of the Okavango are pure, drinkable and relatively void of accumulated salts. This seems impossible considering that more than 90 per cent of the water entering the system is lost through evapotranspiration. With the amount of water leaving the system in this manner, one would be forgiven for thinking that very high salt levels would accumulate in the water. This is not so.

There are two distinct processes that best summarize the dynamics of the system. Simultaneously, they are responsible for the low salinity of surface waters. These are:

•     The concentration of salts in and below islands due to evapotranspiration

Islands act as natural salt removers. The process occurs through evapotranspiration. Most of the water in the delta is not lost through evaporation, but instead through evapotranspiration from the forested edges of islands. The plants rapidly transpire and remove water from the system, but not dissolved substances. This increased movement of water into the island and out with the exclusion of these substances, results in their increased levels. The islands and their trees act as water pumps and salinity filters. These dissolved substances remain behind and are accumulated in the soil. Evidence of this process is the accumulation of white sodium bicarbonate at the surface of many islands, especially at the centre where levels are at their highest. This is also evident in the distinct zoning of vegetation on these islands due to different salinity and toxicity levels.

To summarize these processes, forested islands, especially medium-sized ‘round’ islands, act as salt filters and dumps, cleansing the system of dissolved substances. These islands in turn eventually become ‘toxic’.

•     The redistribution of water through a constant process of new channel formation and death

Water is constantly being redirected and redistributed in the Okavango. This is because of the low gradients of the system. Water moving at very slow speeds does not clear and move sediments from channels. The process is enhanced by Papyrus (Cyperus papyrus), as it contains incoming sandy sediments, whilst allowing water to leak out of the channel. At this stage, Hippo grass (Vossia cuspidata) starts encroaching from the channel fringes. With a further slowdown in the flow rate and increased sedimentation of sandy deposits, Hippo grass grows across the channel and floating debris accumulates in the channel. Eventually the channel bed is elevated above the surrounding swampland terrain; more water is lost and the channel dies. The diverted water forms a new swamp system and in time, with flow, a new channel is created. Adjacent peat deposits dry out and subsurface peat fires spontaneously start. These mombo fires, as they are known in Setswana, result in the formation of highly fertile soils.

Toxic islands also dry up in this process of water redistribution. Rain flushes the toxic substances deeper into the soil and after many years, the islands are renewed and recolonized by grasses, termites and fruit trees.

In summary, this incredible system is filtered by islands and constantly refreshed by changes of water flow, resulting in new channels, swamp systems, islands and a lack of salinity in surface waters.

Aerial view of an older island. The white centre consists of sodium carbonate deposits as a result of evapotranspiration, thus eliminating plant growth. Wild date palms (Phoenix reclinata) fringe the outer less-saline rim.

Low gradients and resultant low water speeds facilitate the deposition of sandy sediments.

Growth of Hippo grass (Vossia cuspidata) on the channel fringes is evidence of sedimentation and the beginning of channel death.

Mammals

Mammal habitats and niches with particular reference to the Okavango Delta

Mammalians all fill specific niches in nature and this discussion states that each animal has an exclusive list of habitat requirements that help the individual exclude competition in its region. Due to a lack of choice in habitats, however, the delta has produced many unique circumstances for animals to exist within. For example, the lion and the leopard are both large nocturnal predators. This is, however, where the similarities stop. They live in different habitats over most of their corresponding range outside of the Okavango, with leopards in dense bush, rocky slope and river valley, and lions on open grassland and mixed woodland. In the delta, these animals overlap in range utilization, with both animals frequently encountering one another in the combination of island and grassland habitat that is available. It is interesting to note that there are no hills, and, therefore, no hillsides, in the delta, other than those created by termites, and there are very few stones, let alone rocks or rocky areas. The leopard focuses on medium-to-small antelope, with the occasional large animal, and the lion on medium-to-large herbivores, with occasional small animals.

In the Okavango, one often finds that the most common prey animal is the Red lechwe, so it is often co-utilized by both these predators as food, even though a decent-sized male Red lechwe can weigh around 100kg, outweighing a female leopard by up to 100 per cent. (It is interesting to record an eyewitness report of a hunt by a solitary Side-striped jackal that successfully killed a female Red lechwe, demonstrating how important a food source this animal is to all the predators in the delta.) The hunting methods of these two large predators are usually adapted to their environment, as are their social structures. In the delta it often occurs that prides of lionesses fragment into smaller hunting parties of two or three to ensure that there is enough food to go round, as a large pride of 10 or 15 females would not be able to feed their fill on a single Red lechwe carcass.

This is how the Niche Theory is effected, contrary to the competitive exclusion principle, when the quantity of food available in a given habitat or range excludes competition for resources in this given area. In order to determine the functions of the animal’s niche it is useful to think along the lines of its competitors and how they avoid over-competing, or, to simplify this further, think of each animal having a particular ‘job’ within its environment, such as a leopard being ‘employed’ within an area, e.g. the delta, to hunt small-to-large antelope by using a stalk-and-pounce technique, within the thickets and among palms of the island fringes.

The Okavango often has its own unique sets of principles, which, due to habitat constraints, push animals into occupying what we might consider marginal niches, or alternatively becoming superbly adapted to what one may consider a totally atypical habitat—lions that swim for their prey; aquatic antelope with specially adapted bodies; Painted wolves that excel at hunting in water and crocodiles that eat leopards; these are but some of these local Okavango anomalies.

The importance of bats within the delta

Insect-eating bats (Microchiroptera) fill a locally exclusive mammalian niche of a nocturnal flying insectivore. It has no competitor in the mammal world and no real competition in the bird world, as no nocturnal birds echo-locate to search for food. Interestingly, the delta provides a perfect habitat for these micro-chiropterans, with its abundance of flying insects and large trees in which to roost. There are also abundant mega-chiropterans: these fruit-eating bats of the delta have evolved wonderful relationships with some tree species, such as the majestic baobab, which depends upon the species of epauletted fruit bats to pollinate them.

Feeding on ants

The aardvark is a nocturnal insectivore. It locates prey by smell and readily digs deep burrows to obtain termites. In the delta the flood plains are seldom utilized by the aardvark, as diggings to only a few feet below the surface often reach the water table. This is also not the type of habitat where termites are successful—they usually occur more on the island formations (which, ironically, they also help to develop). The aardvark digs more readily than the pangolin, however, and the aardwolf does not usually dig deep at all to feed, but makes shallow scrapes on the ground (see the Track and signs section for more information). Aardvarks are usually more common than pangolins, but in the ‘true’ delta regions more pangolins than aardvarks are encountered.

The hippopotamus and its function within the ecosystem

The hippopotamus shares a feeding niche with grazers such as the White rhinoceros, but there is a limit to how many bulk-grazers an area can carry. The delta is very much the domain of the hippopotamus, with very few White rhinoceros found here today. The few seen are the result of a joint venture between the Botswanan government and Wilderness Safaris where several animals were re-released after total regional extinction due to poaching. The hippopotamus occupies a unique daytime niche by resting up during the hot hours in open water and papyrus beds. It plays an extremely important role in the maintenance of the waterways by keeping the channels open and allowing the water to flow, in turn permitting the movement of fish, crocodiles and other life, which would otherwise become trapped in ever-decreasing pools. Without the hippos of the Okavango this system would be dysfunctional and certainly not exist as it does today. Its subsequent permanent dependence on water has led to physiological changes in the hippopotamus, making it more streamlined, but unable to sweat due to a lack of glands designed for this. If a hippo is trapped in direct sunlight for a long time it begins to secrete lymphatic fluid in order to protect itself. This lymph is pinkish in colour, which gives rise to the story that hippos can sweat blood.

The suspensorium in the big cats

Vocalization in the two genera of big cats occurring in the Okavango (Acinonyx and Panthera) is quite different. The cheetah cannot roar—it chirrups, whistles, growls and purrs. This is because the suspensorium, or hyoidean process, is ossified, restricting movement of the larynx, which cannot open wide to permit deep vocalization. The suspensorium is a small group of bones in the throat, positioned in a big cat’s throat roughly where a human male would have his Adam’s apple. In comparison, the suspensoriums of the leopard and lion are soft and cartilaginous, allowing a massive expansion of the voice box, or larynx, to enable roars as well as growls and purrs. The larger voice box allows for deeper sounds. The roar of a lion and the saw-like cough of a leopard are synonymous with the night in the Okavango, as the lion and the leopard are found throughout the Okavango system, both in the wetlands and in the peripheral desert. Cheetahs, however, have a tendency to move out of the seasonally inundated areas when the floods arrive, being seen more commonly in the flood plains during September to April when the waters dry up, facilitating more effective hunting by speed.

The antelope of the Okavango

Throughout most of the southern African subregion the impala is a flagship species, often being the most abundant variety of antelope found in a given area. In the wet delta the impala is more or less restricted to the dry islands, often occurring in very small groups. On the larger islands, the herds may be bigger, but generally they are limited in number within the delta itself. On the periphery of the wetlands, extending into the deserts, they become commoner.

Within the short-grass wetlands of the Okavango the Red lechwe is the most prevalent antelope species. The lechwe is also the staple food of most of the larger predators. This antelope is specially adapted to wet and even totally inundated areas. It has a high rump with long hindlegs to facilitate leaps and bounds in deep water. Its hooves splay open at an impossible-looking angle in order to prevent its feet sinking into muddy or sandy soil under the water in which it grazes.

Yet farther into the watery system of the delta one begins to encounter thick papyrus, reedbeds and channels that may be several metres deep. In this habitat, the lechwe is seldom seen, as this is the domain of the sitatunga. This antelope, which is closely related to the kudu, is a specialist in this environment and can swim well, although it tends to avoid the deeper water. It is much like the kudu in appearance, with slightly shorter, spiralling horns. The body of the male is dark brown with shaggy, longish hair, with white blotches and stripes along the flanks. There may be yellowish blotches on the flanks. Females, by contrast, are reddish-brown in colour, with white blotches and stripes. This extreme sexual dimorphism can also be seen in the nyala. It walks with a clumsy gait and has extremely long, splayed, open hooves. These massively elongated hooves are used to good measure when moving around in the papyrus and reedbeds. It is seldom seen because of the near-impenetrable habitat in which it lives, but may occur in relatively good densities when food is abundant.

The elephants of the Okavango

The number of elephants in the Okavango varies according to the season and local migrations. Furthermore, distributions of the sexes vary, as some parts of the delta are principally the domain of solitary bulls and bachelor groups, whereas other areas are occupied by breeding herds of females with calves, and attendant breeding bulls. The elephants of the delta show a marked preference for the fruits of the Real fan palm during the fruiting season, from September to about February, causing the bulls to herd on islands where these fruits are ripe. They may spend literally months feeding on a single large island. Elephants also spend a large proportion of time in the water and river channels, and regularly feed on reeds and other water plants. Movement from island to island is the norm, however, with intensive feeding taking place among the Fig, Sausage and Jackalberry trees on the islands.

Carnivores (order Carnivora)

Family Hyenidae

Aardwolf

Thukwe

Proteles cristatus

These attractive members of the hyaena family are very dog-like in appearance. This is all the more astounding given that hyaenas as a group are categorized in the cat group of carnivores, the Feliforma. The aardwolf is usually straw-yellow or yellowish-grey in colour, with about seven dark, vertical, tiger-like stripes on the body, hips and shoulders. The legs are also black-striped. The ears are large and the muzzle is dark. There is a long crest of hair along the back, called a saggital crest, which is erected in order to make the animal look bigger when threatened. They also have a very bushy tail. The long claws are permanently out and show in the track. They are fairly small animals, weighing only 10kg on average.

Their dentition is interesting as these animals feed mainly on ants and termites, consuming up to an estimated 400,000 in a single evening. The canines are massive, almost disproportionate, and are used for competing with others of their own species, whereas the premolars and molars are massively underdeveloped, to almost peg-like structures. This development has evolved because the need to chew meat is totally removed by its specialized diet. When feeding it makes shallow scrapes at the entrance to an ant’s nest, and then licks up the confused ants with its very coarse tongue that has backward-pointing papillae to snag or hook up the insects. They are usually seen alone or in pairs, but aggregations of up to three and four are not uncommon, probably a parent pair and their most recent offspring. In areas where there are large numbers of Black-backed or Side-striped jackals the numbers of aardwolf are reduced, as adult jackals are one of the main enemies of aardwolf cubs. In the Okavango they are most commonly seen on Chief’s Island and the large, dry islands adjacent to the Moremi, such as the Duba islands. These island areas are either lightly wooded, or, as in the case of Duba, formed of vast, open rolling plains of Couch grass (Cynodon dactylon). When the aardwolf breeds, mating is a long affair, often lasting for several hours. If the female is impregnated she gestates for almost three months, giving birth to up to four cubs. The aardwolf lives for 13 years or more. These animals are territorial and mark their territory by pasting a secretion on vegetation, which can, remarkably, be smelled by humans for up to nine months after it is deposited.

Spotted hyaena

Phiri

Crocuta crocuta

These highly adaptable, much maligned, but intelligent, hunters are synonymous with the African plains, with an eerie call often heard in the Okavango night. Although these animals are very effective hunters they are definitely better known for their scavenging habits. However, in certain parts of their extensive range they are known to hunt more than they scavenge. They are large predators, weighing up to 90kg in the case of