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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
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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
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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
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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.
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