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THE EFFECTS OF AGRICULTURAL COLONISATION ALONG THE AMAZONIAN HIGHWAY SYSTEM

Started in 1970, the 6 368 km TransamazSnica, the 2 465 km Perimetral Norte and the 1 747 km Cuiab~-Santar~m highways, planned through the Amazon jungle, are now nearing completion.

Peasant agriculture by thousands of government-assisted settlers and large- scale cattle ranching are being developed along these highways by the Brazilian Ministry of Agriculture.

On both sides of the roads, strips 100 km in width have been incorporated, and specifically designated for agricultural colonization. Agricultural settle- ments are imposed on the landscape in a geometric grid wi thout any regard either to physiography or to agricultural suitability. Every 10 km along the TransamazSnica an agrSvila (48--64 houses on a 100 ha site) will be established, each family having an agricultural plot of 100 ha in area. Every 50 km, an agrS- polis (500 houses on a 300 ha site), an agricultural town serving four agrSvilas, will be built. Approximately every 150 km, a rurSpolis, an agricultural devel- opment centre with a populat ion of up to 50 000 people, will be established, each serving two agrSpolis.

Three quarters of the new colonists come from the north-eastern part of Brazil. The government provides them with a house, basic hand tools, crop seeds, seedlings and some rudimentary instruction and clears 2 ha of their plot of forest. During the first 6 months the minimum wage is paid to the new colonists by the Government.

The colonists must feed their family on their own products in order to survive. Nearly all cleared plots are therefore cultivated for manihot and rice. These are all annuals and highly injurious to the ecosystem as they aggravate erosion and loss of soil fertility, while encouraging an increase of weeds and pests. In order to understand the likely environmental consequences of these agricultural activities, some basic information about the ecology of the lowland tropical wet forest has to be given.

This type of forest is the most productive ecosystem in the world, in spite of sterile soils, excessive rainfall and a superabundance of pathogens and pests. Photosynthesis is rapid under the favourable conditions which com- monly prevail, so that only nutrients and herbivores are limiting factors. The vegetation has a very tightly closed nutrient cycle which is rather independent of the soil as the greater part of the decay occurs above the ground. This ecosystem has also evolved effective mechanisms against herbivores, viz, low populat ion density of individual species, repellent or toxic chemicals and a balanced system of predators and parasites.

The main reasons for the decline in crop yield from the cleared forest plots

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in Amazonia in the years subsequent to removal of the forest are: decline in soil fertility, rapid pest build-up, relentless competi t ion with weeds and an increasingly harsh microclimate.

As soon as the forest canopy is removed, rain and sunlight reach the soil surface directly, causing leaching of nutrients and death of the micro-organisms responsible for decay, with the result that the nutrient cycle is broken.

By burning part of the forest, most of the stored nutrients are released; those which are not absorbed by the plants are leached away causing a severe decline in soil fertility. Many animals are killed by burning and deforestation, biological control is absent and phytophagous insects increase rapidly, especial- ly in the presence of crops. These crops are selected for the proport ion and palatability of the harvestable part, but are not well adapted to the attacks of pathogens and insects. Pesticides have little effect as they are washed away by the frequent rains. Due to the tropical climate they must act the whole year round, so that the insects have many generations each year in which to evolve a tolerance or resistance to these pesticides.

Weeds are specialized to survive in harsh microclimates with the result that crops fail to compete with them and inevitably succumb when in competi t ion with them.

After the third crop, if the latter is worth harvesting, it is more profitable to clear another part of the forest and repeat the whole procedure. This kind of cultivation will only be successful if the fallow lasts long enough for the softs to recoup the nutrients carried off in the harvest and lost by erosion and burning, and if t he plots are small in area in comparison to the intervening forest. It is an ecologically sound strategy with low populat ion densities, bu t is totally inadequate to support the huge numbers of officially supported colonists.

There are, however, at least four alternatives for the agricultural develop- ment of Amazonia, related to four distinct ecosystems that are found in this area: (1) the annually f looded v£rzea; (2) forest on patches of rich soft; (3) islands of savanna or cerrado within the forest; (4) the forest itself.

The periodically f looded land (v~rzea), which is fertilized by the deposit ion of rich silt f rom the adjacent rivers, could be manipulated to support an agro- -ecosystem having a sustained-yield if the flooding were also used to eliminate pests and weeds. Rice, sesame, jute, beans and maize could be appropriate crops for this system.

On the little patches of more fertile soft, scattered in the forest, conventional agriculture could be pursued, causing much less damage than agriculture on the poor soils.

The islands of non-forest vegetation within the forest are vegetated with scattered trees and a herbaceous, of ten grassy, ground layer. Here, cattle grazing and browsing could be justified and also the cultivation of fruit trees and pineapples could be applied.

The fourth and most fragile ecosystem, the forest itself, will not sustain

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heavy harvests using the methods known today; these mostly originate from temperate areas. In order to cultivate this system with any success, the following four related precautions have to be respected: (1) the closed nutrient cycle must be maintained; (2) the forest canopy must not be perforated; (3) the extent of nutrients, imported into the ecosystem as rain solutes, dust, fixation by plants and a little from the substrate, should be used to determine the sustainable size of the crop to be harvested; (4) the biotic diversity in physiognomic types, trophic levels, life forms, species composition and age distribution must be maintained above the level at which the activities of pests become a serious factor, for example by estab- lishing mixed tree plantations.

Apart from these four forms of agriculture, the culture of fish would probably be the most sustainable and commercial possibility of the entire region. The Amazon contains the world's richest fish fauna, but this is still an under-exploited resource. Improved fishing technology and cooperatives could multiply fish production severalfold.

Also, the local riparian fauna, if farmed, would be a valuable and sustain- able food source; they could be raised from nutrients mainly of fluvial origin. In addition, each colonist should be encouraged to cultivate a small home garden, largely of vegetable and fruit perennials. Such an ecologically de- signed garden, based on fruit trees with some small mammals and chickens integrated into a closed ecosystem incorporating household wastes, could be much more reliable than the export-orientated agriculture promoted at pre- sent. In conclusion: Ideally the Amazonian forest should be preserved intact until research reveals how it can best be manipulated to give a sustained yield, because it should be remembered that the tropical wet forest is in ecological terms a desert covered by trees!

F.W.

REFERENCE

Goodland, R.J.A. and Irwin, M.S., 1975. Amazon Jungle: Greenhell to Red Desert? Elsevier, Amsterdam, 155 pp.

PROGRAMME ADVOCATED TO MEASURE IMPACT OF AGRICULTURAL PESTICIDES

An international group of experts has agreed on a world-wide programme to measure the impact of residues from agricultural pesticides on the environ- ment. The experts were brought together by the United Nations Environment Programme (UNEP) and FAO at FAO headquarters in early October 1975. They were asked to provide guidelines and advice on the technical points to be covered by such a monitoring programme.

The launching of a programme to improve the competence in measuring residues of pesticides in food, soil, water and other materials had been sug-