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AQA A2 Geography 3 Ecosystems: change and challenge Section A 1 (a) The sites of primary production in the oak woodland food chain are the trees (autotrophs) which convert the sun’s radiant energy to chemical energy through photosynthesis. The remaining organisms in the food chain are consumers (heterotrophs). Caterpillars eat oak leaves and are therefore primary consumers. Blue tits, which predate caterpillars, are secondary consumers. Sparrow hawks, situated at the end of the food chain are the tertiary consumers. A feature of the oak woodland, and all food chains, is the decline in biomass with distance from primary production. The reason is that only a small proportion of food energy is converted to animal tissue. Ninety per cent is used in respiration and is dissipated as heat energy. One consequence of this is that the amount of energy and populations of animal species become smaller at each successive trophic level in the food chain. (b) The evolution of salt marshes from mudflats follows a number of successional stages known as a halosere. Mudflats close to the mean low-tide level are exposed for only a few hours on each tidal cycle. Salinity and long periods of inundation create harsh environmental conditions allowing only specialised pioneer species such as glasswort and cord grass to colonise. However, these seral communities bring about subtle changes to the environment. By slowing the movement of tidal currents they encourage sedimentation, raising the height of the mudflats. As a result the period of inundation is shortened and salinity reduced. Other plant species able to tolerate these less-demanding conditions now invade (e.g. sea blight and sea purslane) and become dominant. They too have a positive feedback effect, increasing the rates of sedimentation and raising the height of the mudflats. Eventually, parts of the inter-tidal zone reach a height where flooding occurs only occasionally on the highest spring tides. This is salt marsh, the final climax stage of succession. The pioneer species have all but disappeared. The salt marsh supports a complete vegetation cover and is biodiverse, with a wide range of species including sea lavender, couch grass, sea aster, sea plantain and scurvy grass. (c) Heather moorland is a plagioclimax formation, in balance with environmental conditions and human activity. If human activity were removed, within a few decades heather moorland would be replaced by scrub (e.g. bramble, gorse, blackthorn) and birch woodland. The overall effect of human activity is to keep heather moorland at an early stage of succession. This is done by periodically burning the heather (e.g. every 7–15 years) by controlled firing. Management creates an ideal habitat for large populations of red grouse, a game bird which supports a lucrative shooting industry in upland Britain. Firing the heather does a Hodder Education © 2012 1 number of things: it eliminates plant species that might compete with heather; it stimulates new heather shoots, the staple food of red grouse; and it encourages the germination of heather seeds. Heather moorland is also managed (sometimes illegally) by eliminating potential predators of red grouse and their chicks such as foxes, stoats, crows and raptors. Light grazing by sheep is also encouraged to prune the heather. 2 (a) The most obvious feature of the rainforest structure is its layering. Layering is a response to light levels in the forest. Thus the forest floor, in permanent deep shade, supports little ground vegetation. As light intensity increases with height, an understorey of small trees and saplings is found between 5 and 20 metres. The stems of the tallest trees, which form the canopy and emergent layer, are festooned with creepers, which grow upwards towards the canopy, and specialised plants such as epiphytes and orchids. Photosynthesis is concentrated in the canopy (20–30 metres). There, leaves, fruits and seeds support a diverse array of animals, including insects, reptiles, birds and mammals. The tallest trees (50 metres or more in height) rise above the canopy to form a discontinuous emergent layer. Forest trees are typically umbrella-shaped to capture the maximum amount of sunlight. This top-heavy structure, together with their shallow roots, explains their buttressed stems. (b) Today only around 100 indigenous tribal groups, like the Yanomani in Brazil and the Penan of Sarawak, survive in the rainforests and practise their traditional way of life. Indigenous rainforest societies relied on hunting animals, gathering edible forest fruits and plants, and shifting cultivation. Ecologically this was a highly efficient and sustainable economy which survived for thousands of years. Because population densities were low, and technology relatively simple, indigenous societies had minimal impact on the rainforest ecosystem. Shifting cultivation made little impact on the forest ecosystem because: (1) It involved temporary (rather than permanent) cultivation of land. After just one or two harvests, yields become depleted, farmers abandon their plots and make new clearings, and the forest recolonises. (2) Burning the branches, leaves and stems from forest clearings provides a natural fertiliser, adding nutrients like potassium and calcium to soils. (3) Farmers grow a mixture of different crops sown in the ash in the clearings — a practice known as polyculture. Polyculture mimics the biodiversity of the rainforeforest; reduces the potential damage from plant pests and diseases; and minimises soil erosion by run-off. (c) The contemporary exploitation of rainforests by technologically advanced societies has been largely unsustainable. At worst this exploitation has destroyed entire ecosystems. Even when development has stopped short of destruction, ecosystems have often been severely degraded. Unsustainable development is linked to large-scale economic activities based on advanced technologies. These activities include modern farming, logging, dam building, mining and road construction. In Amazonia in the 1970s and 1980s, the Brazilian government gave settlers small plots of land for permanent cultivation. They cleared the forest from the plots to Hodder Education © 2012 2 grow food and cash crops such as coffee. However, permanent cultivation is unsustainable in the rainforest; as yields declined thousands of small farmers were forced to abandon their smallholdings. The land either reverted to forest (but with greatly reduced primary production and biodiversity) or was acquired by large ranching enterprises for low-intensity grazing. Elsewhere in Amazonia, soya production (a type of agribusiness) replaced vast tracts of rainforest. Logging companies extracted valuable hardwood trees such as mahogany and rosewood, destroying in the process many non-commercial species and degrading forest ecosystems. Further destruction occurred as roads were cut through the forest to provide access. Huge areas of rainforest have also been destroyed by the construction of dams to generate electrical energy for Brazil’s booming economy. Modern forestry is sustainable when rates of saplings planting matches the harvesting of mature trees. Current investment in forest plantations (often using fast-growing exotics like eucalyptus) is long-term, sustainable and attracts government support and foreign investment. Amazonia has massive mineral wealth. Open-cast mining of iron ore, bauxite, cassiterite and other minerals invariably results in the total, though localised, destruction of rainforest. Moreover, toxic elements and chemicals often leach from these sites, polluting rivers and entering food chains. By definition, mining cannot be sustainable, but better management and government control could greatly help to reduce its impact on the rainforest and its ecosystems. Section C 3 Until the last 100 years or so, human impact on the tropical rainforest biome had been negligible. Indigenous people living at low densities and relying on simple technologies had lived sustainably in the rainforest for thousands of years. The economies of tribal groups such as the Yanomani in Amazonia and the Penan in Indo-Malaysia were based around hunting and gathering in the forest and shifting cultivation. Shifting cultivation is superbly adapted to an environment where soils are naturally infertile and where plant growth is vigorous. It places minimum pressure on ecological resources because forest clearings are cultivated for just 1 or 2 years before abandonment leads to forest regeneration. Indigenous groups have a direct interest in conserving the rainforest because their survival wholly depends on it. In contrast, recent developments in the rainforest, centred on farming, logging, mineral extraction and HEP production, have had severe and damaging impacts on large areas of the biome, especially in Amazonia and Indo-Malaysia. Much of the development has been exploitative, resulting in permanent environmental destruction and degradation. According to the UN Food and Agriculture Organisation nearly 1 million km2 of rainforest was destroyed every year between 2000 and 2005. Two hundred years ago, primary rainforest occupied 14% of the Earth’s land surface. Today, deforestation has reduced this figure to just 6%. Most rainforest development by technologically advanced societies is unsustainable. Modern farming has had the most widespread impact. In western Amazonia, thousands of smallholders, granted small plots of land by governments in the 1960s, 70s and 80s, cleared primary forest for permanent cultivation. After just a few years Hodder Education © 2012 3 cultivation became unsustainable and the land was either sold or abandoned to largescale ranching. Thus the world’s most biodiverse and productive ecosystem was replaced by a monoculture of low-quality grassland that requires 3 ha of pasture to support a single head of cattle. Illegal logging of tropical hardwood trees, together with roads cut through the forest to provide access, have also degraded vast areas of rainforest. On a more localised scale, open-cast mining for minerals such as iron ore and bauxite, has not only destroyed the forest, but has also released toxic elements and chemicals, polluting rivers and food chains. Finally, dam building for HEP generation has flooded vast tracts of forest in countries like Brazil, Malaysia and Laos, destroying both terrestrial and aquatic ecosystems. 4 Environments such as drylands (arid and semi-arid areas) and high mountains are easily damaged and degraded by human activities. These environments are intrinsically fragile because they have high degrees of endemicism and lack biodiversity (i.e. highly specialised plants and animals); slow rates of soil formation and plant growth; and sparse vegetation cover that is easily destroyed, leaving soils vulnerable to erosion. Environmental degradation and desertification has occurred in the fragile semi-arid lands of northern China. Five billion tonnes of topsoil are lost to wind erosion every year. The problems are particularly severe in the Korqin sandy lands. Overgrazing and the deliberate clearance of forests for timber and farmland have exposed the loose, sandy soils to erosion. The outcome is large expanses of degraded and desertified land. In places there are other problems: overexploitation of groundwater has lowered water tables, while excessive irrigation has led to poor drainage and salinity on some arable lands. In response to these problems, in 1978 the Chinese government launched an ambitious programme to combat land degradation and desertification. Its focus was a massive reafforestation scheme covering over 350,000 km2 and due for completion in 2050. However, by 2010, only 13,000 km2 had been planted. In Korqin, the programme aims to protected farmland against wind erosion, restore soil fertility and improve the well-being of local people. Plantations and shelterbelts of native poplar trees, resistant to drought and frost have been established. They also provide a sustainable source of timber. There is an emphasis on conservation, with the introduction of sustainable cultivation involving the recycling of organic material to the soil, integrating tree crops with pasture and cash crops (known as agro-forestry) and planting tree species that provide fodder and timber and improve soil fertility. Meanwhile, the controlled management of grazing lands (i.e. keeping within the land’s carrying capacity) is being introduced for the first time. The Korqin scheme is part of a much larger programme to halt the spread of the desert in northern China. The programme aims at a more sustainable use of resources by reafforestation, integrating cultivation, livestock management and forestry, and reducing pressures on the environment. Although only in its early stages, this programme appears to be sustainable and could provide a blueprint for rehabilitating other fragile drylands damaged by overexploitation. Hodder Education © 2012 4