Large-scale development and its threats to the forest ecosystem

Overview

Main forms of large-scale development that threaten tropical forest and effects of large-scale development on indigenous forest peoples.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What are the main forms of large-scale development that threaten tropical forest?

   Model answer

   Commercial logging for hardwood timber and pulp. Conversion to agriculture, both cattle ranching on large pastures and plantations of crops such as oil palm, soya, rubber and sugar. Mining and quarrying for minerals such as iron ore, bauxite, gold and copper, together with oil and gas extraction. Hydroelectric schemes whose reservoirs flood large areas of valley forest. Road building, which opens the interior to all the other activities. Settlement and resettlement schemes that move populations into the forest, and the expansion of towns that follows.

2. 2

   Why is road building often described as the most damaging single act of development in a tropical forest?

   Model answer

   Because of what follows it rather than the width of the road. A highway through the interior gives access to previously unreachable forest, so logging, mining, ranching and spontaneous settlement follow along it. Clearance spreads outward from the road in a characteristic herringbone pattern of side tracks and plots. The road fragments a continuous ecosystem into blocks, cutting off animal movement and creating long edges where the forest microclimate dries out and species composition changes. It brings people, fire and disease into remote areas, and it makes the export of timber and minerals economically viable where it previously was not.

3. 3

   Even selective logging that takes only a few trees per hectare causes disproportionate damage. Why?

   Model answer

   Because of the physical structure of the forest. Emergent and canopy trees are linked to their neighbours by lianas, so felling one drags down several others. Heavy machinery needs skid trails and log landings, and dragging trunks out gouges the soil and compacts it over a much wider area than the trees actually removed. Every gap in the canopy lets light and drying wind into the understorey, changing the microclimate. And because commercially valuable species are widely scattered rather than growing in stands, extracting a small volume of timber requires access across a very large area. The result is that removing a few per cent of the trees may damage a large fraction of the stand.

4. 4

   Assess the threat that cattle ranching poses to the tropical forest.

   Model answer

   It is among the largest causes of clearance by area, because it requires the complete removal of the forest over very large blocks rather than selective extraction, and the land is usually burned to establish pasture. It is also among the least productive uses of the land: latosols are quickly exhausted of the ash flush, pasture grasses decline within a few years, stocking rates fall, and the ground is compacted by cattle so that erosion accelerates. Ranchers then commonly abandon the degraded plot and clear fresh forest, so the damage is progressive. Because the returns per hectare are low, the economic gain is small relative to the biomass, biodiversity and carbon lost, which makes it particularly difficult to defend on any cost-benefit reasoning.

5. 5

   How does mining damage the forest ecosystem beyond the area actually excavated?

   Model answer

   Overburden and spoil are dumped over adjacent forest, and tailings ponds occupy further land. Processing uses and contaminates large volumes of water: mercury used in gold extraction and acid drainage from sulphide ores enter the rivers and accumulate in fish and therefore in the people and animals that eat them. Sediment from workings smothers river beds and destroys spawning grounds far downstream. Mines need roads, power lines, and often a dam and a town, each of which clears more forest and attracts more people. And a mine has a working life after which the site, on a latosol stripped of its topsoil, is extremely difficult to restore.

6. 6

   What are the consequences of large-scale forest clearance for the water cycle, locally and regionally?

   Model answer

   Interception and evapotranspiration both fall sharply, because there is no longer a huge leaf area intercepting rain and returning moisture to the atmosphere. Locally this means more water reaches the ground surface and, on a compacted or crusted soil, runs off rather than infiltrating, so river discharge becomes flashier, with higher flood peaks after rain and lower flows between them, and reservoirs and channels silt up. Regionally, since a large share of rainfall over a great forest basin is recycled moisture transpired by the forest itself, extensive clearance reduces the moisture supply to the atmosphere and can reduce rainfall over the remaining forest, drying it and making it more vulnerable to fire. That feedback is why clearance in one part of a basin is a threat to forest elsewhere in it.

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