The undesirable effects brought by over-harvesting and
Land-clearance and on the ecosystem
The lands occupy only about 30% of earth’s surface. Only about 10% of the land surfaces are suitable for agricultural practices. It is estimated that the formation of fertile agricultural soil requires 3000years, but humans have already destroy over30% of their farmland. This is mainly due to misuse of agricultural farmlands and inappropriate agricultural practices such as over-harvesting and land-clearance.
1) Over-harvesting
Cultivated fields are relatively unstable ecosystems because all vegetations except the cultivated crop plants are removed. This has already a great impact on the soil quality. Over harvesting further increases the impact on the soil. When crops are harvested, plant tissues together with large quantities of minerals are taken away. Thus the nutrients required by the crops are rapidly depleted from the soil. If fallow periods are not sufficient and fertilizers are not applied appropriately, the soil will eventually become infertile.
2) Land-clearance
The distribution of remnant vegetation in the landscape is dependent upon the protection of land in nature reserves and the distribution of land clearance. Land clearance and reserve selection are often significantly influenced by the distribution of certain physical environmental properties (Pressey and Tully 1994). Without wishing to oversimplify the process, land clearance for agriculture has often targeted high productivity environments whereas low productivity environments have been avoided by clearance or targeted for reserve selection. Thus, environmental and geographical properties influence the propagation of land clearance.
Some thinkers (Goodwin 1994, Coveney and Highfield 1995) are questioning the complex nature of many processes operating in nature and tending more towards non-linear explanations and theories of self-organization. The progression of land clearance and fragmentation may also exhibit qualities of self-organization. Cellular automata are a locally-adaptive, globally evolving n dimensional array of cells which are capable of modeling self-organizing behavior in systems. Cellular automata are iterative and are based upon a set of rules involving the state of each cell and the neighborhood conditions which together define the behavior of the automaton.
In this study the propagation of land clearance in a region is modeled using cellular automata. The end result of the model (the distribution of remnant vegetation) is not important - indeed, it is already known in great detail. Rather, it is the propagation of land clearance in space and over time that are of interest in this study. The propagation of land clearance has ecological implications with respect to the time since isolation of patches and the advancement of species relaxation and implications for nature conservation and restoration.
The main problems affected by
over-harvesting and land-clearance are soil erosion, deforestation and
desertification. The impacts of them on ecosystem are discussed as follows.
Soil
erosion
Agricultural soil erosion reduces soil
quality and degrades water quality. Even relatively small movements—for
example, from the top of a slope to the bottom—cause changes in soil structure
that can reduce fertility and make normal cropping practices difficult. When
soil moves further, eventually ending up in streams and lakes, it causes water
quality problems, in part because eroded sediments often carry both fertilizers
and pesticides. Even without such pollution, sedimentation alone imposes
significant costs on reservoirs and water treatment facilities, navigation, and
other water and waterway users. Erosion, organic matter content, soil
salinity, and soil biological
condition are key indicators of soil quality; changes to crop and soil
management practices affect soil quality.
Deforestation
Deforestation will have many social, economic and ecological effects. Deforestation results in many negative consequences. The loss of forestlands is connected to desertification, a widespread phenomenon. Fewer trees translate into an insecure future for forest workers. Heavy rainfall and high sunlight quickly damage the topsoil in clearings of the tropical rainforests. In such circumstances, the forest will take much longer to regenerate and the land will not be suitable for agricultural use for quite some time. Where forests are replanted, their replacement can mean a loss of quality. As well there is the possibility that the basic elements of potential medical treatments, cures and vaccines may lie undiscovered within these environments. There may be a loss of future markets for ecotourism. The value of a forest is often higher when it is left standing than it could be worth when it is harvested (Dudley et. al. 1995). Some indigenous peoples' way of life and survival are threatened by the loss of forests. Among these groups are the Waorani of the Amazon's tropical rainforest, the Sami of Lapland's taiga and the Kyuquot of Vancouver Island's temperate rainforest (Dudley et. al. 1995). Often, the stakeholders associated with forest areas are not always consulted before clearcutting occurs. This has sometimes led to non-violent and violent confrontation and fueled bitter rivalries between area residents, the forest sector and environmentalists. Consequently anti-environmentalism has intensified and environmental activism can be dangerous.
Deforestation
can cause the climate to become more extreme in nature; the occurrence and
strength of floods and droughts could increase. Forests store large amounts of
carbon that are released when trees are cut or burned. It is projected that
deforestation and the burning of biomass will be responsible for fifteen
percent of the greenhouse effect between 1990 and 2025 (FAO 1993). The ranges
of tree species could shift with respect to altitude and latitude as a result
of global warming. Furthermore, the stress of such environmental change may
make some species more susceptible to the effects if insects, pollution, disease
and fire (FAO 1993). In addition, genetic diversity may decrease and areas of
trees may be lost. Rising sea levels brought on by global warming have the
potential to threaten the locations of many major cities, much fertile
agricultural land, the purity of freshwater supplies and the survival of some
nations. The clearing of forestland results in increased erosion and
landslides. Soil from areas of reduced forest cover can fill reservoirs created
by dams. Thus a dam's ability and future capacity to generate hydroelectricity
and provide irrigation would be significantly reduced. Forests play a crucial
role in the management of fisheries. Logging has directly and indirectly
damaged spawning grounds, blocked river channels, raised water temperatures and
caused water levels in streams to fluctuate dangerously. Therefore, the removal
of trees can reduce the viability of fish stocks in their watershed and
downstream environments. The effects of deforestation discussed are of
considerable magnitudes. Still, with all the present and predicted problems, it
was estimated that one acre of Canadian forest was logged every 12.9 seconds in
1995 (McCrory et. al. 1997).
Desertification
Desertification became well known in the 1930's, when parts of the
Increased population and livestock pressure on marginal lands has
accelerated desertification. In some areas, nomads moving to less arid areas
disrupt the local ecosystem and increase the rate of erosion of the land.
Nomads are trying to escape the desert, but because of their land-use practices,
they are bringing the desert with them.
It is a misconception
that droughts cause desertification. Droughts are common in arid and semiarid
lands. Well-managed lands can recover from drought when the rains return.
Continued land abuse during droughts, however, increases land degradation. By
1973, the drought that began in 1968 in the Sahel of West Africa and the
land-use practices there had caused the deaths of more than 100,000 people and
12 million cattle, as well as the disruption of social organizations from
villages to the national level.
While desertification has
received tremendous publicity by the political and news media, there are still
many things that we don't know about the degradation of productive lands and
the expansion of deserts. In 1988 Ridley Nelson pointed out in an important
scientific paper that the desertification problem and processes are not clearly
defined. There is no consensus among researchers as to the specific causes,
extent, or degree of desertification. Contrary to many popular reports,
desertification is actually a subtle and complex process of deterioration that
may often be reversible.
Conclusion
As there are so many undesirable results brought by over-harvesting and land-clearance, people should their land
seriously since land is of course a limited resource in the world and the
governments should take required actions.