Located within the miombo and mopane woodlands of the Central African Plateau, Zambezian flooded grasslands are an anomaly of productivity and abundance in a landscape characterised by nutrient deficient soil and associated vegetation. Unlike the surrounding woodlands that generally support animals only in rather low densities, the wetlands and floodplains of this ecoregion provide habitats to sizable faunal populations, since food and water are abundant throughout the majority of the year. Large populations of waterfowl gather during the rainy season, and numerous herd animals and carnivores frequent the landscape. However, in a climate of rapid population growth and increasing need for land and natural resources, this fragile ecoregion is destined to face increasing ecological threat. Several conservation measures are urgently required for the ongoing survival of these wetland ecosystems and their biota.eec

Location and general description

Embedded predominantly within miombo and mopane woodlands, the Zambezian flooded grasslands ecosystem forms a discontinuous ecoregion distributed between northern Botswana in the south, to northern Tanzania. The Kilombero Valley, Moyowosi/ Malagarasi system and the Ugalla River in Tanzania; the Okavango River Alluvial Fan in Botswana; Lake Chilwa in Malawi; the Barotse Floodplain, the Kafue Flats, Busanga and Lukanga Swamps, Lake Mweru, Mweru Marsh and Bangwuelu/Luapala/Chambezi system in Zambia and a number of smaller floodplains and wetlands comprise this ecoregion.

Mostly scattered across the Central African Plateau between 1000 and 1200 millimetres (mm) above sea level, the underlying geology of this ecoregion consists largely of Precambrian volcanics, granite, serpentine and sandstone rocks. The gentle warping of the plateau surface is largely responsible for the seasonal flooding of the region that has given rise to the wetlands that make up this ecoregion. Alluvial soils are the predominant underlying wetland substrate, and often consist of heavy clays. These soils are generally waterlogged for part of the season, with a pH ranging from weakly alkaline to weakly acidic. Several of the wetlands are known to have a peaty organic horizon that has a pH as low as 3.5.

This ecoregion experiences a seasonal tropical and hurricane climate, with most of its rainfall concentrated in the hot summer months between November and March, to the south, and November to the end of May in the north. Harsh droughts are normal since recorded history, lasting up to seven months mark the cooler season. Mean maximum temperatures range anywhere between 18o and 27oC, depending on altitude, while mean minimum temperatures are typically between 9o and 18oC. The variation in rainfall across the ecoregion is significant, with Lake Bangweulu experiencing annual rainfall in excess of 1400 mm, while rainfall as low as 450 to 600 mm per annum is characteristic in the Okavango Delta of Botswana, and along the Ugalla River and Moyowosi system in Tanzania. Inundation of these wetlands is, however, contingent not only on localized rainfall, but on the climatic conditions occurring in the upper sections of the catchments.

This ecoregion falls within the Zambezian Phytochorion, the largest of the centres of plant endemism within Africa. Although mostly characterized by a mosaic of edaphic grassland and semi-aquatic vegetation, local variation in flora between wetlands is notable due to the wide distribution of the flooded areas that constitute this ecoregion. In addition, the seasonal fluctuations in water levels and ranges in tolerance of different plants and plant communities to water-logging result in a complex, constantly changing mosaic of edaphic grasslands interspersed with permanent swamp vegetation as well as grassland and thicket communities found only on better-drained substrates. Grasses belonging to the genera Acrocera, Echinochloa, Leersia, Oryza, Phragmites, Typha and Vossia, together with Cyperus papyrus dominate most of these wetlands. Herbaceous freshwater swamps and aquatic vegetation are also common. In some swamps, it appears as if growth of swamp vegetation itself has caused sufficient obstruction to retain the water. In some of the wetlands in this ecoregion, halophytic vegetation is also found.

Elevated topographic areas within these wetlands support various types of vegetation. These include pervasive termite mounds with distinctive patches of grassland and thicket, Zanzibar-Inhambane lowland rainforest in the Kilombero valley, and woodland vegetation in Ugalla. Miombo or mopane woodlands often surround these wetlands, although dry forest, secondary grasslands and Itigi thicket also occur locally.

The more extensive floodplains of this ecoregion have historically provided important habitat for humans. For example, the Lozi people have traditionally occupied the Barotse floodplains. Their livelihoods have been closely linked to the seasonal flooding of the area. The Kafue Flats have been used by the Tonga people for cattle rearing and limited cultivation. However, much of the area comprising this ecoregion falls into areas affected by tsetse flies, vectors of sleeping sickness or trypanosomiasis, which affects both livestock and humans. This factor, in combination with the fact that many of the wetlands are at least afforded partial protection, may explain the relatively low overall human populations in many of these floodplain regions. However, human populations do tend to concentrate on the edges of floodplains. For example, just under half the population of the Barotse plain districts occurs in close proximity to the floodplain. Reliable data on actual human population densities for this ecoregion are, however, difficult to find.

Biodiversity Features

In general, there are rather few endemic species in this ecoregion, but there are high levels of species richness. For example, there are no endemic mammals in the ecoregion. However, it supports high mammal species richness, including huge herds of large mammals that still undertake some seasonal migrations. Red lechwe (Kobus leche) populations are known to exceed 20,000 in Moremi Game Reserve, while more than 35,000 individuals of the endemic subspecies, the Kafue lechwe (Kobus leche kafuensis) have been recorded in Lochinvar National Park. Moreover, the largest remaining population of Puku (Kobus vardoni) is found in the Kilombero Valley and more than 20,000 Cape buffalo (Syncerus caffer) occur in the Moyowosi delta of northwestern Tanzania. Many of the ungulates that inhabit this ecoregion move seasonally through the floodplain in response to the fluctuating water levels. Tsessebe (Damaliscus lunatus), which are mostly restricted to seasonally flooded grasslands, are known to follow the receding waters in the dry season and to retreat to higher ground when the waters rise. Wildebeest (Connochaetes taurinus) and Oribi (Ourebia ourebi) also widespread and extensive floodplains as well as grasslands, although the latter favor less waterlogged areas such as termitaries, where herbs and woody growth provide food and cover. Waterbuck (Kobus ellipsiprymnus), puku (Kobus vardoni), Southern reedbuck (Redunca arundinum), and Sitatunga (Tragelaphus spekei) are also common inhabitants of the floodplains, although these species tend to prefer the reed beds or more wooded vegetation on the margins of the floodplains. Greater kudu (T. strepsiceros), Hartebeest (Alcelaphus buselaphus), Steenbok (Aepyceros melampus), Sable (Hippotragus niger) and Roan antelope (H. equinus) are found in fewer numbers, generally preferring the woodland margins of inundated grasslands. Other herbivores attracted to these verdant pastures include elephant (Loxodonta africana), Burchell's zebra (Equus burchelli) and Eland (Taurotragus oryx). Hippopotamus (Hippopotamus amphibius) is found in almost all of the ecoregion’s prominent floodplains, with a population numbering between 2000 and 3000 individuals in Mweru Marsh.

Although many mammals are common to the entire ecoregion, the disjunct nature of the Zambezian flooded grasslands has resulted in fairly distinct species compositions. For example, black lechwe (Kobus leche smithermani), tsessebe, and sitatunga dominate the Bangweulu basin, while the Kafue lechwe and Burchell's zebra are amongst the most prominent herbivores in Kafue Flats. Similar patterns are evident with the top predators, where different species tend to predominate in different areas. These large carnivores include Lion (Panthera leo), Leopard (P. pardus), Cheetah (Acinonyx jubatus), Spotted hyaena (Crocuta crocuta) and Painted hunting dog (Lycaon pictus). Smaller predators characteristic of this inundated ecoregion include the water mongoose (Atilax paludinosus), the Cape clawless otter (Aonyx capensis) – common at Lake Chilwa – and the Spotted-necked otter (Hydrictis maculicollis), an animal that requires clear waters and shows preference for rocky undisturbed areas without turbid inflow.

This ecoregion supports a high avifaunal diversity, and provides important habitat to a range of wetland birds. The ecoregion also provides habitat for endemic and near- endemic bird species, as well as five others that are considered globally rare. The only strict endemic is the Kilombero weaver (Ploceus burnieri, VU)). It is known from one locality along the Kilombero River, where it is seasonally common in the riverside swamps that are fringed with tall Phragmites mauritianus reed beds. Two other birds are nearly endemic to the ecoregion. Chaplin's barbet (Lybius chaplini) is endemic to south central Zambia, concentrated in the Kafue basin between Kafue National Park and Lusaka. Reliant on miombo woodland or open country bearing fruiting trees, this species is mostly found on the woody margins of flooded grassland areas. The Tanzania masked weaver (Ploceus reichardi) occurs only in a few swamps in western Tanzania, northern Zambia, and adjacent areas of the Democratic Republic of Congo (DRC). Two further restricted range birds, the Katanga masked weaver (Ploceus katangae) and the grey-crested helmet-shrike (Prionops poliolophus) occur in this ecoregion. In addition, this ecoregion falls within the center of distribution of the globally threatened slaty egret (Egretta vinaceigula, VU). Largely restricted to this ecoregion, this species is an uncommon resident of the marshes and floodplains of the Okavango, Chobe, and the Caprivi Strip as well as from the Zambezi Valley northwards to the Bangweulu swamps. Other globally threatened species recorded in this ecoregion include wattled crane (Bugeranus carunculatus,VU), which has its main breeding populations in the wetlands of Zambia, including the Kafue Flats and the Bangweulu and Busanga swamps, corncrake (Crex crex, VU), lesser kestrel (Falco naumanni, VU), great snipe (Gallinago media) and shoebill stork (Balaeniceps rex). Other rare birds found in these wetlands include the long-toed flufftail (Sarothrura lugens) and the white-headed plover (Vanellus albiceps).

Amphibian and reptile species richness and endemism are not high, with only two strictly endemic species. The merera toad (Bufo reesi) is known only from the Kihanzi-Kilombero floodplain of the Mahenge District of southern Tanzania. A number of amphibian associates are found within the ecoregion,including the Beira Toad (P. beiranus), African clawed toad (Xenopus laevis), African ornate frog (Hildebrandtia ornata), Boettger’s metal frog (Cacosternum boettgeri), Common reed frog (Hyperolius viridiflavus), and the Lukula grass frog (Ptychodena taenioscellus).

Other The Barotse water snake (Crotaphpeltis barotseensis) only occurs on the Kalabo Floodplain of Barotseland. The Nile crocodile (Crocodylus niloticus) and the slender-snouted crocodile (Mecistops cataphractus) are also common in certain floodplains of the ecoregion.

Information on plant and invertebrate diversity of these wetlands is relatively sparse. However, it is known that sixteen butterfly species are strictly endemic to the Zambezi River Basin.

Current Status

Areas of largely untouched habitat still exist in many of the wetlands of this ecoregion, particularly in the Okavango Delta, the Bangweulu, Moyowosi and Kilombero swamp systems and the Kafue Flats.

Six of the wetlands comprising this ecoregion are designated as protected areas according to IUCN criteria, and three floodplains (Okavango, Bangweulu Swamps, Lake Chilwa) are designated as Ramsar sites. Other portions of the ecoregion are contained within Game Controlled Areas or similar designations that allow controlled hunting. There are a number of additional areas proposed to be Ramsar sites.

The most prominent wetland in this ecoregion is the Okavango Delta. Mainly protected within the Moremi Game Reserve and two adjacent wildlife management areas, the delta remains relatively unspoiled and contains a unique array of fauna and flora. This inland delta covers approximately 68,640 square kilometers (km2) of northern Botswana and consists of stretches of pristine clear waters, wooded islands, inundated grasslands and extensive reed and papyrus beds. It is home to approximately 1060 plant species; the animal diversity of the area is relatively high with roughly 32 large mammals recorded. Designated as a Ramsar site of international importance, the most outstanding feature of this vast area is its prolific birdlife, with more than 650 species recorded.

Zambia contains several wetland protected areas belonging to the ecoregion. Lochinvar and Blue Lagoon National Parks and the Bangweulu-Kafinda Game Management Areas are considered key locations for threatened antelopes in sub-Saharan Africa. In addition, Bangweulu is also designated as a Ramsar site, with more than 400 bird species inhabiting its roughly 2500 km2 area. This game management area contains 17 rivers that flow into the Bangweulu River Basin, where the waters spread out to form expansive floodplains and marshes. Bangweulu, meaning "where the water meets the sky", this is one of the few remaining true wilderness areas within this ecoregion.

In Tanzania, there are three important wetlands in the ecoregion. The Moyowosi and Ugalla systems are particularly worthy of mention. During the peak flood season these wetlands cover roughly 10,000 km2, stretching for about 300 km from north to south and 70 km east to west. These floodplains include large areas of miombo woodlands, with permanent swamps being restricted to the margins of major rivers and their associated lakes. Both these rivers with their associated game reserves are proposed as Ramsar sites in Tanzania. The Kilombero Valley system is also mainly contained within a Game Management Area and is a proposed Ramsar site; its wetlands cover up to 3000 km2.

Types and Severity of Threats

The combination of historically low human population densities, largely due to water-borne diseases, and the presence of tsetse flies – in and around the wetlands of this ecoregion, and the more recent establishment of parks and reserves centered on several of these floodplains, has ensured that many natural habitats remain. Even in areas that have been home to significant numbers of people, the disturbances and impacts of human activity needs to be seen in the context that wetlands and their surrounding landscapes are inherently unstable and are continually evolving and changing. Wetland organisms and communities have altered and adapted over millennia to thrive in an environment that is in continual flux.

However, with rapidly expanding native human populations and increasing use and manipulation of natural resources, this ecoregion is facing increasing threat from a variety of sources. These include: increased land clearance and deforestation (Land-use and land-cover change) of swamp forests and surrounding woodlands, poaching, pollution and eutrophication, and the modification of natural flow regimes.

While several of the wetlands such as the Barotse Floodplain and the Kafue flats have been occupied by humans for centuries, large changes are becoming evident in many of the areas as human activities and land-use intensifies. These changes are largely a result of increased human populations, increased levels of technology available, increased government and donor inputs and better control of tsetse fly. Many of the seasonally flooded grasslands have historically been used for grazing during the dry season. However, several areas that only used to be seasonally grazed are now permanently occupied and cultivated. Although large-scale cultivation is relatively uncommon, subsistence agriculture is practiced by as much as 75 percent of the population of certain areas. Growing staple and cash crops such as maize, cassava, millet, sugarcane and tobacco, therefore, pose significant threats to areas of the ecoregion. Increased wealth and government/aid inputs, as well as extensive tsetse fly control have contributed to a large increase in cattle numbers over the past few decades; as many as 250,000 head of cattle are said to graze only in the Kafue Flats. Not only does livestock overgrazing eliminate the herbaceous components of the vegetation, high numbers of cattle generally displace game, as there is less suitable natural habitat available.

Fire has always played a role in the ecology of grasslands and woodlands. However it has most probably never been as frequent as it is now, due to the increase of fire setting by humans, including widespread practises of slash and burn agriculture. The increased incidence of repetitive fires may eliminate fire-sensitive species, thereby reducing species diversity and can also result in over-utilization of recovering areas, such as on the Barotse Floodplain.

While most of the ecoregion consists of open grasslands, deforestation and habitat fragmentation of swamp forest patches and surrounding woodlands still has a considerable impact on the healthy function of this ecoregion. Opening up the tree canopy increases the risk of fires. In addition, it reduces the habitat for many mammals such as greater kudu, hartebeest, sable and roan antelope, as well as birds that rely on the woodland-grassland interface for food or shelter.

Poaching and illegal hunting for bushmeat are continuing and increasing problems in several areas of the ecoregion. Hunting of large animals and their gradual replacement by domestic cattle, goats and sheep threaten many mammal populations in the wetlands. This is a problem in all of the wetland areas, but especially in those with higher population densities where there is little effective protection. In the Kafue Flats in Zambia, for example, large mammals are now confined almost exclusively to two national parks (Blue Lagoon and Lochinvar National Parks, a combined area of only 860 km2). In other areas, the local human populations have removed these species for food or to reduce crop-raiding problems. Furthermore, although many of these floodplains are officially protected, ineffective management and lack of funds has resulted in uncontrolled poaching of animals such as hippo, elephant, and rhino. In addition, overfishing is also becoming an issue of increasing concern. For example, over 50 percent of the fish production for Zambia comes from the Bangweulu basin and Kafue Flats.

The biodiversity and integrity of many of the wetlands of the ecoregion are further threatened by pollution, especially close to urban centers, where industrial, domestic wastes and agricultural runoff flow unchecked into the water catchments. It is estimated that these land-based activities contribute as much as 80 percent of the aquatic pollution in Tanzania and pose a major threat to biodiversity. In some of these wetlands and floodplains, eutrophication of waters has led to serious infestations of aquatic weeds such as Salvinia molesta, Eichornia crassipes and Pistia stratiodes, which has resulted in decreased fish productivity, as well as displacement in natural fauna and flora, and overall loss of biodiversity. Pollution from pesticides such as DDT (dichloro-diphenyl-tricholoroethane) and chemical sprays used in tsetse fly control may also be severely affecting the biota of this ecoregion. Floodplains occurring in the vicinity of mines face serious problems relating to water pollution both in terms of chemical wastes and accumulation of heavy metals. Reports from the Copperbelt have shown that the copper content in rivers found in affected areas was up to 80 times higher than the acceptable level. The Kafue River is known to be contaminated with heavy metals and agricultural run-off. There is therefore considerable concern regarding the effects that bioaccumulation of these contaminants may have on animals that depend on the Kafue Flats for their sustenance.

Changes in the frequency, magnitude and variability of rainfall due to climate variation since the beginning of the Holocene era, as well as the anthropogenically induced modification of natural flow regimes through water abstraction and dam construction are of considerable threat to tghis ecoregion. Water diversion for irrigation and hydroelectric dams has already affected some floodplain systems. For example, the Itezhitezhi Dam on the Kafue River has changed the flood regime such that unseasonal flooding occurs on the flats, threatening the breeding sites of the wattled Crane, Grus carunculatus, and grey crowned (Balearica regulorum) cranes. The area of land available for wildlife and for human uses such as fishing and recession farming has been further reduced by dam operations. Prior to the dam, about 6000 km2 of land flooded in wet years. Now the flooding generally covers 2000 km2 in normal years and 5000 km2 in wet years. Non native weeds have invaded grazing areas and cattle herders must travel much further to water their flocks because of the altered flood regime. The population of Kafue lechwe that numbered about 100,000 in 1971 before construction of hydroelectric dams dropped to nearly half that number in 1987, after dam construction due to the altered flood regime.

A similar fate could await the Okavango Delta. Botswana, Namibia, Angola and Zambia would like to extract large quantities of water from the Okavango and Kwando rivers and their tributaries for irrigation and urban water use. Namibia is currently negotiating drawing an estimated 20 million cubic metres of water annually from the Okavango River system via the Eastern National Water Carrier project. Although this is less than ten percent of the yearly flow, it is a very significant quantity of water during the dry season. Excess abstraction from the Okavango River could have deleterious effects on functioning of the delta ecosystem, as well as other effects such as the transfer of fishes from the Okavango to more southerly drainage systems.

More recent damming of rivers, and conversion to agricultural systems is resulting in loss of habitat in parts of the ecoregion. In the Okavango Delta, concern was raised that the erection of a fence to control the movement of cape buffaloes, which carry disease, would fragment the migrations of large mammals in the area. However, the fence apparently has prevented the movement of cattle into the region and, thus, has benefited local wildlife populations.

Justification of Ecoregion Delineation

All the seasonally flooded areas within the Zambeziean Phytochorion area were mapped within this ecoregion. The borders of each area roughly follow the standards for a ‘herbaceous swamp and aquatic vegetation’ unit.

This ecoregion forms part of larger complex of Caesalpinoid woodland ecoregions that support wet and dry miombo, mopane, thicket, dry forests, Baikiaea woodland, and flooded grassland habitats, among others. The dominance of Caesalpinoid trees is a defining feature of this bioregion (i.e., a complex of biogeographically related ecoregions). Major habitat types (e.g., mopane and miombo) and the geographic separation of populations of large mammals are used to discriminate ecoregions within this larger region. All of these ecoregions contain habitats that differ from their assigned biome or defining habitat type. For example, patches of dry forest occur within larger landscapes of miombo woodlands in several areas. More detailed biogeographic analyses should map the less dominant habitat types that occur within the larger ecoregions.