| Date de sortie | 27/03/2009 |
|---|---|
| Donateur | Ibrahim DIAKITE |
| Couverture géographique | Mali |
| Mots clés | perte de diversité biologique |
Chapitre V CAUSES DE LA PERTE DE DIVERSITÉ BIOLOGIQUE DIVERSITÉ BIOLOGIQUE
Pour assurer une bonne conservation de la diversité biologique, il est indispensable de bien comprendre les facteurs responsables de la dégradation de cette diversité. C'est en maîtrisant ces facteurs que nous assurerons la conservation et la pérennité des ressources biologiques. L'extinction de certaines espèces ainsi que la perte de souches génétiques et de types d'écosystèmes compromettent les opportunités de développement durable.
Le défi majeur qui se pose est la satisfaction des besoins en ressources biotiques d'une population de plus en plus nombreuse par des systèmes de production restés extensifs et peu productifs. Relever ce défi consiste à faire face à un ensemble de facteurs complexes dont les principaux sont climatiques, notamment les sécheresses récurrentes et les défrichements anthropiques, l'exploitation anarchique du bois comme source d'énergie, les incendies de brousse, la récolte excessive d'arbres ligneux et herbacés. (fruits verts, jeunes pousses, mutilation), surpâturage, braconnage, pêche illégale, agriculture « minière », pauvreté, utilisation excessive de produits chimiques, introduction d'espèces exotiques, etc.
5.1 SÉCHERESSES RÉCURRENTES
Over the past thirty years, the climatic aridity of Mali has increased and recorded rainfall, punctuated by periods of extreme drought, has been below average (Cf. Map 2). The isohyets have moved about 200 km to the south (PNAE, 1998).
These frequent droughts, more than any other factor, have contributed to further weakening ecosystems, making them more vulnerable to the slightest disturbance and accelerating the rate of degradation of biological resources. The resulting water deficits have led to a reduction in primary production, a change in the structure of the vegetation cover and a massive reduction in wildlife and livestock.
The deficit was large enough according to Hiernaux (1994) to decimate stands of ligneous plants, limit the germination of seeds of annual plants and reduce the cover of herbaceous plants. However, the decrease in primary and secondary production, the reductions recorded for perennial plant species, in livestock and in wildlife turn out to be reversible trends when climatic conditions improve. In general, regeneration is not immediate, rather it goes through a series of processes and stages. Some structural modifications and changes in perennial species associations are not reversible, at least in the medium term.
Figure 2: Comparative rainfall for the periods 1950-1969 and 1970-1999
Broad niches, often associated with high intra-specific genetic diversity, help species adapt to large spatial and temporal fluctuations in soil moisture regime, a determining factor in plant production.
These characteristics have certainly contributed to forging a certain resistance of the vegetation to climatic variations. Drought can cause species to disappear from the arid margins of their range, or cause the range to flap towards the wet margins. For example, Andropogon gayanus, classified as an endangered species north of Gourma (Ag Mahmoud, 1994), is found in abundant quantities south of Gourma after droughts.
5.2 CLEARING
The high rate of population growth and the economic activity linked to traditional production systems strongly contribute to the decline in the quantity of biological resources and the erosion of biological diversity. With an annual growth rate of 3.2%, it is estimated that more than 100,000 ha of natural formation areas are cleared each year to meet the increase in food needs alone. The agricultural area would increase by an average of 4.7%/year contributing to a strong reduction in natural formations. This increased pressure from human populations is considered the most serious threat to the biological diversity of species in Mali. The economic damage linked to the loss of forest formations in 1997 represented 5.35% of GDP, i.e. 79 billion FCFA (Pillet and Dabo, 1997). In addition, the extension of cultivated land, necessary to meet the food needs of an ever-expanding population, has considerably reduced the fallow areas and the duration of fallows. The clearing of land for cultivation has a very significant impact on the structure of the vegetation and on the environment. Land losses can, for example, reach 10 tons/ha (Bishop and Allen, 1989). Most woody plants are destroyed, with the exception of a few protected species (Vitellaria paradoxa, Acacia albida and Parkia biglobosa), and only bushy species such as Guiera senegalensis, Piliostigma reticulatum and Anona senegalensis resist repeated annual cuts. The reduction in fallow has shortened the time required for regeneration processes and increased the fragmentation of the islands of natural vegetation that constitute the “seed banks”. Its impact is even more serious when the increase in demographic pressure and social changes lead to the advance of crops on fragile land with low yields, with shallow and erodible or waterlogged soils.
5.3 ANARCHIC LOGGING
Wood and charcoal are the cheapest and most accessible forms of energy due to the fact that forest maintenance costs are not taken into account when establishing their selling price. Around 5 million tonnes per year in 1989-1990 (i.e. the equivalent of the production or standing stock of 400,000 ha of forest formations in the southern regions of the country), the removal of wood for energy purposes is close to 7 million tonnes in the year 2000, further amplifying the loss of biological diversity. Some species are particularly threatened because of the calorific value of their wood: Combretum glutinosum, Pterocarpus erinaceus, P. lucens, Acacia nilotica. of
The exploitation for artisanal and industrial purposes of certain species such as Khaya senegalensis, Prosopis africana, Bombax costatum, Dalbergia menaloxylon (Nioro ebony) and Sclerocarya birrea leads to disturbances in the dynamics of stands.
The uncontrolled exploitation of timber for commercial purposes now threatens all formations located near cities. Very lucrative and unorganized, the timber trade requires few means and costs, thus allowing a large number of people to engage in this activity.
This situation causes the rush of people on this product which sells very well and everywhere. The production of charcoal, more popular than round wood by city dwellers, empties the forests of their resources.
5.4 MISGATHERING OF FOREST PRODUCTS
Harvested products such as shea, néré, doum, jujube, kapock, gum arabic, baobab, etc., are mainly harvested by women and young people. They are used for several purposes: food, medicine, trade and crafts. But often, the manner and timing of these withdrawals dangerously compromises the development or regeneration of the parent resource. Thus these harvested products are often taken, either in the immature state, or by mutilating the mother tree. Fruits picked in the immature state will not be able to give birth to other trees. Mutilated individuals have little chance to regenerate and grow.
5.5 OVERGRAZING
Pastures are constantly diminishing due to the concentration of herds around water points and villages, the removal of livestock corridors, etc. This has led to a gradual shift from pastoral land in the north (Sahelian zone) to the Sudanian and Guinean zones in the south, accentuating in particular land conflicts between farmers and herders.
Grazing affects the interactive processes of plant physiology and soil biology. The strong seasonality that characterizes the use of pastoral resources in the Sahelian zone limits the risks of overgrazing and the resulting environmental damage to short periods and restricted areas. Locally, although there are very few herbaceous plants totally disdained by livestock, the intensive and selective nature of grazing during the growth period favors edible short-cycle annuals ( Zornia glochidiata, Alysicarpus ovalifolius , etc.) and the less palatable long-cycle perennials ( Cymbopogon gyganteus). Therefore, intensive grazing, which is observed only in limited territories, only affects plant diversity locally.
The limbing of species such as Acacia seyal, Acacia senegal, Balanites aegyptiaca , exposes stands to the devastating effects of bush fires and termites. When there is no such delimbing, the effects of grazing on shrub stands remain minor compared to the effect of interannual climatic fluctuations. Thus, the high stocking rates did not prevent the natural regeneration of Acacia seyal stands which had perished following the 1983-1984 drought in the Gourma. The regeneration and dissemination of certain woody species such as Balanites aegyptiaca and Acacia raddianacan be favored by the passage of seeds in the digestive tract of animals. The annual vegetation in the Sahelian zone seems very unstable, but resists pastoral aggression well thanks to the dynamic vigor of annual seed production, the efficiency of dispersal and the germination modes of the species.
5.6 BUSH FIRES
Bushfires are commonly practiced in savannah regions and in the lively Niger Delta. They would destroy approximately 14.5 million ha of pasture per year, or 17% of the national territory (CNRST, 1997), thus affecting the soil, fauna, flora and vegetation.
The loss of plant organic matter, which results from fires, reduces soil fertility, leading to a drop in productivity. Herders light fires in the savannah and in the flooded plains with perennial grasses to stimulate the resumption of growth and to have green grass of high nutritional quality. When lit in the southern Sahelian zone, where herbaceous fodder consists of annual species, they completely destroy the herbaceous cover, thus depriving livestock of pasture.
Fires have adverse effects on woody vegetation. The fragile annual shoots die, thus compromising the regeneration of the vegetation. Cissé (1986) indicates that after the passage of fire, there is a high mortality of seedlings of Guiera senegalensis, Acacia seyal and Ziziphus mauritiana . Early fires consume the fruits and seeds on the stems or on the surface of the ground, causing a change in the floristic composition of the herbaceous cover from one year to the next. This is how we observed (PPS, 1982) the replacement of a herbaceous cover based on Cenchrus biflorus sensitive to early fire by a cover dominated by Eragrostis tremula .
By destroying herbaceous vegetation and reducing woody cover, fires degrade the habitats essential to wildlife. The populations of reptiles, birds, amphibians, insects which depend on the micro-environments of the herbaceous layer are reduced. Small animals such as walking insects that cannot escape fires are killed.
5.7 POACHING
The study on the impact of the closure of hunting in Mali (1978) revealed a clear decline in the numbers of large ruminants. It showed that certain species of mammals, reptiles and birds have disappeared or are threatened with extinction (Appendix 2). Furthermore, it revealed a disturbance and a reduction in the natural habitat of several species of wildlife in areas such as the Boucle du Baoulé and the Gourma.
Of the four small herds of elephants in Mali (Boucle du Baoulé, Gourma), only the Gourma remains. This reduction would be linked either to the reduction in habitat, or to the development of commercial circuits, or to the modern means used by poachers, or to the demand for protein following the demographic explosion. The situation is similar for giraffes in the Sahel, chimpanzees in the Fina area, Derby eland in Baoulé and gazelles, antelopes, manatees and lions elsewhere.
In 1997, the economic damage linked to the loss of wildlife resources represented 7.25% of the GDP, or 107 billion FCFA (Pillet and Dabo, 1997).
And yet, hunting in traditional society was an activity reserved for a social group (the society of hunters), governed by a very strict hierarchy, subject to long-term initiation/training and a set of rules. These rules, known to all practitioners, aimed at the balance of nature while allowing the population to be supplied with meat. This traditional society has gradually crumbled. The hunting rights, granted by modern legislation, which are difficult to apply, have not been able to effectively replace these customary rules which are less and less respected.
5.8 ABUSIVE FISHING
First a self-consumption product, fish has now become a very valuable element of exchange. The sector concerns more than 800,000 people, 200,000 of whom live directly from it. The Bozo, Somono and Sorko are the traditionally fishing ethnic groups. Today the activity attracts other ethnic groups. This results in an intensive fishing practice following the increase in needs, the improvement of fishing gear and the use of toxic substances and explosives. The production renewal capacity proved to be low compared to the withdrawals to which it is subject.
In addition to illegal fishing methods (prohibited gear, poisons, lighting gear, small-mesh nets, etc.), droughts with the early drying up of ponds and lakes, low flood levels, embankments linked to the creation of numerous Irrigated perimeters reducing spawning grounds are all causes of the decline in fish production and the loss of certain species.
5.9 POLLUTION
The Niger River receives more than 2,200 m 3 of industrial wastewater per day in Bamako. The factories of tanneries, textiles, soap, oil; the chemical industries, mining and slaughterhouses discharge pollutants in an uncontrolled manner, which largely contribute to the degradation of terrestrial and aquatic ecosystems.
The exploitation of gold mines (Syama, Sadiola, etc.) and numerous gold panning sites has a significant impact on biological resources. Cyanide, the excavation and dumping of ores, sources of pollution, lead to the destruction of plant cover and the loss of the biological diversity of the surrounding environment.
In addition, it receives more than 32,000 m 3 of domestic waste water and 16,000 m 3 of waste water from dyeworks containing dyes, chemical pollutants, heavy metals which degrade aquatic flora and fauna.
In the absence of being able to use less polluting organic fertilizers, the massive use of chemical fertilizers is becoming widespread and is causing a degradation of the ecosystem, due, among other things, to the acidification of the soils that it entails. The use of pesticides, in addition to the targeted enemies, unfortunately destroys fauna (eg pollinating insects including bees) and flora that are very useful for maintaining the balance of ecosystems. The agricultural development companies in their areas of intervention use large quantities of pesticides and mineral fertilizers (for example, 970,000 liters of insecticide, 73,935 tonnes of mineral fertilizers in 1995-96 in the CMDT zone 6,700 liters of pesticides and 52,396 tons of mineral fertilizers in 1993-94 at OHVN; 330 tonnes of pesticides for the National Plant Protection Service, etc.), which threaten flora and fauna. In 1986, an estimated 15,800,000 ha of areas treated with pesticides.
5.10 INTRODUCTION OF ALIEN SPECIES
The introduction of certain species in Mali has endangered local species with which they had not evolved. This is the case of the water hyacinth ( Eiclornia erassipes ). It has become a source of growing concern. Originally from the Amazon, this floating aquatic grass was introduced around 1990 around Bamako. It constitutes the biotope of numerous aquatic organisms that are vectors of diseases and invades ponds, the infrastructures of Energie du Mali, the Office du Niger, and the irrigated perimeter of Baguinéda. By covering bodies of water, it poses a threat to aquatic animal and plant species, which they asphyxiate.
The need for crop intensification has led to a reduction in the genetic diversity of local cultivated plants, as the few varieties selected for their best yield have replaced the wider range of less productive varieties.
The introduction, for example, of Caudatum sorghum limits the development of local varieties; that of Asian rice is to the detriment of local Glaberrima rice, whose areas are in clear decline.
5.11 “MINING” AGRICULTURE
Some methods of operating cash crops are counter-productive and very harmful to biological resources due to the mechanized or motorized clearing of large areas. The gradual clearing of forest areas and the abandonment of those rendered sterile and unproductive, the excessive use of chemicals (fertilizers, fungicides, herbicides, insecticides, etc.) harmful to both animal and plant species are major causes of degradation. ecosystems and the disappearance of adapted local breeds and varieties.
5.12 POVERTY
Difficulties in accessing land, inputs and agricultural equipment are the main bottlenecks for the poor. In addition to the fact that they have difficulty accessing land, the poor can only use marginal and unproductive land.
The consumption of poor populations is based almost exclusively on the exploitation of natural resources. The lack of means to valorize the natural resources pushes the poor to a mining type exploitation and often abusive of these resources. Indeed, primitive production systems without inputs are characterized by a high consumption of land resources, vegetation, and by itinerant practices of breeding, agriculture, fishing, etc.
Poor populations, in their precarious situation, often have no other recourse than to carry out abusive or illicit taking of resources whatever their state: poaching, excessive fishing, mutilation of trees, anarchic clearing, illicit exploitation forests, overgrazing, etc.
L'information et la formation des populations sont une nécessité pour la réussite des actions de conservation que l'Etat, les collectivités locales ou les particuliers devront entreprendre.
5.13 AUTRES CAUSES
Pour diverses autres raisons, la diversité biologique du pays s'érode. On peut citer entre autres l'urbanisation rapide et incontrôlée du pays, l'envasement des rivières, lacs et étangs du nord.