Analyse the role of human activity in plant succession

There are many factors that can affect a climax vegetation. These include climate, hydrology and soil type. Human activity also plays a major role. If succession is modified or interrupted by a human influence then a plagioclimax is reached. Human activities include grazing, burning and deforestation and these processes can be collectively known as arresting factors.

A plant succession begins on bare ground with light-seeded grasses, legumes and flowering plants. Over time small trees, brush and briar thickets shade out the sun loving grasses. Ultimately, light-seeded trees shade out the brush and make conditions favourable for shade loving mast producers like oaks.

Deforestation involves the cutting down, burning and damaging of forests. If the current rate of deforestation continues then the world's rainforests will disappear completely within 100 years - causing unknown effects on global climate and eliminating a majority of plant and animal species currently alive on the planet.

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The reasons for deforestation are complex. It is mainly carried out for agricultural reasons, e.g. grazing cattle and planting crops. Poor farmers may chop down a small area (usually a few acres) and burn the trunks in a process known as 'slash and burn' agriculture. Cattle pastures often replace the rainforests in order to grow beef for the world market.

Another type of deforestation is known as commercial logging. This is the cutting down of trees in order to sell off as timber and pulp. This process uses heavy machinery, such as bulldozers and road graders etc to remove trees and this is detrimental to the forest and plant successions overall.

Competitive global marketing drives the need for money in economically challenged tropical countries. At national level, governments sell logging concessions to raise money for projects or to pay international debt. For example, Brazil had an international debt of $159 billion in 1995 on which it must make payments each year. Logging companies seek to harvest forest and make profits from sales of pulp and valuable hardwoods e.g. mahogany. This raises the money required in order to pay back some debts.

A case study of an area where deforestation has affected succession is in the Philippines. For the past 50 years the Philippines has lost 2.4 acres of hardwood forest every minute leaving only 21% of forest cover.

This has been due to increasing agriculture and illegal logging. Reports of deforestation-related declines in rainfall totals and disruption of rainfall regimes have also been reported for parts of the Philippines (Alfonso, pers. comm., 1988). During the 1960s there was considerable deforestation on Mount Apo, for instance, due to the establishment of coffee plantations; and the apparent result is increasing drought.

The short-term gain of deforestation, i.e. the profit to be made, is all well and good but there have been a large number of adverse effects brought about by the removal of large areas of forest. The main effects are increased soil erosion (which leads to increased flooding in most areas), reduction in biodiversity, the Greenhouse Effect, reduced fertility of land in deforested areas and disruption of the hydrological cycle.

Major roads are being built through the forest to provide easier access to the underground resources like iron and aluminium ores: more trees need to be removed to mine these commodities. Inhabitants of shanty towns around Major cities in Brazil are being encouraged to move to rural areas and more land has to be cleared to accommodate these people. In third world countries forests are cut down and used to provide firewood, since this is practically the only source of fuel available to people living there.

There are also a large number of new markets opening which are set to increase the demand for products, which have their origin in the tropical rainforests. These include body creams, bath oils, sweets, fruits and nuts, but as these products come into greater demand the future of the forest will be more secure because to produce a large yield of these products a large number of trees need to be grown. About 16-20 million hectares of tropical rainforests are removed each year.

When trees are removed no water gets transpired back into the atmosphere from the soil. Because of this there is reduced rainfall and surrounding areas of forest are threatened with dessication and faster soil erosion because the soil is no longer held together by moisture.

Another recently discovered side effect of deforestation is that it may actually help to spread diseases like malaria and river blindness. Anopheles darlingi, a mosquito that effectively spreads malaria parasites, breeds in pools of water created in deforested land, open cast mines, and on eroded land after trees have been removed. Deforestation has favoured a population explosion for this species and up to a quarter of the people living along the Transamazon highway in Brazil are affected by malaria each year.

However damaging to plant succession human activities can be, the effects provide excellent conditions in which for a secondary succession to take place. A secondary succession differs from a primary succession in that it occurs in an area where a community of plants and animals has recently been wiped out, i.e. the land is already suitable for a wide diversity of species to establish themselves on. A secondary succession can happen on an area of land where a forest fire or parasite wiped out a previous climax community or which has been disturbed in some way by humans e.g. deforestation or arable farming.

A secondary succession takes place much more quickly than a primary succession for two main reasons, the most obvious one being that a suitable medium already exists for the new organisms to grow in, and the other being that some seeds or spores from the last plant community can still be present in the soil after the community is destroyed. This is a positive aspect as new plant species can grow and thrive.

As well as big processes such as deforestation, humans can contribute to vegetation in smaller ways. There are now varied types of species in Britain due to human interferences, modification and management. Without interference vegetation would be typically temperate forest. Today, in Britain, there are no natural woodlands at all, only semi woodlands, and there is only 2% of that. In Spain the figure for natural woodland is 28% and Sweden has a 64% woodland covering. Other interferences have caused the introduction of foreign plants such as the rhododendron or sycamores. The rhododendron was in fact imported from Japan. Other introductions to this country include the rabbit, which was imported from Spain by the Normans.

Urbanisation also causes damage to plant life. High demand for housing has caused the destruction of forest and woodland, especially in the north. A rising birth rate in the twentieth century called for an increase in housing, and this needed to be conducted as cheaply and effectively as possible. As a result, miles of forests were cut down in order for building to go ahead. This s detrimental to our health as well as destroying areas of natural beauty. . Recent investigations suggest that over a third of the world's oxygen supply is produced by the rainforests, the only larger source being algae in the sea. So cutting down trees is very damaging to our well being.

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