Describe how a global vegetation system you have studied has been modified by human activity

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L.Tollman

Describe how a global vegetation system you have studied has been modified by human activity

        Human activity affects rainforest ecosystems in many ways, some of which are negative and some of which are positive. One impact of human activity is the loss of nutrients from the soils of the rainforest. This occurs as tropical vegetation is interfered with or removed by deforestation, this leads to changes in nutrient availability and transfers. In the rainforest most of the nutrients are stored within the biomass and the ground litter reservoirs, the main transfers are between these two reservoirs. Dead and decaying matter decomposes rapidly and plant roots then take up nutrients. Under human influence of for example a ‘slash and burn’ regime with a three year cropping limit and a long fallow period in between the nutrient levels in the litter and the biomass decrease and the nutrients in the soil increase. This is however counteracted by heavy rain which causes loss of nutrients through leaching, soil erosion and to the atmosphere. Soil fertility increases during periods of fallow should sufficient time be allowed. Rainforest ecosystems under permanent cultivation show depletion in nutrient stores in the biomass, litter and soils. Soil fertility therefore reduces rapidly. Areas that have been intensely cultivated in the past might never return to their former forest cover and are much more likely to be covered in heathlike vegetation.

        Soil erosion is also an effect of human activity in rainforest areas. Soils of the tropical rainforest are ferralitic soils or latosoils. Once again it is deforestation that causes this problem, as under natural conditions, the tree canopy would provide an effective shield against heavy rainfall. Ground litter would usually absorb some of the falling rainfall therefore protecting the soil further. Tree roots and animal burrows also allow rainwater to percolate through the soil with the result that levels of soil erosion are low under the natural vegetation cover. Without this rapid removal of the topsoil occurs, leaving behind an impermeable laterite layer that hardens when exposed to air. Soil erosion also causes deep gullies to form, which cause problems for the construction of roads and buildings. The heavy machinery used for this may also compact the soil, reducing infiltration rates and possibly causing mudslides on steep slopes.

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        Silting and river flooding may also occur as soil that has been eroded from deforested areas is carried downstream, contaminating water supplies and increasing suspended sediment loads. River flooding however might occur after deforestation because a smaller amount of rainwater is lost via evaporation from the trees causing an increase in runoff. Infiltration is reduced through laterisation and compaction therefore large quantities of soil-laden water flow over the surface and directly into the river channels causing flash floods. Landslides may occur and agricultural land may be lost.

        The rainforest may also undergo a loss of biodiversity. As rainforests are cut ...

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