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close this bookThe Fragile Tropics of Latin America: Sustainable Management of Changing Environments (UNU, 1995)
close this folderPart 1 : The ecological outlook
close this folderRich and poor ecosystems of Amazonia: an approach to management
View the document(introductory text...)
View the document1 Introduction
View the document2 Characterization of the oligotrophic environment
View the document3 Characterization of eutrophic forests
View the document4 Management of oligotrophic areas
View the document5 Management of eutrophic areas
View the document6 Conclusions
View the documentAcknowledgements
View the documentReferences

4 Management of oligotrophic areas

There is much we can learn about the proper management of Amazonian ecosystems from the region's indigenous population, which coped with its limitations and in some cases rose above them by changing the composition of forest and soil. What we conclude from our examination of these two ecosystems is that native populations used both areas, but that they restricted their manipulations of the ecosystem to the "islands" of terra firme forest in the oligotrophic regions and focused on the eutrophic "islands" within the vaster nutrient-poor areas of terra firme.

Human populations have inhabited the upper Rio Negro for at least 6,000 years. Ceramics and anthropogenic soils have been identified in the Rio Negro, dated at 3750 BP (Clark and Uhl, 1987: 7; Saldarriaga and West, 1986; Sanford et al., 1985). The anthropogenic soils are found not in the caatinga areas but in upland tropical forests growing on oxisols, suggesting a very ancient preference for the patches of tropical forest, rather than caatinga, for occupation and agriculture.

Populations of the upper Rio Negro practise slash-and-burn agriculture, clearing areas of between 0.5 and 2.0 hectares between September and November each year. It has been observed that contemporary native populations generally avoid locating their swiddens in areas of caatinga, which through ethno-ecological taxonomies are identified as inappropriate for agriculture (Hill and Moran, 1983). Clark and Uhl (1987) estimated that in the region near San Carlos de Rio Negro, in Venezuela, only about 20 per cent of the soils were not spodosols. This extremely restricted availability of soils capable of supporting crops for even a couple of years runs counter to very old assumptions about the ease with which native populations could relocate in Amazonia and the lack of "environmental circumscription" in the basin (Carneiro, 1970). There have been no reports of nutrient-rich alfisols (terra roxa estruturada eutrófica) in the Rio Negro Basin. Galvão (1959: 24) noted another factor limiting the agricultural potential of the upper Rio Negro: the apparent avoidance of areas which required penetration into the forest and a preference for areas along river banks by horticultural populations. Chernela (1983) observed a similar preference among the Uanano, a Tukanoan population in the Vaupés, as have those who have studied the Bara Makú (Silverwood-Cope, 1972) and the Hupdu Makú (Reid, 1979). This tendency to limit territorial occupation to the river-banks may represent a compromise response to the poverty of the terrestrial ecosystem, the availability of restricted areas of fertile levees, and the importance of fisheries based on ancient territorial claims.

Similarly, flooded forests (igapós) are avoided for agriculture, given their importance for fisheries (Chernela, 1983; Clark and Uhl, 1987; Dufour, 1983). Many of the fish in Amazonian river channels enter the flooded forest during the rainy season to gain weight and to spawn (Goulding, 1980, 1981). When fishing gives poor results, the Wakuenai in Venezuela say that the fish are spawning and locate their swiddens distant from the flooded forests. Chernela (1982, 1986a) noted the same explicit avoidance of flooded forest for agriculture among the Uanano in Brazil.

The length of the cultivation period and the size of clearings is of special significance in these oligotrophic habitats. The smaller the area cleared, the easier it will be for seeds from the native vegetation to recolonize the area. The length of the cultivation period affects the levels of nutrients available to the incoming seeds and the growth rate of secondary vegetation. In black-water ecosystems, the return of the original vegetation may take over a hundred years (Uhf, 1983; Uhl et al., 1982). Uhl et al. (1982: 319) found 271 seeds per m² in an area studied at the end of the cultivation cycle, 90 per cent of them secondary successional species. It is quite likely that secondary successional species are better adapted to low levels of nutrients and can thrive where domesticated plants cannot. Succession is much slower in black-water ecosystems: above-ground biomass after three years was only 870 g/m² compared to 2,000 g/m² in areas of oxisols on upland forest. It appears that oligotrophy, as well as the flooding, are responsible for this lower level of above-ground biomass production (Uhf et al., 1982: 320). After sixty years, the above-ground biomass is only 40 per cent of that of the original vegetation (Clark and Uhl, 1987: 12; Jordan and Uhl, 1978), compared to 90 per cent of above-ground biomass in eight years on ultisols in the Peruvian Amazon (Sanchez, 1976: 351).

One of the ways in which native peoples help accelerate the recolonization of cleared areas is by planting fruit trees in the swiddens. Not only does this increase the utility of the land, but it serves to attract birds and bats, which are the principal agents of primary forest seed dispersal in the humid tropics. In a controlled experiment, areas planted in this manner had nine times the number of seeds of native trees than an area which was not planted with fruit trees at the end of the cultivation cycle. The shade of the fruit trees serves to provide the needed shade to primary tree species of slow growth, and reduces leaching of nutrients.

The lack of a marked dry season in this region would lead us to expect that burns would be of poor quality. Generally, it is the quality of the burn that determines the yield of slash-and-burn cultivation in non-volcanic areas of the humid tropics (Moran, 1981). That is clearly the case in areas with high above-ground biomass. But in areas with lower biomass, high insolation, and high albedo resulting from reflection from the white sands, the biomass dries sufficiently to burn so well that, in fact, areas of xeromorphic vegetation tend to experience burns beyond the areas cleared. Clark and Uhl (1987) documented the problem of natural burns in this habitat, where hundreds of hectares can catch fire when twenty rainless days occur. The destructive impact of fire is a real threat in this ecosystem, in contrast to other areas of the humid tropics, where fire rarely extends beyond the area cleared (or in areas scarred by logging activities). Planting follows, and is dominated by bitter manioc.

Dufour (1988) found more than a hundred varieties of bitter manioc among a Tukanoan population in the Colombian Vaupés. Very few varieties of sweet manioc were known and cultivated. Chernela (1986b) found that repeated efforts by Uanano peoples in the Vaupés of Brazil to introduce sweet cultivars of manioc failed and only bitter cultivars persisted, when she did a study of their manioc varieties. Bitter manioc resembles the native vegetation of blackwater regions by its toxic quality, which serves to conserve nutrients for the plant through reduction of herbivory. Montagnini and Jordan (1983) found that insects consumed less than 3 per cent of the tissue of bitter manioc plants due to the cyanogenetic glucosides present.

Bitter manioc cultivation solves one of the great problems of Amazonian populations: how to cultivate soils that are extremely poor in nutrients, extremely acid, and have toxic levels of aluminium. Manioc, a plant that appears to have evolved in just such areas of South America, can produce impressive yields in areas where nothing else will grow (Moran, 1973). One of the few limitations to its cultivation is its inability to withstand waterlogging, which explains why it is cultivated on higher ground. Manioc is even adapted to drought, during which it loses its leaves and goes into dormancy, gaining its leaves again with the return of soil moisture. Beans and corn, by contrast, are unable to produce a predictable crop in these nutrient-poor areas, and unable to cope with even short-term droughts. Galvão (1959: 24) noted that corn had been abandoned by the populations of the Içana and that it probably never had much importance in the Rio Negro Basin.

Bitter manioc produces the bulk of the calories for black-water basin populations. Dufour (1983) showed that among the Tukancans she studied in Colombia, 70 per cent of the energy came in the form of manioc flour and manioc bread (casabe or beijú), tapioca, manioc beer, and other forms of prepared manioc. The energy efficiency of manioc is impressive: it yielded 15.2 calories for every calorie spent on its production. Seventy percent of the production costs occur during processing. Nevertheless, the oligotrophy of the black-water regions depresses the total yields. Yields vary between 3 and 8 tons per hectare, with a mean of 4.7 (Clark and Uhl, 1987). By comparison, the world mean is 8.4 tons per hectare, and reaches 12.7 tons in Brazil. The relatively low mean harvests confirm the nutrient-poor conditions of the environment.

Clark and Uhl (1987: 19) estimated the fish productivity of the Rio NegroCasiquiare-Guaima rivers to be between 6.6 and 13.2 kg/ha/ year - one of the lowest values for any tropical basin. In Africa and Asia the mean values are 40-60 kg/ha/year and Goulding (1979) estimated the productivity of the Madeira River in Brazil at 52 kg/ha/year.

The lower mean productivity results, in large part, from the absence of some particularly large species like pirarucú or paiche (Arapaima gigas), aruanã (Osteoglossum bicirrhosum), several large species of the genus Colossoma, and several large catfishes or pimeloids (Goulding, 1979: 15). The absence of aquatic grasses in black-water rivers, an important food source for many of the larger fishes, influences the species composition of these rivers and favours smaller species. This is not to say that black-water rivers are species-poor. Goulding et al. (1988) have shown that the lower Rio Negro is among the most species-rich rivers in the world, with approximately seven hundred species. Fish species in the Rio Negro, however, are dominated by smaller species with a mean length of only 40 mm with over one hundred species of less than 30 mm length (Goulding et al., 1988: 109).

Success in fishing in black-water ecosystems depends mostly on territorial control over cataracts and flooded forest, where the fish volume is greatest. The most successful method of fishing is through the use of large fixed traps which require considerable investment in their construction and maintenance. The importance of control over the best fishing spots is evident in such a situation, and reaches a considerable level of sophistication in the Rio Negro (Chernela, 1986a; Moran, 1990). Fish was found to be a part of meals in 78-88 per cent of cases sampled by Dufour (1987: 389).