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Chapter 3 TSETSE ECOLOGY BANDS, A NORTH-SOUTH GRADIENT


The effect of climate and vegetation on tsetse ecology has been studied in detail since the 1930s (e.g. Nash, 1937; Buxton, 1955; Ford, 1963). In West Africa, as depicted in Figure 2, a series of climatic bands follow a north-south gradient ranging over less than 2 000 km from extreme arid (Sahara desert) to tropical rainforest and humid coastal conditions (Golfe de Guinée). The various tsetse species are adapted to different ecoclimatic conditions and this north-south climatic gradient affects their distribution dynamics and seasonal fluctuations that vary from:

In the north (A), arid conditions prevent fly spread. Locally riparian vegetation constitutes suitable niches for localized, welldemarcated pockets of tsetse populations. Outside these favourable microclimates tsetse hardly survive and it would appear that there are no links between pockets (i.e. there are no population exchanges or population migratory fluxes) except occasionally and in spatially limited neighbouring areas during the rainy season.

In the intermediary band (B), climatic conditions and vegetation become gradually more suitable. Distinct fly pockets tend to merge and tsetse distribution patterns become more linear along main streams. Tsetse populations still remain concentrated in pockets during the dry season, but during the rains flies spread over larger parts of the river systems including important tributaries and savannah buffers.

FIGURE 2
Hypothetical north-south tsetse ecology gradient in West Africa, linked to climatic bands

The maps on the right show length of growing period layers, i.e. number of vegetative days per year, as a surrogate for prevailing ecoclimatic conditions. The distribution of the three main tsetse species present in the dry northern band is also given.

In the humid south (C), there are no climatic limitations to fly distribution. Flies are no longer restricted to river systems but are also present in surrounding humid woodlands and forests. Seasonal variation has a greater affect on fly numbers than actual spatial distribution patterns.

In the more humid parts of West Africa, the wide range of favourable climatic conditions (rainfall, humidity and temperature) and the ubiquitous availability of water favour a great variety of suitable tsetse habitats. Though it has been shown that minimal changes in climatic data sets may influence both tsetse distribution limits (Rogers, Hay and Packer, 1996) and abundance (Hendrickx et al., 2001b), tsetse tend to be more widespread when compared to the dry northern part. In the latter area the presence of tsetse, within their distribution range, is more markedly restricted to suitable vegetation patches and by seasonal vegetation changes. In addition, the number of tsetse species (biodiversity) tends to increase with more luxuriant vegetation conditions (see Figure 3).

FIGURE 3
Relationship between vegetation and number of tsetse species
(sub-Saharan Africa)

A strong positive correlation between increasing vegetation activity and biodiversity is shown.

Furthermore, these favourable conditions, combined with the impact of agricultural activities and the number of tsetse species involved, make the dynamics of various fly populations complex. Savannah and forest flies are more sensitive to encroachment of their habitat by agricultural activities, like fuelwood gathering, as well as the reduction of natural host populations because of poaching (Nash, 1948). With regard to riparian species, it has been shown in Togo (see case study reported in Chapter 4) that while in the northern part of the country higher agricultural density activity contributes to reduce significantly the presence and abundance of Glossina tachinoides, in the humid south, at comparable levels of agriculture intensity, G. palpalis palpalis is far less affected (Hendrickx et al., 1999a).

FIGURE 4
FAO-IAEA desk study to select priority areas for area-wide vector eradication in West Africa
See Box 3 for additional information

In turn, human activities including crop and livestock agriculture may create new habitats favouring the persistence of peridomestic fly populations that are difficult to detect and may act as major foci for both human and animal trypanosomiasis. Fly populations may also be preserved locally in protected holy forests (Hendrickx et al., 1999a) and in areas where the soil structure is not suitable for agriculture (de la Rocque, 1997) thus maintaining epidemiological hot spots (see the case study reported on page 35).

Different settings between humid southern and dry northern West Africa call for an adapted approach. While area-wide vector and disease management may be part of the answer in the north, in the south more complex situations may need more integrated answers. This viewpoint was adopted in an FAO-IAEA-funded desk study aiming at identifying priority areas for trypanosomiasis management. A summary of results obtained in the study is shown in Figure 4. This desk study was based on a set of assumptions (see Box 3). Areawide vector suppression eventually leading to elimination should focus on areas where:

BOX 3

Selection of areas for area-wide vector removal (FAO-IAEA desk study)

Tsetse vulnerability

Climate: tsetse populations are likely to be more fragmented and easier to remove permanently in the northern dry band. In addition, reinvasion is less likely because gaps of adverse conditions prevent fly dispersal. For this study the limit of this “dry band” was empirically set to a Length of Growing Period (LGP) of 170 days.

Human population and land-use pressure: with increasing human population, the number of natural tsetse hosts will drop and an increasing amount of the fly’s habitat will be cleared through cultivation and fuelwood consumption. As a result tsetse populations are expected to drop and/or previously continuous distributions may become fragmented. This was modelled by overlapping areas of (i) expected high human population impact (Reid et al., 2000) and (ii) predicted high cropping intensity and livestock densities (PAAT-IS). It is important to mention that agricultural practices might create new habitat types, especially for riparian tsetse. Nevertheless such habitats are expected to be relatively isolated and therefore vulnerable once identified.

Reinvasion risk

Reinvasion pressure in at least part of the northern band of the fly belt is likely to be low due to: (i) vector populations being restricted to isolated pockets in an otherwise marginal habitat, (ii) interrupted linear vector distribution patterns due to habitat restrictions and (iii) reduction of reinvasion pressure due to high land-use pressure.

Historically, similar conditions have occurred in northeastern Nigeria where flies were removed from a large area, and even without barriers reinvasion has been very limited; human-induced activities and land pressure de facto prevented reinvasion (Bourn et al., 2001).

Fly reinvasion risk was modelled for West Africa based on observed climatic and mixed farming patterns of the Nigerian cleared areas.

Potential benefit

  • Cereal production: FAO-IIASA data.

  • Livestock production: potential impact on livestock numbers should tsetse be removed (PAAT-IS).

Selected river basins

River basins were identified as “vector control” units. River basin clusters within areas of high tsetse vulnerability, low reinvasion risk and high benefit were earmarked as priority areas for area-wide vector eradication.


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