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Trente ans de lutte contre l’onchocercose en Afrique de l’Ouest. Traitements larvicides et protection de l’environnement

 | 
Laurent Yaméogo
, 
Christian Levêque
, 
Jean-Marc Hougard

Environmental Impact Assessment of Settlement and Development in the Upper Léraba Basin

4. Ecological Impact of Human Activity using Medium and Local-Scale Assessments

Texte intégral

Medium Scale Land Utilization Analyses, 1972, 1983 and 1993

1From the maps presented in Figures 4.1, 4.2 and 4.3, it is to be noted that there have been appreciable extensions of settlement and of cultivation in the Land Settlement Analysis area (LUA) since 1972, and corresponding decreases in the savanna woodland. More detailed analytical information on these developments is presented below.

The Situation in 1972

2From the data presented in Figure 4.1, it is evident that in 1972, before onchocerciasis control measures were introduced, farming communities were largely confined to villages on the higher ground, 10 km or more from the center-line of the Léraba valley: Nadera, Létiéfesso, Katierla, Nafona, Mambiré, Titédougou and Danguandougou in Burkina Faso; Mambiadougou, Katierkpon and a village whose name is not known (denoted by NNN on the map) in Côte d’Ivoire. From an epidemiological viewpoint, many of these settlements were classified as “front-line villages,” and some of them (such as Danguandougou) suffered the devastating consequences of unabated blinding onchocerciasis.

3However, there were some isolated plots of cultivation, or smallholdings closer to the main rivers, especially in the hypo-endemic areas that flanked the L.or; much less so in the Southern (hyper-endemic) part of the area where vector Annual Biting Rates (ABR) and onchocerciasis Annual Transmission Potentials (ATP) were extremely high. In this context, it has to be recalled that, according to WHO (1987), an area under vector control could only be considered safe for resettlement if the ABRs were less than 1000 and the ATPs less than 100 for two consecutive years.

4Not surprisingly, there were no settlements at the sites of these smallholdings, and we must conclude that, either from fear of contracting onchocerciasis or because of the intensity of blackfly and tsetse bites, these fields were cleared, sown and harvested by farmers who commuted to them from the relative safety of the villages located well away from the rivers.

Table 4.1 ABR and ATP values at Léraba Bridge, 1975-1993

Table 4.1 ABR and ATP values at Léraba Bridge, 1975-1993

• Phasing out of vector control operations.

4.1 Map of the LUA Area Showing Areas Cultivated in 1972. burkina faso/cote d’ivoire. LERABA BASIN / BASSIN DE LA LERABA1972

4.1 Map of the LUA Area Showing Areas Cultivated in 1972. burkina faso/cote d’ivoire. LERABA BASIN / BASSIN DE LA LERABA1972

4.2 Map of the LUA Area Showing Areas Cultivated in 1983. burkina faso/cote d’ivoire. LERABA BASIN / BASSIN DE LA LERABA1972-1983

4.2 Map of the LUA Area Showing Areas Cultivated in 1983. burkina faso/cote d’ivoire. LERABA BASIN / BASSIN DE LA LERABA1972-1983

The Situation in 1983

5By 1983 the LUA had been subjected to nine years of OCP vector control operations, which had significantly reduced vector density and onchocerciasis transmission. Although ABR and ATP values did not meet the specified requirements for safe resettlement (WHO 1987) until 1986 (see Table 4.1), there had clearly been significant developments between 1972 and 1983, both with regard to settlement and to agriculture (see Figure 4.2).

6With regard to human settlement, it was noted that over the decade 1972-1983 four new villages came into existence: three in Côte d’Ivoire and one in Burkina Faso (Lérabadougou, which was established in 1981). In addition, Danguandougou had moved to a more southerly location. With one exception, all these villages were located along the main surfaced road connecting Burkina Faso with Côte d’Ivoire. They were all at the center of local zones of more or less intensive cultivation (which also extended to the east of the main road in some cases).

7The exception was a village located in the headwaters of a southbank tributary of L.oc in the western part of the LUA, about 3 km from L.oc, and about 19 km north of the town of Kaouara (located to the south on the main surfaced road, and too far south to be shown on the maps), to which it was connected by an unsurfaced road. Motorable tracks connecting upstream settlements and cultivated areas with that road were noticeable features of the increased settlement infrastructure in the area.

8With regard to agricultural development, there had been a significant extension of land under cultivation over the decade preceding 1983; mainly in the savanna woodland, but also on the banks of some of the Léraba tributaries. The area of land utilized in 1983 was at least 300% larger than that utilized in 1972.

9It was also noted that smallholdings, distant from any village, still continued to account for the greater proportion of land under cultivation in 1983. However, not all of this land was being managed on a commuter basis, which appeared to have been the case in 1972. By 1983, many of the smallholdings were being tended by family groups living on-site, in small homesteads, interconnected by networks of footpaths.

10The evidence suggests that one of the factors contributing to the great expansion of agricultural land during the pre-1983 decade, was the introduction of cotton as a cash crop.

The Situation in 1993

11OCP vector control operations in the LUA ceased in early 1990, meaning that the land utilization pattern observed in early 1993 reflected the situation in the third post-control year. Although the blackfly population of the Léraba Bridge area had returned to its pre-control level of density, the ATP remained at an acceptably low level.

12Figure 4.3 clearly illustrates that over the period 1983-1993 there had been further significant developments, both with regard to settlement infrastructure and land utilization.

13It was very noticeable that over the decade preceding 1993, there had been a marked expansion of the area supporting homestead settlements and associated smallholder farms, particularly on the eastern side of L.ss (in Burkina Faso). It was equally apparent that on the western side of L.ss in Côte d’Ivoire, there had been a considerable increase in the number of clearly-defined villages surrounded by zones of intensive cultivation, and an expansion of the road network (not shown on the map). Ten years previously, there had been only one village in the upstream part of this area, i.e. away from the main road. In 1993, there were six.

14It was also noteworthy that some of the villages that were considered as “new” in 1983, had expanded very considerably over the following decade, and in this respect it is pertinent to digress by briefly describing the evolution of Lérabadougou.

15The first settlers (only a few small family groups) arrived at the Lérabadougou site in 1981, and occupied a surface area of less than one hectare. In early 1993 the village housed about 400 people and covered an area of at least 5 ha. Initially, farmers were only concerned with subsistence crops, but since 1984 became more committed to cotton-growing.

Ecological Significance of the Observed Changes in Land Utilization

16It is now evident that progressively increasing demands for new farmland, for land for settlements, for wood (for building timber and as domestic fuel) and for improved pasture (to provide thatching material and better grazing for livestock), have resulted in some major ecological changes in the LUA over the last two decades.

4.3 Map of the LUA Area Showing Areas Cultivated in 1993. burkina faso/cote d’ivoire. LERABA BASIN/BASSIN DE LA LERABA1983-1993

4.3 Map of the LUA Area Showing Areas Cultivated in 1993. burkina faso/cote d’ivoire. LERABA BASIN/BASSIN DE LA LERABA1983-1993

4.4 Map of the LUA Area Showing Areas Cultivated, Remnants of Savanna Woodland and Floodplain Grassland, 1993. burkina faso/cote d’ivoire. LERABA BASIN / BASSIN DE LA LERABA1993 LAND USE

4.4 Map of the LUA Area Showing Areas Cultivated, Remnants of Savanna Woodland and Floodplain Grassland, 1993. burkina faso/cote d’ivoire. LERABA BASIN / BASSIN DE LA LERABA1993 LAND USE

17Physical changes in the two most extensive components of the terrestrial environment, woodland savanna and floodplain grassland, are elaborated below.

18Woodland savanna. From the data presented in Figure 4.4 it is evident that by 1993 as much as 75% of the original savanna woodland had been destroyed or was in the process of becoming deforested, and it is reasonable to suppose that in the not too distant future, the woodland resource will become totally depleted, with three important consequences:

  • overall environmental deterioration and a diminution of the carrying capacity of savanna land,

  • threatening of the riverine forests of the main Léraba rivers, through settlers’ further search for wood,

  • increased albedo.

19Floodplain grassland. As illustrated in Figures 4.4 and 4.5, there had been little agricultural incursion into the floodplain grasslands of the main Léraba rivers, except in the case of L.or, where some rice was grown. Certainly in the Southern part of the area there had been only minor incursions, which were not likely to have been of any major environmental consequence.

Medium and Local-Scale Assessments of the Status of Riverine Forests

20The detailed vegetation analysis presented in Figure 4.6, constitutes the baseline for the stretch of the main Léraba river immediately above and below the OCP Hydrobiology Station at Léraba Bridge, and has been made in accordance with guidelines established by the Ecological Group of the OCP (WHO, 1992). By comparing this map with data to be collected during future vegetation surveys, it will be possible to monitor accurately any changes which may have taken place over the intervening years, and assess the impact of those changes on the aquatic environment.

21At this juncture, it is appropriate to recall the important roles played by riverine forests in maintaining the ecological well-being and productivity of large perennial rivers (such as the Léraba and its two principal tributaries) and their adjacent floodplain grasslands. Although some (albeit, limited) publicity has been given to the ecological importance of the riverine forests of Africa—see Balk & Koeman (1984), Koeman & Dejoux (1990), and WHO (1992)—in general the reasons for attempting to conserve Africa’s riverine forests are not always fully appreciated, either by the general public, or by authorities concerned with agricultural and natural resource development.

22It cannot be overemphasized that riverine forests, as exemplified by the Léraba, constitute the most concentrated vegetal biomass of the whole river basin ecosystem, arranged like close-fitting protective shields along the rivers. And, it is now well known that, on a regional scale, loss of riverine forest has contributed to increased albedo, with resultant reductions in rainfall, water tables and duration of river flow, and to increased river flow velocity and soil-surface evaporation. The more specific regulative functions of the arboreous and subterranean components of riverine forests are summarized in Annex 2.

23From the vegetation analysis presented in Figure 4.6 and on the basis of aerial surveys of the main R. Léraba, it appeared that there had been no significant disturbance of riverine forest vegetation, except where limited forest clearing had been necessary to make space for the new road bridge to the west of Lérabadougou. This general impression is probably equally applicable to L.or and L.oc.

24The practical implication of these observations is that greater quantities of leachable substances, e.g. agrochemicals, can be safely tolerated in the savanna, than if these forests had been significantly reduced, or destroyed, and it is only to be hoped that a similar situation will remain in ten years time.

25Unfortunately, the same encouraging conclusion does not apply to the smaller rivers and streams in the LUA. It is clearly discernible in some of the maps that in many parts of the LUA, preference has been given to the cultivation of the banks of the tributaries that drain into the main Léraba rivers. What is not portrayed by the maps, but can easily be seen on aerial photographs, is that some of these tributaries have been so completely cleared of their natural vegetation and are now being so intensively cultivated, that it is now barely possible to discern their courses; at least under dry season conditions. In such instances, there are immediately increased risks of riverbank soil erosion, which can already be detected in a few places, and of agricultural chemicals being conveyed more directly into the main Léraba rivers, during the wet season.

4.5 Schematic Vegetation Profiles of the Léraba Valley Immediately to the North of Léraba Bridge

4.5 Schematic Vegetation Profiles of the Léraba Valley Immediately to the North of Léraba Bridge

4.6 Map Showing the Vegetation of the Léraba Valley Adjacent to Léraba Bridge

4.6 Map Showing the Vegetation of the Léraba Valley Adjacent to Léraba Bridge

Table des illustrations

Titre Table 4.1 ABR and ATP values at Léraba Bridge, 1975-1993
Légende • Phasing out of vector control operations.
URL http://books.openedition.org/irdeditions/docannexe/image/28725/img-1.jpg
Fichier image/jpeg, 40k
Titre 4.1 Map of the LUA Area Showing Areas Cultivated in 1972. burkina faso/cote d’ivoire. LERABA BASIN / BASSIN DE LA LERABA1972
URL http://books.openedition.org/irdeditions/docannexe/image/28725/img-2.jpg
Fichier image/jpeg, 676k
Titre 4.2 Map of the LUA Area Showing Areas Cultivated in 1983. burkina faso/cote d’ivoire. LERABA BASIN / BASSIN DE LA LERABA1972-1983
URL http://books.openedition.org/irdeditions/docannexe/image/28725/img-3.jpg
Fichier image/jpeg, 396k
Titre 4.3 Map of the LUA Area Showing Areas Cultivated in 1993. burkina faso/cote d’ivoire. LERABA BASIN/BASSIN DE LA LERABA1983-1993
URL http://books.openedition.org/irdeditions/docannexe/image/28725/img-4.jpg
Fichier image/jpeg, 612k
Titre 4.4 Map of the LUA Area Showing Areas Cultivated, Remnants of Savanna Woodland and Floodplain Grassland, 1993. burkina faso/cote d’ivoire. LERABA BASIN / BASSIN DE LA LERABA1993 LAND USE
URL http://books.openedition.org/irdeditions/docannexe/image/28725/img-5.jpg
Fichier image/jpeg, 88k
Titre 4.5 Schematic Vegetation Profiles of the Léraba Valley Immediately to the North of Léraba Bridge
URL http://books.openedition.org/irdeditions/docannexe/image/28725/img-6.jpg
Fichier image/jpeg, 156k
Titre 4.6 Map Showing the Vegetation of the Léraba Valley Adjacent to Léraba Bridge
URL http://books.openedition.org/irdeditions/docannexe/image/28725/img-7.jpg
Fichier image/jpeg, 178k

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