The impact of humans on the African environment
Trypanosomiasis control under different land-use systems
The author is Director of the Tsetse Research Laboratory, University of
Bristol, Langford House, Langford, Bristol BS18 7DU, UK.
The direct effects of tsetse control on the environment have been relatively well studied and documented (Douthwaite, 1992) but the importance and complexity of the indirect effects of trypanosomiasis control are far from being fully understood. Although tsetse control has been the main approach to the control of trypanosomiasis for many years in Botswana and Zimbabwe, elsewhere in Africa today the main approach to controlling this disease, except in zones where trypanotolerant breeds of livestock are kept, is by the use of trypanocidal drugs.
In many countries, the administration of drugs to domestic livestock, primarily cattle, is mainly carried out by the livestock owners rather than the veterinary authorities. For the purposes of this article, unless stated otherwise, no distinction is made between combating trypanosomiasis by vector control and by the use of drugs. The environmental implications are generally the same whatever means is adopted to reduce the risk of disease, and they therefore represent a constraint to the keeping of domestic livestock.
Although concern for the African environment and the possible role of the tsetse fly as a protector of this environment has come to the fore in recent years, it is by no means a new concept. The threat that this fly poses to humans and their domestic animals was recognized by the early explorers - for example, Richard Burton considered that anything which could exterminate the fly would be "the greatest benefactor that central Africa ever knew" (Burton, 1860). However, the converse was already appreciated by the 1920s and 1930s: "the tsetses are the most potent preservers of the natural flora and fauna. Drive out the tsetse and the whole landscape changes" (Swynnerton, 1936). Although workers in Africa since then have remained aware of this dilemma, until relatively recently the debate had rather receded from the public eye, at least partly because of the arguments associated with the use of insecticides for tsetse control and their perceived threat to the environment. To some extent these arguments were counterproductive, as they diverted attention from the most important environmental and ecological issues associated with trypanosomiasis control.
It is necessary to put these issues into context. Africa today is facing two closely linked crises: a high rate of human population growth and environmental degradation exacerbated in some countries by political instability. Both of these crises, not directly related to tsetse or trypanosomiasis control, nevertheless have major implications for the development of appropriate strategies for the control of livestock diseases.
The main factor affecting the ecology of Africa today is its rapidly increasing human population. In countries that are already densely populated this increase is causing the removal of tsetse habitats as well as the consequent disappearance of the tsetse over large areas (Jordan, 1986). The classic cases are Nigeria, the Gambia, Burkina Faso, Rwanda, Malawi and parts of the United Republic of Tanzania and Zimbabwe. The Glossina morsitans and G. fusca are the main groups to be affected by humans, but even the G. palpalis group is affected, especially in drier areas. In sparsely populated countries, such as Mali, the southern Sudan, Zaire, Angola, Mozambique and Zambia, tsetse flies are as yet little affected by humans and, in some places, are even invading new land.
It is against this background of varying demographic pressure on land (and the tsetse) and of the consequential, widespread land degradation, that strategies for the control of trypanosomiasis need to be developed and assessed in terms of whether or not they are likely to exacerbate or reduce the risks of environmental degradation. In principle, as a component of land-use planning, effective trypanosomiasis control in previously unsettled territory provides a unique opportunity for the orderly development of land. In practice, this has often been a pipedream since, despite effective control of trypanosomiasis and sound land-use planning, plans have not been implemented and people have simply moved on to the cleared land and taken it up in a random, and often non-sustainable, way. In the context of expanding populations, the prognosis is not favourable. What is clear is that the environmental impact of tsetse and trypanosomiasis control is likely to be insignificant against this background of human population growth, but this is no reason for complacency. When vector or disease control is a component of other development activities, it is important that every effort be made to ensure that the growth of livestock populations resulting from a reduced incidence of trypanosomiasis does not exceed the carrying capacity of the land. Sustainability must be the objective, bearing in mind that different types of land will be able to sustain different intensities of use.
When trying to predict and control the likely impact of tsetse or trypanosomiasis control on the environment, it is important both to comprehend the role of cattle in Africa (most are kept primarily for social reasons - with numbers rather than quality being important - and not for meat, milk or other products which contribute significantly to a market economy) and to take account of existing livestock systems in the control area or, if they are non-existent, the type of system that would develop following the successful control of trypanosomiasis.
Following is a general classification of circumstances under which animal trypanosomiasis may occur in Africa. Conditions vary from areas of wilderness, often densely infested with Glossina spp. and where domestic livestock cannot be kept (mainly because of the presence of trypanosomiasis), to areas where arable farming is the main agricultural activity but where occasional domestic livestock are exposed to a low risk of trypanosomiasis. Under some of these circumstances, the successful control of trypanosomiasis may be followed by an escalation in domestic livestock numbers and consequent overgrazing and environmental degradation (see Fig. 1). Elsewhere, on the other hand, there are virtually no adverse implications for the environment. It is pertinent to examine some of these individual circumstances and to determine the potential environmental risks of interventions to control trypanosomiasis.
Wilderness - no domestic livestock
It is when successful control of trypanosomiasis is achieved over extensive areas that are densely infested with tsetse flies that the most profound changes in land use - and risks to the environment - are to be expected. In such cases, particular care must be taken to try and prevent excessive pressure on what is often already a fragile ecosystem. A highly successful tsetse eradication campaign was carried out in northern Nigeria from 1955 to the late 1970s, mainly with ground spraying of insecticide (Maclennan and Na'isa, 1971).
In the short term, the campaign released extensive new areas for cattle grazing and reduced pressure on the arid, naturally fly-free traditional grazing areas in the far north of the country. The immediate outcome was thus entirely environment-friendly. In the longer term, the outlook for the environment is less favourable. There are already indications that the occupation of new land, formerly infested by tsetse flies, is only postponing a crisis as humans continue to increase in numbers and still more land is required. As a result of this process the livestock owner is often coming out second best, as the most suitable land for pasturage is increasingly encroached upon by subsistence farmers. Land-use studies undertaken prior to tsetse control foresaw this potential conflict of interests and recommended the drawing up and enforcement of reservation ordinances to protect the graziers but, in practice, little was achieved. One lesson is clear from this experience: tsetse control should not be held responsible for the present rate of environmental change. The original objective of relocating the national herd away from overstocked areas was achieved but this success is being overtaken by pressures associated with rapidly expanding human populations - pressures beyond the scope of tsetse control.
Because of its high and increasing human population density, Nigeria is perhaps a special case, but populations are expanding elsewhere in Africa, albeit usually from a lower baseline. Lessons are to be learnt from events in Nigeria and they should be taken into account when planning trypanosomiasis control in wilderness areas elsewhere. Subsequent environmental change is inevitable (and necessary if expanding populations are to be fed), but every effort must be made to ensure that the changes are beneficial and sustainable and that they do not result in degradation.
National parks and reserves
In many respects, tsetse-infested national parks and reserves are similar to the preceding category of land in that most parks and reserves are located in wilderness areas. The major difference between the two categories is that, whereas domestic livestock are permitted to utilize areas of wilderness, provided constraints such as trypanosomiasis and a lack of water are overcome, they are forbidden on reserved land. For this reason, tsetse control in national parks and reserves - even by nonpolluting methods - is a particularly emotive issue. Those who oppose this method argue that the fly's presence is a major factor in preventing the encroachment of domestic livestock. Those in favour, while not advocating the takeover of reserved lands by livestock owners, see the removal of the fly as essential to the protection of domestic livestock in the periphery of reserves. Very often, the origin of infestations affecting large numbers of domestic livestock is to be found in reserved land, and a realistic policy to control trypanosomiasis cannot allow the protection of extensive tsetse infestations in such zones.
It is perhaps ironic that tsetse infestations are ill-protected in some national parks. If animals are in balance with the habitat, the tsetse is safe, but there are examples from many of the great East African parks (Ford, 1966) where, at various times, elephant populations have been allowed to increase freely, causing so much damage to the woodland that they have been highly effective agents in the control of tsetse, particularly of the G. morsitans group (see Fig. 2). If it is decided to control tsetse in reserved areas, all possible precautions should be taken to prevent the encroachment of livestock into what are often areas of great economic as well as aesthetic value. It would be defeatist to retain a disease risk in areas surrounding a park just because the authorities felt they could not protect the park in any other way.
Traditionally, African pastoralists own cattle and other domestic livestock but do not own land. Again traditionally, they occupy the tsetse-free zones between the sub-humid zone and the great African deserts (see Fig. 3). The presence of tsetse flies has prevented the spread of cattle into the woodland wilderness, except for some areas where losses from disease are balanced against the seasonal necessity to locate herbage and water. For many years pastoralists resisted the efforts of colonial authorities, and later those of African governments, to settle them. In many areas today, however, they are settling in response to the realities of life - grazing land is no longer limitless. Arable farmers are increasingly encroaching and settling on the traditional grazing grounds of pastoralists. One result of this is that pastoralists are being pushed into closer contact with the tsetse.
It is because of this seasonal or periodic contact between pastoralists and the tsetse fly that this group has been, and still is, a major user of trypanocides. Pastoralists were also intended to be the main beneficiaries of the tsetse eradication campaign in northern Nigeria. Another pastoral group, the Masai in Kenya, have actively and successfully cooperated in a small-scale project over some 120 km2 (Dransfield, Williams and Brightwell, 1991), aimed at reducing the trypanosomiasis risk to their livestock by catching the local tsetse with home-made traps. All of these very different methods can succeed in their immediate aim of reducing the risk of trypanosomiasis but all have attendant risks of subsequent, uncontrolled increases in cattle numbers,- thus leading in turn to degradation of the rangeland.
This is an important form of land use in many countries and is becoming more so as pastoralists settle in response to pressures on their traditional grazing lands. It generally involves individually farmed arable plots and communal grazing (see Fig. 4). Cattle ownership provides social status and financial capital which can be called upon when required. The animals provide milk and manure but beef production is not important. Of increasing importance in such farming systems is the use of animals, especially cattle, for draught power - there has been an enormous expansion in the last 30 years or so -in many African countries (see Fig. 5). In many rural farming communities, particularly those in more marginal areas, the economics of animal draught power are extremely attractive (Barrett, 1989). Tsetse or trypanosomiasis control should be much less controversial under such circumstances. In many cases it is protecting and making more efficient an existing agricultural system rather than permitting the opening up of new land.
Nevertheless, in the context of expanding human populations, there is often a tendency to overstock. In the Zambezi Valley in Zimbabwe, Barrett (1989) has shown that, as the population increases, there will be a growing number of households wanting to own cattle. As a result, human population density could exceed the level at which there are adequate grazing resources to maintain the herd, particularly as some of the grazing land designated at present will inevitably be turned into arable land. Although a more "efficient" use of livestock is possible - draught animals typically account for 30 percent or less of a cattle herd in the Zambezi Valley ultimately, the choice must be made between overstocking (and, consequently, land degradation) and keeping an insufficient number of animals for each household.
Many African cattle are owned by people whose chief concern is cultivation but in cases where the two activities are less integrated than in traditional agropastoralism. True mixed farming is uncommon, but many authorities consider that a move in this direction would assist in maintaining soil fertility and reducing land degradation.
Trypanosomiasis is not usually a major constraint in areas of intensive arable farming where some domestic livestock are present. There are exceptions, however, particularly in the humid African zones where the palpalis species, which is less affected by dense human populations, occur. Control of trypanosomiasis under such circumstances can be beneficial to the environment by encouraging the development of sustainable mixed farming (see Fig. 6). The implementation of control measures is generally not a controversial issue.
1 Land in the United Republic of Tanzania, formerly infested with Glossina morsitans, now overgrazed, virtually denuded of woody vegetation and showing early signs of erosion - Terre autrefois infestée par Glossina morsitans en Tanzanie, aujourd'hui surpâturée, pratiquement sans végétation ligneuse et montrant les premiers signes d'érosion - Tierra antes infestada por Glossina morsitans en la República Unida de Tanzania, ahora prácticamente carente de vegetación leñosa, sometida a sobrepastoreo y con los primeros signos de erosión
2 Elephants have destroyed this former habitat of Glossina pallidipes in a national Dark in Kenya - Un ex-habitat de Glossina pallidipes détruit par les éléphants dans un parc national du Kenya - Los elefantes han destruido un hábitat anterior de Glossina pallidipes en un parque racional de Kenya
3 Zebu cattle, owned by pastoralists in West Africa, occupy the tsetse-free zones between the sub-humid zone and the Sahara Desert but they seasonally penetrate tsetse-infested areas for pasture and water - En Afrique de l'Ouest, les zébus appartenant à des éleveurs occupent les zones exemptes de tsé-tsé entre la zone subhumide et le désert du Sahara, mais ils font des incursions saisonnières dans les régions infestées, en quête de pâturages et d'eau - Los cebúes de los pastores de Africa occidental ocupan las zonas libres de la mosca tsetsé entre la zona subhúmeda y el desierto del Sáhara, pero efectúan penetraciones estacionales a zonas infestadas en busca de pasto y de agua
4 Communally grazed cattle around the edge of the fly belts in southern Zambia manure individually owned arable plots but have to be kept off the crops in the growing season - Les zébus qui paissent sur pâturages communaux à la lisière des zones infestées par la mouche tsé-tsé, dans le sud de la Zambie, fument les parcelles appartenant aux cultivateurs, mais ils doivent être éloignés des champs cultivés pendant la période de végétation - Los vacunos que ocupan pastos comunales alrededor de la franja protectora contra la mosca tsetsé en Zambia meridional abonan parcelas cultivadas de propiedad privada, pero hay que mantenerlos fuera de los cultivos durante el periodo vegetativo
5 There is a long tradition of using cattle for draught power on the highlands of Ethiopia; they are being introduced into the tsetse-infested lowlands as the vegetation is cleared for human settlement - Sur les hautes terres d'Ethiopie, on utilise de longue date le bétail pour le trait; à mesure que les terres sont défrichées dans les nouvelles zones de colonisation, les animaux sont conduits dans les basses terres infestées par les tsé-tsé - En las tierras altas de Etiopía es tradicional el uso de vacunos para el tiro; a medida que los asentamientos humanos eliminan la vegetación, se los está introduciendo en las tierras bajas infestadas por la mosca tsetsé
6 Trypanotolerant N'Dama cattle feed on crop residues in the Gambia where there is a close integration of the arable and livestock sectors - Bovins N'Dama trypanotolérants pâturant des résidus de recolle en Gambie, où l'agriculture et l'élevage sont étroitement intégrés - Vacunos N'Dama tripanotolerantes alimentados con residuos de los cultivos en Gambia, donde existe una estrecha integración de los sectores agrícola y pecuario
A ranch occupying a Glossina morsitans habitat in the United Republic of Tanzania where cattle numbers are kept within carrying capacity and environmental degradation is not a problem - Ranch dans un habitat de Glossina morsitans en Tanzanie où le cheptel bovin ne dépasse pas la capacité de charge et la dégradation de l'environnement n'est pas un problème - Explotación ganadera en un hábitat de Glossina morsitans en la República Unida de Tanzania, donde el número de vacunos se mantiene dentro de los limites de la capacidad de pastoreo y no se plantean problemas de degradación del medio ambiente
Intensive livestock production systems
Attempts have been made in most African countries to manage tsetse-infested rangeland and other types of land for livestock production by setting up commercial or state-owned ranches. In a few countries commercial ranches are highly successful, but they make little contribution to the meat supply of most tsetse-infested countries. The controlled use of trypanocidal drugs or tsetse control, especially by low-cost methods, can be a major help in increasing productivity economically. In such managed systems, where human and animal populations are generally kept well within carrying capacity, environmental degradation is not an issue (see Fig. 7).
The complex issues affecting the sustainable use of land after successful tsetse or trypanosomiasis control are poorly understood. Perhaps the most important point to emphasize is that the potential impact of control measures on the environment can vary from highly damaging, if stock numbers are subsequently uncontrolled, to highly advantageous. This article has made some attempt, albeit necessarily simplistic, to categorize the levels of risk of environmental degradation in different ecological circumstances. The risks vary according to existing or potential livestock management practices which are, in turn, related to climatic, edaphic, social and other factors.
Particularly rigorous precautions are necessary when previously unoccupied land is opened up to domestic livestock as a result of effective tsetse or trypanosomiasis control. If there is perceived to be a major risk that overstocking will occur, a stand against trypanosomiasis control might be justified until everything possible has been done to ensure that future land use is sustainable. However, this option is often not feasible. Past experience in a number of countries has shown that, if sufficient population pressure exists, the land will be taken and settled, irrespective of what planners might say. This can take place even before the habitats and hosts of the tsetse have been destroyed - at the cost of death for many cattle.
The risk of environmental damage from control interventions carried out to protect well-established farming systems from trypanosomiasis are very much less. In fact, control under such circumstances is generally entirely beneficial.
The possibility that trypanosomiasis control, whether it involves combating the vector or using trypanocidal drugs, could contribute to rapid and irreversible environmental degradation is, quite rightly, of great concern to all those associated with the funding and implementation of such programmes. Planners and those who have to ensure that plans for sustainable development are enforced must walk a tightrope between reaping the undoubted benefits of control and preventing the aftereffects of control from getting out of hand.
Nevertheless, a word of advice to decision-makers is appropriate. All the available evidence clearly shows that expanding human populations are the prime cause of much environmental degradation in Africa. With or without trypanosomiasis control and in response to expanding populations, much spontaneous or planned settlement is going to occur in many parts of Africa where the disease is a risk. Inevitably, environmental degradation will continue - sometimes as a direct result of overstocking or from natural resource abuse commonly associated with farming under population pressure. It is all too easy to place the blame for degradation on trypanosomiasis control and, in so doing, condemn procedures which, when properly applied and controlled, can make an enormous contribution to orderly rural development in tropical Africa.
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