Itigi-Sumbu thicket

The Itigi-Sumbu Thicket is a unique but poorly understood ecoregion, best known for its impenetrably dense vegetation. It is found in two small areas in Tanzania and Zambia. The thick, primarily deciduous vegetation is so dense that people can scarcely penetrate it, and elephants forcing their way through these thickets barely leave tracks, as the shrubs spring back to their original positions. While little research has been conducted, the Itigi-Sumbu Thicket’s vegetation is unique and contains a number of endemic plants. It was once vital habitat for the black rhino, although poachers have eradicated the rhino in this ecoregion. Human populations in the area are rapidly increasing and even the thicket contained in protected areas is converted for agricultural purposes. In fact, the Itigi-Sumbu Thicket is being transformed so quickly that the Zambian portion may be lost if conservation action is not taken.

  • Scientific Code
  • Ecoregion Category
  • Size
    3,000 square miles
  • Status
  • Habitats

Location and General Description
Itigi-Sumbu Thicket is a unique but poorly understood ecoregion found in two isolated patches in Tanzania and Zambia. Its most outstanding characteristic is its dense, primarily deciduous, thicket vegetation (White 1983; Wild and Fernandes 1967). In Zambia the ecoregion lies between Lakes Mweru Wantipa and Tanganyika (Wild and Fernandes 1967) and barely extends over the border into the Democratic Republic of Congo. Tanzania's Itigi-Sumbu thicket occurs in the center of the country near its namesake town of Itigi. Itigi-Sumbu Thicket is found nowhere else, although related thicket vegetation types occur elsewhere in Zambia and Zimbabwe (White 1983; Wild and Fernandes 1967).

This unique vegetation is mapped as ‘Itigi deciduous thicket’ by White (1983). Floristically, Itigi-Sumbu Thicket is almost totally unrelated to the surrounding vegetation types (Almond 2000). Virtually no transition zone exists between the thickets and the surrounding Brachystegia-Julbernadia woodlands of the largest neighboring ecoregion, the Central Zambezian Miombo Woodlands, although the trees in the latter may be stunted where they border the thickets (White 1983). This decisive break could be due to the nature of the soils and topography, as the miombo woodlands surrounding Itigi-Sumbu Thicket tend to occur on plateaus and hills where the soils are rocky, while the thickets reside on alluvial soils lying over a hard "duri-crust" (White 1983).

The ecoregion’s climate can be divided into three distinct seasons: the cool dry season from May to August, the hot dry season from August to November, and the rainy season from November through April (Almond 2000). Available information indicates that rainfall differs between the Tanzanian and Zambian portions of the ecoregion. A long-term average of precipitation of northern Zambia indicates annual rainfall to be 1,400 mm (Moyo et al. 1993, cited in Almond 2000), although in any given year this figure may vary by as much as 50 percent (Almond 2000). In Tanzania’s central region of Singida, where the town of Itigi is located, annual precipitation is less than 500 mm ( 2000), although rainfall within Itigi itself is 600 mm per annum (FAO undated). The central region of Tanzania experiences dry season temperatures ranging between 20° to 27° C, with wet season temperatures attaining 30° C and higher ( 2000).

Zambian Itigi-Sumbu Thicket lies between 950 m and 1,200 m above sea level in the depressions between Lakes Mweru Wantipa and Tanganyika (Almond 2000; White 1983). Geologically, the ecoregion is underlain by a basement floor of granite (White 1983). Above the granite bedrock is a characteristic soil structure, consisting of seasonally well-aerated and well-watered sandy soils of 0.6 m to 3 m in depth that desiccate and harden during the dry season, with an impermeable "duricrust" of cement-like consistency beneath (White 1983).

This ecoregion falls within the Zambezian regional center of endemism and is characterized by extremely dense vegetation that is difficult for people to traverse. The vegetation grows so densely that branches of the canopy are intertwined (White 1983). Little is known about the ecology of the Itigi-Sumbu Thicket, but it has been conjectured that the thickets represent a pre-climax state that is enforced and maintained by unfavorable environmental conditions (Fanshawe 1971, cited in Almond 2000).

The Itigi-Sumbu Thicket contains at least 100 woody species (Fanshawe 1971, cited in Almond 2000), which are mostly spineless, multi-branched shrubs of 3 m to 5 m in height, characterized by Baphia burttii, B. massaiensis, Bussea massaiensis, Burttia prunoides, Combretum celastroides subsp. orientale, Grewia burttii, Pseudoprosopsis fischeri, and Tapiphyllum floribundum (White 1983, Wild and Fernandes 1967). Albizia petersiana, Craibia brevicaudata subsp. burtii, and Bussea massaiensis occasionally emerge from the lower canopy (White 1983) to form an open, upper canopy of 6-12m height (Edmonds 1976, cited in Almond 2000).

Itigi-Sumbu Thicket vegetation is generally deciduous for a 4 month period (White 1983), although within the lower canopy there are nuclei of evergreen shrubs surrounded by the more numerous deciduous species (Edmonds 1976 cited in Almond 2000). The density of the canopy prohibits the penetration of sunlight, and a herbaceous layer is therefore precluded, although the grass Panicum heterostachyum and several small herbs are found (White 1983). Both the succulent and climbing floras are depauperate (White 1983).

Biodiversity Features
Itigi-Sumbu Thicket ecoregion is unique due to the presence of strictly endemic species (Kideghesho 2001, National Forestry Programme, undated), although specific information is largely unavailable. It should also be noted that other vegetation types, such as grasslands, open savanna, and closed miombo woodlands, are interspersed with the Itigi-Sumbu Thicket, (Edmonds 1976, cited in Almond 2000). The savanna woodland components of the ecoregion also contain endemic species. Termitaria within the thickets harbor species otherwise foreign to the community, such as the large candelabra euphorbia, Euphorbia bilocularis (White 1983). From an evolutionary perspective, Itigi-Sumbu Thicket may represent a relict vegetation type from a period when the climate was cooler and moister, and therefore more favorable for evergreen species (Smith, pers. comm. 2000, cited in Almond 2000).

Three near-endemic reptile species are known from the ecoregion: Urungu beaked snake (Rhinotyphlops gracilis), four-fingered skink (Sepsina tetradactyla), and Latastia johnstoni. The Itigi-Sumbu Thicket ecoregion was once an important stronghold of black rhino (Diceros bicornis) (Kideghesho 2001), before their eradication by poaching.

No information is available on migration patterns through the ecoregion, predator-prey interactions, intact biotas, or natural disturbance regimes. Intense fires are known to destroy thicket vegetation, although these are unable to penetrate to any significant depth (White 1983). Aerial photos of Itigi-Sumbu Thicket in Zambia have shown the occurrence of both a uniform and a clumped vegetation structure (Fanshawe 1971), and it is possible that the latter is the result of modification by elephant and buffalo (Syncerus caffer).

Current Status
The ecoregion is endangered, with 50 percent of the Tanzanian portion already cleared (Kideghesho 2001), and as much as 71 percent of the Zambian portion cleared (Almond 2000). Although large parts of the ecoregion are conserved in Zambia, it appears that thicket clearing takes place even within protected areas (Almond 2000). The part of the ecoregion occuring in Tanzania is completely unprotected (Kideghesho 2001, Stuart et al. 1990). The largest known blocks of intact Itigi-Sumbu Thicket vegetation are found on the northern shores of Lake Mweru Wantipa (Almond 2000), and in the eastern portion of the Zambian Itigi thicket, where little clearing had taken place as of 1992.

The protection status of this ecoregion differs markedly between the Zambian and Tanzanian portions. In Zambia, elements of the Zambian Itigi-Sumbu Thicket are protected within two protected areas on the shores of Lake Mweru Wantipa, in Mweru Wantipa National Park and Tabwa Reserve on the northern and eastern shores, respectively (Almond 2000). Another portion falls within Nsumbu National Park, although the extent of thicket vegetation within these areas is not known.

Unfortunately, protected status has not prevented the removal of significant amounts of Itigi-Sumbu Thicket within Mweru Wantipa National Parks and Tabwa Reserve (Almond 2000). Analysis of satellite imagery has shown that the majority of existing thicket vegetation within these two protected areas was cleared between 1976 and 1999, indicating a lack of effective policing (Almond 2000). The remaining Itigi Thicket in Zambia has become increasingly fragmented (Almond 2000). Each of the plots assessed within Almond’s (2000) satellite image analysis of Itigi-Sumbu Thicket show the vegetation to be in some stage of clearing, which has created a mosaic pattern of cleared and intact thicket. Tanzania’s portion of the ecoregion is presumably similarly fragmented, as 50 percent of it has already been removed (Kideghesho 2001).

Types and Severity of Threats
The greatest threats to Itigi Thicket come from rapid human population growth, which amounts to 3.7 percent growth per year in Zambia (Moyo et al. 1993, cited in Almond 2000), and around 2.8 percent per annum in Tanzania, and the attendant demand for land and resources (Almond 2000).

In northern Zambia, fuelwood requirements account for 2,000 km2 of deforestation annually, while a shifting form of cultivation known as chitemene causes the further conversion of 9,000 km2 (Moyo et al. 1993, cited in Almond 2000, Almond 2000). Although most of this clearance comprises miombo and related woodland habitats, the Zambian portion of Itigi-Sumbu Thicket is located in this region of rapid habitat transformation. As a result, the majority of Itigi-Sumbu Thicket has already been removed within the Zambian section of the ecoregion. Although Almond’s (2000) study, defined the boundaries of Itigi thicket somewhat differently than this account (only the eastern half of his study area is relevant to this report), it reveals that as much as 71 percent (628 km2) of Itigi thicket was removed between 1976 and 1999. Transformation rates within Mweru Wantipa National Park and Tabwa Reserve were as high as 66 to 67 percent for the same time period, indicating that protection has not been strictly enforced (Almond 2000). During this period, the margins of thicket on the eastern shores of Lake Mweru Wantipa retreated between 1.3 m to 7 km as a result of deforestation. Even the most significant remaining block of intact thicket on the northern shore of the Lake is receding by 20 m per annum (Almond 2000).

In summary, the Itigi-Sumbu Thicket in Zambia is reduced by 3 percent each year, a figure closely tied to the 3.7 percent annual population growth rate of northern Zambia (Almond 2000). Thicket deforestation patterns are correlated with proximity to human settlements, which have grown on average by 15 percent between 1984 and 1999 within the vicinity of Zambia’s portion of the ecoregion (Almond 2000). If this pattern of destruction continues unabated, it is estimated that the total extinction of Zambia’s Itigi-Sumbu Thicket will occur between 2009 and 2019, although the more intact segments such as that on the northern shore of Lake Mweru Wantipa may persist to 2050 (Almond 2000). Unfortunately, there is no hope of the thickets regenerating following the abandonment of cultivated areas, as the highly sensitive thicket vegetation is replaced by Lake Basin chipya vegetation (Fanshawe 1971, cited in Almond 2000), which White (1983) termed Zambezian ‘chipya’ woodland and wooded grassland. Chipya vegetation can have a highly varied structure that is strongly influenced by fire and dominated by species not found in either miombo woodlands or Itigi-Sumbu Thicket (White 1983).

Justification of Ecoregion Delineation
Itigi-Sumbu thicket vegetation occurs on specialized soils that result in dense deciduous thicket. Although it occurs throughout Caesalpinoid woodlands, only the two larger areas mapped by White (1983) are delineated at the continental scale. This ecoregion is based on White’s ‘Itigi deciduous thicket,’ and includes the adjacent Mweru swamps.

This ecoregion is 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.

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Fanshawe, DB. 1971. The vegetation of Zambia. Forest Research Bulletin, Number 4, Government Printer, Lusaka.

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Wild, H. and A. Fernandes, editors. 1967. Vegetation map of the Flora Zambesiaca area. Supplement: Flora Zambesiaca. Salisbury.

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Prepared by: Mike Bingham, Lyndon Estes