|The Fragile Tropics of Latin America: Sustainable Management of Changing Environments (UNU, 1995)|
|Part 1 : The ecological outlook|
|Rich and poor ecosystems of Amazonia: an approach to management|
The Amazon always brings out the grandiose vision in us. It is by any measure an enormously vast area. The drainage basin encompasses about 4 million Km², an area the size of the continental United States. Contrary to the popular opinion, the Amazon Basin is not mostly tropical rainforest. It is made up of a patchwork of different vegetations, reflecting environmental conditions as well as past use and abuse. In it we find deciduous forests, seasonal forests, vine forests, palm forests, flooded forests, moist forests, well-drained and poorly-drained savannas, and xeromorphic scrub forests or caatigas amazônicas.
In this paper I will focus on two extremes of the environmental gradient in Amazonia. This gradient is based not only on the criteria of nutrient availability in soils and their pH, but relies also on assessment of other environmental and social processes (such as ABOVEGROUND biomass and vegetation transformations by prehistoric populations) relevant to the future use and management of these areas. The two extremes chosen are the oligotrophic black-water river basins - of which the Rio Negro Basin is the best known - and the anthropogenic forests found associated with nutrient-rich soils in the lower Xingú, lower Tapajos, and lower Tocantins basins and in portions of the state of Rondônia like the Guaporé valley.
This contrast departs from the traditional one commonly made between the terra firme (uplands) and várzeas (floodplains) of Amazonia. I do not dispute the validity and usefulness of the contrast between the floodplain and the uplands - a contrast that has served to highlight the differences between those areas enriched by Andean alluvial deposition and areas lacking these favourable conditions. However, this contrast has also hidden the heterogeneity present within the terra firme, an area that accounts for 98 per cent of the entire basin. As this paper hopes to show, the terra firme is very diverse in environmental conditions. The two extreme points of the gradient which I will discuss serve simply to suggest a more extensive exploration that makes more detailed discriminations within the terra firme (Moran, 1990,1993). This heterogeneity has implications to the management of the region.
Within the terra firme, to which I will confine myself in this paper, annual precipitation varies from a low of 1,500 mm to a high exceeding 3,600 mm (Salati, 1985: 33), with considerable micro-ecological variation. The number of meteorological stations is still very small and the variability is probably even greater than figure 3.1 suggests.
Average monthly temperatures do not fluctuate annually by more than 3ºC but the daily variation can be greater than 15°C.
An important ecological distinction was made long ago between the rivers of the Amazon (Sioli, 1951). Three types were distinguished: black-water, white-water, and clear- or blue-water rivers. See figure 3.2 for illustration of the areal extent of these three types of rivers. White-water rivers drain the Andean area and commonly, but not always, carry sediments of high fertility. Black-water rivers drain areas with white sand, podzolic soils, and are extremely acid and of poor optical quality owing to suspended undecomposed organic matter. Clear-water rivers drain areas of the Guiana and Brazilian plateaux and are of medium quality in terms of nutrients and acidity.
The areas drained by black-water rivers can be considered to fall at the poorest extreme of the ecosystems of Amazonia. They are dominated by the most nutrient-deficient and acid soils found in the basin. The forests have such a tight nutrient-cycling that hardly any nutrients escape the forested areas, and the river water has been described as being of near-distilledwater quality in terms of nutrients (Sioli, 1951). The productivity of blackwater lakes is 15 to 19 times less than in várzea, lakes and the fish show signs of nutrient deficiency in their vertebrae (Geisler and Schneider, 1976; Smith, 1979).
The extremely low levels of nutrients or oligotrophy lead to other responses that create additional difficulties for organisms exploiting such areas: organic matter decomposes more slowly because of the extreme acidity of the soils; native plants have evolved high levels of polyphenols and other toxic substances that appear to reduce nutrient loss through herbivory but which require specialized processing to detoxify the plant material. The extreme cycles of flood and drought have led to the selection of plants with highly specialized adaptations to these conditions, in the form of unusually high root biomass and leaves that are either leathery or spiny. The poor optical resolution of the rivers makes techniques like the use of bow and arrow in fishing less productive than elsewhere. These areas require careful description by ecologists and ethnographers because of their patchy nature and the coexistence of low above-ground biomass productivity and high biotic and abiotic diversity.