Contents - Previous - Next

This is the old United Nations University website. Visit the new site at http://unu.edu

2. Environmental threats

The myth of virginity
Climatic change
The environmental impact of smoke
Soil erosion and floods
Hydroelectric dams
The environmental impacts of mining
Petroleum extraction
A blizzard of cocaine
Habitat destruction and the loss of biodiversity

Amazonia has a long history of ecological change under human agency. Hunters and gatherers probably penetrated the region tens of thousands of years ago and artificially enriched their campsites with fruit and nut trees (Smith in press). Hunters undoubtedly fired woody savannas in Amazonia, such as in Roraima, to flush game long before farmers started to clear the forest.

Slash-and-burn farming in Amazonia probably began at least 10,000 years ago, based mostly on root crops, thereby creating a rich texture of forest interlaced with second-growth communities of various heights and ages. At first, such interventions were on a minor scale, but, as the population grew denser, more and more of the forest fell to the axe.

The myth of virginity

Contrary to the prevailing idea of a pristine Amazonian forest little disturbed by human activities until recently (Moran 1993a; Revkin 1990: 39; Richards 1977), much of the region has felt the influence of hunters and gatherers and farmers for a considerable time. Many of the forests of tropical America, including the Amazon, are anthropogenic (Denevan 1992; Turner and Butzer 1992). Assertions that the Amazon is being ravaged by development and floods of land-hungry colonists need to be put within the context of a region that has a long history of settlement and human modifications of the environment.

Archaeological research and unusual concentrations of certain economic plants in the forest suggest much higher human population densities in the past than has hitherto been accepted. Even with relatively inefficient stone axes, aborigines have cleared substantial tracts of Amazonian forests in the past (Huber 1910). Charcoal layers in soils of the upper Rio Negro have been dated to 6,000 years B.P. and some ceramic shards mixed with anthrosols are approximately 3,750 years old (Sponsel 1986). Along the Bragantina coast of Pará, charcoal and potsherds have been dated at 5,000 years B.P. (Simões 1981). The oldest recorded pottery in the New World is from the middle Amazon: ceramics from the Taperinha site near Santarém have been dated at 8,000 years B.P. (Roosevelt et al. 1991). Maize reached the Ecuadorian Amazon at least 6,000 years ago, providing farmers with another food option. Upland and flood-plain forests have thus been altered by farming activities for millennia (Bush, Piperno, and Colinvaux 1989).

Population densities reached high levels, particularly along siltladen rivers, well before contact with Europeans (Moran 1990: 148, 1993b: 114). Estimates of human populations in Amazonia around 1500 range from 1 million to 6 million, or even higher (Smith 1980). Only recently has the region's population regained its former numbers, but with a distinct difference: a sizeable proportion of today's population lives in towns and cities. In pre-contact times, the population was much more rural than at present, and therefore more engaged in farming.

The indigenous population did not raise cattle, however, so cleared areas were devoted to crops and managed fallows. The higher population density in rural areas was thus possible with perhaps the same amount of cleared area as today. The overall cleared area in 1500 was probably close to that prevailing in 1990.

If pre-contact aboriginal populations of Amazonia reached in excess of 6 million people, forest fires in the region were probably as common as at present, but on a smaller scale. Indians would not have cleared fields the size of some ranches and plantations being opened up in the region, particularly in southern Pará, northern Mato Grosso, Rondônia, and Acre, but they likely lit many more smaller fires to prepare fields.

Instead of massive fires concentrated in active colonization zones, particularly in the south-eastern and south-western fringes of Amazonia (fig. 2.1), hundreds of thousands of smaller fires would have been scattered over the basin in pre-contact times. Agricultural activities in Amazonia before the arrival of Europeans were akin to a buckshot event (Oldeman 1989); localized, small-scale clearings spread out over a large area. A Dante's inferno characterizes colonization zones today in the dry season, whereas the twinkling of innumerable small fires would have dotted the nocturnal landscape after the main rains in prehistoric Amazonia.

The shift to clearing larger sections of forest is ecologically more damaging than the scattered, insular fires of the past. When thousands of hectares are cleared for a single ranch or plantation, the ecological fabric of the area becomes simpler (Uhf, Buschbacher, and Serrão 1988). Instead of a patchwork quilt of various plant communities, more homogeneous landscapes emerge. Also, seed sources for forest regeneration became scarcer and soil nutrient recycling systems can be disrupted (Buschbacher, Uhl, and Serrão 1987; Nepstad, Uhl, and Serrão 1990,1991; Serrão et al. 1979).

The implications of a dense, pre-contact human population in Amazonia are far-reaching. A salient lesson here is that Amazonia's diverse environments can support relatively large populations, even on nutrient-poor oxisols and ultisols, if resources are managed wisely. Although some have argued that nutrient deficiencies prevent continuous crop production on highly weathered ferrallitic soils of many parts of the lowland, humid tropics (Weischet and Caviedes 1993: 278), pre-contact indigenous populations probably deployed a wide variety of swidden systems that permitted relatively dense populations, even in inland areas.

People in Amazonia have greatly altered plant and animal communities and the distribution and population densities of certain plants and animals, and have probably triggered increased soil erosion and aggrading of smaller rivers and streams for millennia. The notion of a vast, undisturbed wilderness in Amazonia is an artefact of the indigenous population crash after contact with Europeans and the unleashing of introduced diseases such as smallpox and influenza.

Climatic change

The impact of deforestation on regional and global climate has received the most attention when environmental change is discussed in Amazonia (Bunyard 1987; Collins 1990; Dickinson 1987, 1989; Leopoldo, Franken, and Matsui 1985; Myers 1988; Prance 1986; Reis 1972; Wood 1990). Tropical deforestation is often pinpointed as a major culprit in the purported global warming trend and, since Amazonia is the largest stretch of tropical forest, its fate is thought to have an important bearing on the future of the world's climate. Amazonia was probably also heavily cleared in pre-contact times, without triggering the greenhouse effect.

Global warming

Increased atmospheric levels of carbon dioxide and other gases, such as methane, nitrous oxide, and chlorofluorocarbons (CFCs), can potentially trigger a greenhouse effect (Ravel and Ramanathan 1989). Many in the scientific community, and much of the reporting in the media, suggest that we are on a global warming course propelled by human activities such as burning forests and fossil fuels. But claims that a global warming has already begun may be premature (Byrne 1988; Flavin 1989; Schneider 1989). No firm evidence has yet emerged that the world is becoming significantly warmer (Abelson 1990a; Blinder 1992; Hansen and Lacis 1990; Ray 1993: 12; Solow and Broadus 1989; Spencer and Christy 1990). Indeed, surface temperatures over the western Arctic Ocean have become significantly cooler during the 1950-1990 period (Kahl et al. 1993). Such inconsistent reporting on "global warming" points to major defects in current models of global climate and suggests a tenuous basis for drawing policy conclusions (Bryson 1989).

Although some data suggest a recent global warming trend, no valid correlation with greenhouse gases can be made, nor can we be sure how long this trend will last (Barrett 1990). Even if such changes will soon be confirmed, it will be difficult to separate natural climatic cycles from any greenhouse effect (Mitchell, Senior, and Ingram 1989). For example, the subsurface thickness of ice around the North Pole varies markedly from year to year, and no significant trend emerged during the 1977-1990 period (Langereis, Van Hoof, and Rochette 1992). Also, the effects of clouds, volcanic dust, and oceans on any possible greenhouse effect are imperfectly understood (Abelson 1990b; Jarvis 1989; Kerr 1989; Slingo 1989).

If evapotranspiration rates are substantially reduced in Amazonia as a result of landscape changes, less latent heat may be exported from the region in water vapour (Motion 1987). Whether reductions of the moisture level in warm air circulating from tropical regions to temperate areas in the Hadley cell would affect global climate is unclear. Reduced moisture levels could result in less latent heat being released during condensation, thereby cooling climates. Such a mechanism would help to counteract any greenhouse effect. Deforestation also tends to increase the albedo affect, further reducing energy for atmospheric heating.

In the event that the greenhouse effect takes hold, tropical deforestation will be only partly at fault (Radulovich 1990). Deforestation accounts for less than 20 per cent of greenhouse gas emissions (Flavin 1989: 13). The amount of carbon in initial, undisturbed ecosystems may have been overestimated, thus exaggerating the impact of deforestation on the release of carbon dioxide to the atmosphere (Post et al. 1990).

Carbon dioxide from the burning of fossil fuels, which occurs mostly in temperate countries, is the largest component of greenhouse gases. Just seven industrialized countries produce 40 per cent of carbon dioxide emissions worldwide (Turner et al. 1990a). The industrial countries are responsible for approximately 85 per cent of carbon dioxide build-up in the atmosphere (Parikh 1992).

The release of CFCs, used to make aerosols, refrigerants, and solvents, is responsible for a larger proportion of greenhouse gases entering the atmosphere than CO2 emissions from burning forests. Industrial countries are responsible for most of the CFC emissions. The notion that developing countries must take a large share of the blame for any global warming has been ascribed to environmental colonialism (Agarwal and Narain 1991).

Deforestation in Amazonia may not be implicated in excessive emissions of nitrous oxide, a potent greenhouse gas, as previously thought. Although higher levels of nitrous oxide are released in pasture soils in the first 10 years after forest clearing, emissions of the gas subsequently decline to lower levels than those coming from tropical forest (Keller et al. 1993). This does not mean that cattle-raising is necessarily an appropriate land use for much of Amazonia from the policy standpoint; rather the ecological impacts of different land uses must be weighed according to a "basket" of criteria, and more longterm research is often needed to decipher the environmental effects of habitat change.

The idea that Amazonian countries should arrest forest clearing to save the world's climate while North Americans and Europeans continue to drive their increasingly more powerful cars and burn natural gas and coal does not rest well in Brasilia, Bogota, or Lima (Nisbet 1988). Developed countries will need to do more to reduce their own carbon dioxide emissions if they expect third world countries to tackle the issue (Caccia 1991). Some scientists have urged policy makers to separate "survival emissions," such as resource-poor farmers practising slash-and-burn agriculture, from "luxury emissions," particularly gas-guzzling cars plying the streets of major cities, particularly in the industrial countries (Agarwal and Narain 1991). Some industrial countries, such as Germany, the Netherlands, and Japan, have adopted carbon dioxide stabilization or reduction targets (Miller 1991), but many others are apparently waiting for a global consensus to emerge on appropriate action.

A cautious approach to formulating environmental and economic policy to address global warming has been adopted by several governments. Three main factors account for this wait-and-see attitude (Riebsame 1990). First, climate change predictions are too uncertain, particularly at the regional level. Second, current systems are thought to be capable of absorbing climate change without major disruption, at least for the next few decades. Third, technologies can be deployed to mitigate or compensate for some of the changes wrought by global climate change.

Sceptics about global warming can also point to the fact that C crops, such as rice and wheat, would likely produce higher yields with a doubling of atmospheric carbon dioxide levels if sufficient water and nutrients were available. The C crops, such as sugar cane and maize, are unlikely to be affected by increased carbon dioxide levels, at least for the foreseeable future. Also, some regions, such as the drier tropics, might benefit from a global warming since they could receive more rain.

The ability of countries to respond to global warming will depend in part on the strength of their agricultural research and extension systems to deliver new technologies. Areas with a shift to wetter climates will probably need crop varieties more resistant to fungal and bacterial diseases. Unfortunately, as the need for research institutions to be primed and ready to confront new challenges increases, their scientific capacity is at a low ebb.

In the 1970s and early 1980s, Brazil had one of the strongest agricultural research programmes among the developing countries, with an annual budget of close to US$200 million. Since the mid-1980s, however, high inflation and severe financial constraints have hampered EMBRAPA's (Empresa Brasileira de Pesquisa Agropecuária) ability to raise and sustain agricultural productivity (Ruttan 1991). The agricultural research systems of other countries with territories in Amazonia have also weakened over the past decade or so.

Another constraint on the deployment of technologies to meet the challenge of a possibly warmer world is the loss of biodiversity. As discussed in more detail later, the loss of habitats, particularly in the humid tropics, could have grave consequences for agriculture. The loss of genetic resources for crop improvement, and the disappearance of new crop candidates, could "tie the hands" of plant breeders trying to develop crops adapted to changing environments.

In spite of uncertainties about global warming trends and hazards, pressure is mounting for governments in both industrial and developing countries to take concrete steps to halt the build-up of greenhouse gases in the atmosphere. Tree planting is seen as one way to counteract the greenhouse effect, by providing a carbon sink (Myers and Goreau 1991). Although this popular notion empowers people to do something about a widely perceived problem, the impact of tree planting on the build-up of carbon dioxide in the atmosphere pales compared with what could be accomplished by the more efficient use of fossil fuels. At least 100 million ha would have to be planted to fast-growing trees to sequester a little over 10 per cent of the current annual build-up of carbon in the atmosphere (Myers and Goreau 1991). Once the trees reached maturity, they would no longer act as carbon sinks. That is an area equivalent to Britain, France, and Germany that would then cease to serve as a trap for carbon. The conversion of old-growth forests to fastgrowing plantations would release carbon to the atmosphere, in spite of the greater net photosynthesis of younger trees (Harmon, Ferrell, and Franklin 1990).

Although rehabilitating degraded areas with trees would be desirable, simply filling the landscape with "greenhouse" trees without regard to cultural and economic needs could be counter-productive. Cultural landscapes could not easily accommodate hundreds of millions of hectares of tree planting without disrupting food production and other economic activities, even if the "greenhouse gas" trees formed integral parts of agro-forestry systems. Cleared lands are often fully occupied, and the logistical and managerial implications of massive tree planting would need careful study (Churchill and Saunders 1991).

Reduction of methane emissions, an often overlooked contributor to the greenhouse effect, could help mitigate any global warming. Methane is a far more potent greenhouse gas than carbon dioxide and landfills in the developed world are a major source of methane emissions (Hogan, Hoffman, and Thompson 1991). Recovery of methane from landfills not only would reduce the greenhouse effect, but could supply gas to generate electricity. Improved coal-mining and oil-production techniques would also reduce methane emissions. Ruminant livestock and rice cultivation are also significant sources of methane. Deforestation and burning also increase methane concentrations in the atmosphere, but, again, a large share of the onus for reducing this greenhouse gas rests with the industrial countries.

The spectre of parched deserts

In addition to temperature changes, deforestation has the potential of adversely affecting rainfall regimes. Half of the rain that falls in Amazonia is thought to come from evapotranspiration (Molion 1975: 101; Salati 1987; Salati and Vose 1984; Salati, Marques, and Molion 1978). Accordingly, it has been assumed that continued deforestation might lead to a drier regional climate (Hecht and Cockburn 1989: 43). A linkage between the loss of forests and reduced rainfall has been surmised for centuries, and was much discussed in India and parts of the Caribbean in the nineteenth century (Glacken 1967; Grove 1990). Cattle-ranching and the destruction of Latin America's tropical forests have been blamed for reduced rainfall and increased droughts (Salati1992; Shane 1986: 23). Some computer models predict a sharp drop in rainfall with continued, large-scale deforestation in Amazonia, thus raising the spectre of dust-bowls and desertification (Anderson 1972; Barros 1990: 20; FAO 1991b: 3; Goodland and Irwin 1975; Modenar 1972; Paula 1972; Roddick 1991: 206; Sioli 1987). Cattleranching is sometimes singled out as most likely to provoke desertification (Wesche 1974). But such rainfall models assume that Amazonia will be turned into a barren landscape (EMBRAPA 1989: 6).

It is highly unlikely that substantial areas of Amazonia will be converted to asphalt or a desert. Second growth soon begins the regeneration path to forest in all but the sandiest soils (Moran 1993a). The widespread struggle to keep pastures and crops free of weeds is a testament to the striking speed of secondary succession. Weeds, not deserts, are a major headache for farmers, ranchers, and plantation owners in Amazonia. How soon mature forest returns depends primarily on the texture and fertility of the soil and the proximity of seed sources. Also, more rainfall may be derived from the flux of water vapour from the Atlantic than has previously been supposed (Paegle 1987).

How much forest can be removed without affecting rainfall is not known. Realistic predictions of climatic change as a result of landscape changes in the humid tropics are fraught with difficulties (Henderson-Sellers 1987). Current models of forest and climate interactions in Amazonia are too imprecise to predict with any degree of certainty the impacts of deforestation on rainfall (Salati 1992). Evapotranspiration from groves of perennial crops and silvicultural plantations may be close to that of forest. Even pastures release substantial quantities of water to the atmosphere during the rainy season. During the dry season, though, pastures transpire less water than forest and experience significantly warmer mean surface temperatures (Nepstad, Uhl, and Serrao 1991; Nobre, Sellers, and Shakla 1991).

No evidence is available to prove that deforestation in Amazonia has led to reduced rainfall. The dry season around Manaus was accentuated in 1976, and again in 1979, when no rain fell for 73 days (Fearnside 1986: 50). The "summer" (verão) of 1992 was also severe, with many farmers experiencing reduced yields or the loss of seedlings of perennial crops. But it is hard to separate little-understood climatic cycles from long-term trends. To keep matters in perspective: after two particularly heavy burning seasons in Amazonia in 1987 and 1988, 1989 was a very wet year. So much rain fell in eastern Amazonia in 1989 that the dry season virtually disappeared.

Rainfall patterns are highly variable in Amazonia. In 1774, a drought assailed the Rio Negro watershed when deforestation rates were much lower than at present (Hemming 1987). River levels in Amazonia were unusually low in 1860 owing to poor rainfall (Chandless 1866). In 1958, 64 days passed without any rain in the Bragantina zone east of Belém (Penteado 1968: 138). The Amazon River was particularly low again in 1963. The big "push" to develop and open up the Amazon started only in the late 1960s.

Unusual weather patterns also prevailed in the southern United States in the 1980s. Record heat and drought seared the southern and eastern parts of the United States during 1987 and 1988, thereby provoking widespread concern about global warming. Summer 1989 and spring 1992 in the eastern United States, however, were wetter and cooler than usual, and talk of global warming in the media subsided. In Western Europe, several hurricane-force gales during the winters of 1988 and 1989 led to speculation among some politicians that global warming was under way (Maddox 1990). Some in the environmental movement may be concerned that a return to more "normal" weather will cool the ardour of politicians to tackle the issue of global warming.

Predictions about how dry the Amazon might become with continued deforestation are fraught with shaky assumptions. With satellite imagery, it should be possible to document the area of forest cleared each year. Through its remote sensing agency (INPE - Instituto Nacional de Pesquisas Espaciais), the Brazilian government monitors forest burning in Amazonia. But what happens to the land after it is cleared is crucial to the question of climatic drying. Landsat and Spot imagery can separate second growth, cropland, grassland, and forest, but it cannot readily differentiate between secondgrowth stages or types of crops. Aerial photography from aircraft would be too expensive on the scale needed to document annual vegetation changes in the region as a whole. More sophisticated satellites in the future may be able to help. Also, more information is needed about evapotranspiration rates in various vegetation communities, including croplands.

The environmental impact of smoke

Smoke from forest cut for agriculture and ranching may have global in addition to regional and local impacts. One component of biomass burning, methyl chloride, attacks ozone, and deforestation in Amazonia has been implicated as partly responsible for holes in the ozone layer over Antarctica (Cutrim 1990). Human-induced fires account for 5 per cent of the ozone-destroying chemicals in the atmosphere (C. Anderson 1990). Nevertheless, industrial countries release far more ozonedepleting aerosols into the atmosphere than do farmers and ranchers in the tropics.

Excessive smoke could lead to temporary climatic disruptions. Smoke reflects some incoming radiation back into the atmosphere, thereby helping to mitigate any greenhouse warming. A reduction in the solar energy reaching the earth's surface may reduce convectional activity and rainfall in some areas. The burning season in most of Amazonia extends from July to October; little smoke is generated during the rainy months.

Smoke from burning fires is thick enough temporarily to close some regional airports during the dry season, particularly in Rondônia and Acre. Temperature inversions can exacerbate the haze. In the dry season, cloudless skies facilitate radiative cooling of the land, thereby helping to create a layer of warmer air above that traps pollutants. Prior to 1970, the Brazilian Air Force had to close airports in parts of the Amazon because of the excessive haziness due to smoke. Although the shutting down of airports because of smoke is not new in Amazonia, the problem has become more acute for airfields in parts of Rondônia and Acre in the past two decades.

Smoke from cleared fields has been implicated in poor harvests of Brazil nut (Bertholletia excelsa) in Pará (Miller 1990). Smoke is believed to interfere with bee pollinators of Brazil nut trees, but such a linkage has not been demonstrated conclusively. Variable yields of Brazil nuts are more likely related to the severity of the dry season when flowers are formed. If the dry season is not pronounced, flowering is reduced and fruit production will be poor the following year. The dry season in 1989 was exceptionally wet in eastern Amazonia, and the 1991 harvest of Brazil nuts was correspondingly poor. At Tomé-Açu, for example, 220 mm of rain fell in October 1989, normally a dry month.

Brazil nut harvests in the municipality of Marabá, the major centre for the Brazil nut trade, have varied from a low of 2,000 tons in 1985 to a high of 17,732 tons in 1970. Marabá was connected to the BelémBrasilia Highway along PA 332 (then PA 70) in 1970, so the peak of production in that year might be attributed in part to improved transportation. Traditionally, the Brazil nut harvest had been taken out by river and along a now defunct railroad from Jatobal to Tucurui on the Tocantins.

Brazil nut harvests have often been highly variable, ever since collecting in earnest began in the Tocantins watershed in the latter half of the nineteenth century. In 1895, for example, some 450 tons of Brazil nuts were collected along the Tocantins, whereas in 1896 the harvested amounted to only 250 tons. In 1987, however, 1,000 tons of nuts were gathered, and the harvest would have been closer to 2,500 tons if the Tocantins had not experienced a massive flood (Moura 1989: 153). Such oscillations in Brazil nut harvests could not be attributed to smoke since few people lived along the river at that time.

If smoke from fires was the principal cause of variation in Brazil nut harvests, one would expect a sharp decline during the 1970s and 1980s. Although harvests appear to have dropped during much of the 1980s, a shift of Brazil nut gatherers to more lucrative occupations, particularly gold mining, probably accounts for lower harvests (Smith et al. 1991a). Fluctuations in Brazil nut prices on world markets have also probably influenced collecting efforts.

Soil erosion and floods

Soil erosion is one of the most serious threats to the sustainability of agriculture, silviculture, and forestry in Amazonia. Although many of the region's soils are deep, extending down several thousand metres in some cases, fertility is usually concentrated in the first few centimetres of topsoil. The need to protect the soil is a major reason that perennial crops, silviculture, and properly managed pastures are among the more viable options for rural development. Soil erosion is a contributing factor in the decision of many farmers to abandon their fields and clear a fresh plot from the forest.

Soil erosion can lead to larger-scale environmental problems by aggrading river beds. Increased run-off from cleared land deposits silt and sand on river beds, thereby provoking more severe floods. A rising and lowering of water levels along streams and rivers in Amazonia is part of the normal seasonal pulse of wet and dry seasons (Sternberg 1975). Some unusually heavy floods along the Amazon in the mid-1970s raised the spectre that deforestation in the foothills of the Andes was having a tangible impact downstream (Fowler and Mooney 1990: 106; Gentry and Lopez-Parodi 1980; Smith 1981a: 122). But statistical analyses of flood peak levels do not reveal any trend to more intense flooding along the Amazon (Richey, Nobre, and Deser 1989; Sternberg 1987b). The greatest flood ever recorded along the Amazon was in 1953, well before major development projects were unleashed in Amazonia. The Amazon River reached unusually high levels again in 1993, although deforestation rates have abated since the 1980s.

Similar findings have been made along the Ganges, where flooding in Bangladesh is not a result of deforestation in the Himalayas, as is commonly thought (Ives and Messerli 1989). An important variable in runoff from deforested lands is the infiltration rate of the soil (Newson and Calder 1989). Some compaction of the soil is likely after the forest is cut, but how adversely infiltration is affected over the long term depends on land management and soil type.

Destruction of forests along streams and some river banks is surely affecting water quality and flow on a local scale. But the vast scale of Amazonia's forests appears to be masking the impact of deforestation on smaller watersheds. Landscape changes are not currently radical enough to affect Amazon hydrology on a large scale (Bayley 1989). Also, the variability of rainfall in different parts of the Amazon basin could lead to premature conclusions that floods are more pronounced along the Amazon.

Hydroelectric dams

Disruption of fisheries as a result of dam-building could be a serious threat to the livelihoods of many rural and urban folk in Amazonia (fig. 2.2). A number of fish important in commerce and subsistence, such as jaraqui (Semaprochilodus spp.) and catfish (Brachyplatystoma flavicans, B. filamentosum), migrate from the Amazon to spawn in tributaries (Barthem, Brito, and Petrere 1991; Goulding 1981, 1989). Changes in water quality and flood cycles are likely to interfere with the reproduction and feeding of many of the 2,000 or more species of fish inhabiting the myriad waters of Amazonia.

Little is known about the impact of dams built thus far on Amazonia's fisheries (Bayley and Petrere 1990). On 2 October 1992, a school of jaraqui (Semaprochilodus sp.) was observed swimming back and forth along the foot of the dam at Curuá-Una in a fruitless at tempt to move upstream. The dam closed in 1976, so it is curious to see fish still trying to move upstream. Several other species have apparently disappeared above the dam, such as pirapitinga (Colossoma bidens) and jatuarana (Brycon sp.), but one interviewed farmer felt that fishing yields overall had not declined. Tucunaré (Cichla ocellaris) and charuto, in particular, are reportedly plentiful in the reservoir. The Curuá-Una reservoir spawned a population explosion of piranhas (Serrasalmus spp.) during the first two decades of operation (Ferreira 19B4; Junk et al. 1981). For all their fame as dangerous fish, piranhas are eaten by locals and sell briskly in urban markets.

Fig. 2.2 Major hydroelectric darns in Amazonia

Some fish were killed by the lack of oxygen and hydrogen sulphide when the Tucurui dam closed in 1984 (Sioli 1986), but the reservoir has become a significant fishery for the highly prized tucunare. Tucunaré from the Tucurui reservoir are marketed at least as far south as Carajás. The 2,100 km Balbina reservoir on the Uatumã River, completed in 1987 to provide electricity for Manaus, has also become a significant fishery for this spirited predator.

Known as peacock bass to English-speaking sport fishermen, tucunaré may accumulate mercury released by gold miners. Also, tucunaré in the Curuá-Una resrvoir near Santarém, Pará, became so heavily infested with parasitic nematodes that some locals declined to eat the highly prized fish (Junk and Nunes de Mello 1987).

Some fisheries downstream from the Tucurui appear to have suffered from the dam (Dwyer 1990: 44; Magee 1989). The productivity of fisheries appears to have declined mostly in the lower regions of the Tocantins in the vicinity of Cametá. One migratory species, Anodus elongatus, has virtually disappeared from the lower Tocantins (Merona, Carvalho, and Bittencourt 1987). Populations of Curimata cyprinoides have also diminished, at least temporarily. The Tucurui dam contributed to the collapse of the mapará fishery on the lower Tocantins by closing off a spawning route and reducing plankton biomass (Goulding, Smith, and Mahar in press).

Although the composition of fish communities has shifted below the Tucurui dam, the overall impact of this formidable barrier on fisheries has not proved especially serious from the perspective of local nutrition. A shrimp fishery based on Macrobrachium amazonicum was waning along the lower Tocantins well before the Tucurui dam closed; furthermore, this freshwater shrimp, which is used in a variety of regional dishes, thrives in the Tucurui reservoir (Odinetz-Collart 1987). The rapid turnover of water in the Tucurui reservoir, some six or seven times a year, helps avoid drastic changes in water chemistry and thermal stratification, thus reducing the danger of intoxicating fish (Barrow 1987).

Turtles, especially Podocnemis expansa, were abundant along the Tocantins in the seventeenth century and served as an important food for locals (Heriarte 1964: 30), but this resource had dwindled considerably long before construction of the Tucurui dam. The resulting reservoir, however, has covered many former nesting beaches and will probably preclude the chances of re-establishing sizeable populations of P. expansa along the Tocantins.

One concern about reservoirs in the Amazon is their potential role in favouring the population build-up of disease vectors. Soon after the Tucurui dam closed, some inhabitants and their livestock near the margin of the reservoir were plagued by swarms of mosquitoes (Mansonia titillans) and, to a lesser extent, horseflies (Lapiselaga grassipes). The former are known to carry two arboviruses, but no outbreaks of disease attributed to M. titillans have occurred near the Tucurui reservoir (Marques 1992). Populations of both flies appear to have dwindled, presumably as the new lacustrine ecosystem and surrounding areas have become more stable.

The Samuel dam on the Jamari River in Rondonia, which filled in 1989 to supply electricity to Porto Velho, has reportedly disrupted the upstream migration of some large catfish (João Paulo Viana, pers. comm.). Fisheries have also allegedly suffered downstream from the Balbina dam, but no quantitative data support such claims (Gribel 1990).

Some farmers have apparently benefited downstream from the Tucurui dam, whereas others have lost fertile planting ground. Regulation of water flow facilitates the irrigation of rice at the mouth of the Tocantins, but the reduced sediment load has resulted in a loss of flood plain for agriculture along the lower Tocantins (Barrow 1988).

No reservoirs created for hydroelectric dams in Amazonia are in imminent danger of losing electrical generating capacity because of siltation. The oldest dam, Curuá-Una, is nearly 20 years old and is still operational (fig. 2.3). All the other hydroelectric dams were built in the late 1970s and in the 1980s (table 2.1). Although the Tocantins appears to be getting cloudier as a result of forest clearing, "storage pockets" abound in the reservoir bed and this dead space will take some time to fill with sediment.

Table 2.1 Major hydroelectric dams operating in Amazonia

Sources: Barrow (1988); Ledec and Goodland (1988); Serra (1992); Sioli (1986); field notes of NJHS at the Curuá-Una dam on 2 October 1992.

a. See fig. 2.1 for dam locations.

b. The Tucurui dam is expected eventually to generate 4,000 MW.

The Tucurui reservoir has drowned tens of thousands of Brazil nut trees (fig. 2.4). The Tocantins valley has always been the most important centre for the Brazil nut trade, and the 2,000 km reservoir has destroyed some valuable plant resources. It is difficult to measure the impact of such losses on yields, but some unique germplasm has surely been lost. As the Brazil nut's genepool shrinks, genes that could be useful for future improvement efforts also vanish.

Amazonia is relatively flat, so dam-building along major rivers leads to the drowning of substantial tracts of flood plain and upland forest. Iquitos on the Upper Amazon is 3,600 km from the Atlantic, yet only 80 metres above sealevel (Irion 1989). From Iquitos to the confluence with the Negro, the Amazon drops only 57 metros. Some dams are more "cost effective" in terms of kilowatts generated per flooded area (Ledec and Goodland 1988: 60). The Tucurui dam generates 30 kW/ha of reservoir, whereas Balbina delivers only 2 kW/ha of flooded area (table 2.1). Both dams are low in electricity generated per flooded area when compared with rivers with steeper valleys, such as the Itaipu dam on the Paraná (77 kW/ha) or the Grande Coulee on the Colorado (63 kW/ha).

The apparent and hidden ecological costs of building major dams in Amazonia must be weighed against the benefits hydroelectric dams bring to the region. Brazil's desire to tap the hydroelectric potential of waters in Amazonia is understandable in view of the burdensome bill for imported petroleum. Brazil produces less than one-third of its petroleum needs, and most of the electricity generated in Amazonia has historically come from diesel-powered turbines. The Tucurui dam has benefited Belem and environs with reliable electricity and has created jobs, such as at the aluminium smelting plant at Bacarena.

Nevertheless, a series of smaller, more environmentally benign hydroelectric projects might prove more suitable over the long term. To supply power to the pulp mill at Jari, for example, a proposed hydroelectric plant at the Santo Antonio Falls will divert part of the river through a turbine and thus will not involve any flooding.

Continue

Contents - Previous - Next