Most ecosystems must have taken hundreds or thousands of years to establish a dynamic balance between the plant and animal communities and their environment. The balance is very easily destroyed sometimes intentionally as in agriculture and sometimes unwittingly as with excessive hunting or fishing.

Described below are just a few of the ways man has interrupted he natural cycles and disturbed the balance of the ecosystems he exploits for food.


Trees may be cut down to make way for agriculture or to use the timber. If carried out in the right areas to the correct extent and with provision for reafforestation a balanced community can be maintained. The soil on sloping ground, however, is often thin and once the tree cover is removed, it is no longer protected by the leaf canopy from the forces of wind and rain.

The top-Soil is washed away, silting up rivers and lakes and causing floods.

In temperate forests the decomposition of organic matter in the soil is slow. This is due probably to the low average temperatures and also perhaps to the absence of termites. In tropical forests, the higher temperatures and the activities of termites in removing and eating dead vegetation results in a low level of organic matter in the soil. Although the two ecosystems contain about the same amount of organic carbon compounds, in tropical forest abou of the trees and only 20 per cent in the soil. In the temperate forest, 50 per cent of the organic carbon is in the soil as decaying organic matter. When a temperate forest is cut down and the trees removed there is thus a fair amount of nutrient matter left 75 per cent of the carbon is in the wood in the soil. When a tropical forest is cut down and the trees removed, the remaining soil is poor in nutrients and those which are present are soon dissolved out by the increased rain now reaching the exposed soil.

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Ecosystems, Energy flow in an ecosystem.


A natural environment usually has a wide variety of vegetation at different levels, flowering and fruiting at different times. This vegetation is exploited in different ways by the animals living there, e.g. giraffe browse on the leafy branches, antelope crop the turf, birds take berries and nuts from the trees, and worms consume the leaves which fall from them.

Agricultural practice involves removal of the natural plant and animal community and its replacement with large populations of a single species of plant or animal: arable fields given over to cereal, pastures supporting cattle exclusively. This practice obviously makes the environment unsuitable for the majority of its original inhabitants, indeed it is meant to; a mixed population of cereal and "weeds" is commercially undesirable. The practice of monoculture, however, has its disadvantages.

Parasites and pests, which in a mixed community find their hosts well spaced, in a monoculture can spread rapidly since Suitable hosts are growing closely together. ln many parts of be Africa there is evidence to suggest that moe dned by harvesting the mixed populations of wild ani ga natural environment than is derived trom the herds of sicky cattle which replace them and destroy their habitat.


To protcct the plants in a monoculture trom the depredahons Of insects the crops are of ten sprayed with insecticides such a5 the chlorinated hydrocarbon DDT. This prevents the loss ol hundreds of tonnes of f00d but in some cases it upsets the dynamic balance ol lile in unexpected ways. For example, thee hemicals affect harmful and beneficial insects alike, so that after spraying fruit trees to eliminate a moth whose larvae burrow into apples, enormous numbers of red mites apPpearea because the spray had also killed the spiders which normally preyed upon them.

For a few years after it had been discovered and developed, DDT seemed to be the perfect insecticide. Used to kill body lice and mosquitoes it must have saved thousands of lives by eradicatng typhus and malaria, spread respectively by these insects in certain areas and conditions. The concentratons used seemed harmless to man and other animals though, in higner concentration, it was known to be poisonous, particulariy to nsh. Unfortunately, however, when DDT is taken in with food and water, it is not all eliminated from the body. a proporuon being retained and accumulated in the fat dcposits ot the body When, in some animals, the fat is mobilzed for respiratio, harmful quantities of DDT may be released into the biood.

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The animals at the end of food chains are particularly vulnerable when DDI is used to kill insect pests on a large scale.

In America, DDT was used to kill the beetle which transmits dutch elm disease. The spray and the sprayed leaves reached the soil where the DDT was taken up by worms. When birds, notably the American robin, ate the worms, they accumulated lethal doses ot DDI and whole populations ot birds were wiped Out.

A Similar event occurred when an insecticide was used to kill gnat larvae in Clear Lake, Calilornia. At a concentration of 0-015 part per million of insecticide in the lake water the fish were unharmed. After hve years, however, the western grebes on the lake were dying in large numbers. Although the water contained only U-0115 ppm, the plankton living in the waters had accumulated the compound to a level of 5 ppm. The small fish which fed on the plankton contained 100 ppm and the predatory tish even higher concentrations. The grebes which fed on the larger hsh had as much as Ib00 ppm in their body lat.

DDT is a stable compound and its elects last for a long time; good property for an insecticide but potentially disastrous for the balance of nature. When applied to the crops, it reaches the soil and destroys the insect life there. Eventually it reaches the rivers, lakes and oceans where, if suincIent accumulated, it could begin to poison the fish and other marine life.

Hazards of using chemical fertilizers

Long-ternm experiments show that by applying to the soil a programme ot crop rotation and using larmyard manure, the yield ot crops can be increased over a fhlty-year period. The long-lerm effects ot using chemical tertilizers, in particular soluble salts of nitrogen, are tar less satislactory. Whereas organic nmanure contributes to the bumus content of the soil, helping to mainlain its crumb structure and hence its porosity and permeability to air, the continued exclusive use of chemicals leads to a loss of the organic humus, a deterioration of crumb structure and a decrease in porosity. As a consequence the plant roots are deprived of oxygen in the poorly aerated soils and cannot absorb the salts effectively. The unabsorbed nitrates are washed by rain from the soil and eventually drain into rivers and lakes where they cause eutrophication (see below).

In the short term, heavy application of nitrates to crops has raised the level of free nitrates in the food plants to a point where it may constitute a hazard to health.

Another harmful long-term effect of exclusive use of chemical fertilizers, particularly on light soils, is that in destroying the crumb structure, the soil is much more likely to become dry and powdery and be blown away by the wind, when it is not protected by a plant cover. This leads to loss of valuable top-soil or in extreme cases to dust-bowls and deserts.

Two possi ble solutions to these problems are (a) the development of chemical fertilizers which are less soluble and release their nitrogen slowly into the soil at a rate to suit the demands of the plants and (b) more efficient use of organic manure to help to conserve the humus and maintain the soil structure.

Eutrophication. This is the overgrowth of aquatic plants resulting from an excess of salts reaching rivers. The salts may come from farmland where heavy application of soluble nitrogenous fertilizers is taking place or from the sewage effluents of cities, intensive animal rearing units and the phosphates present in certain detergents. When the nitrates and phosphates reach lakes and rivers they provide abundant nutrients for microscopic green plants whose populations are usually limited by a shortage of these nutrients. As a result, the algae grow and reproduce at a tremendous rate, much faster than they can be eaten by the first order consumers. The algae eventually die and decompose but the vast numbers of aerobic bacteria which are responsible for their decay take so much oxygen from the water that the oxygen supply is depleted and fish and other aquatic animals suffocate and die. The semi- decomposed organic remains of the algae form an oxygen- deficient mud.

Lake Erie in America receives water draining from 30,000 square miles of farmland and the effiuents of large cities such as

Detroit and Cleveland. In its waters eutrophication has produced an organic mud with a large oxygen deficit. A layer of iron oxide separates the organic mud from the water above but some scientists think that further removal of oxygen could dissolve this layer, allowing the organic matter to mix with the water and so produce anaerobic conditions which could not support life.

Soil Erosion 

Soil erosion means the removal of top-soil, usually by the action of wind and rain.

(a) Deforestation. The soil cover on steep slopes is usually fairly thin but can support the growth of trees. If the forests are cut down to make way for agriculture, the soil is no longer protected by a leafy canopy from the driving rain. Con- sequently, some of the soil is washed away into the rivers which become choked with silt and are liable to overftlow their banks.

(b) Poor farming methods. Ploughing loosens the soil and destroys its natural structure. Failure to replace humus after successive crops, excessive use of artificial fertulizer and burning the stubble or weeds, reduces the water-holding properties so that the soil dries easily and may be blown away as dust. On sloping ground, such soil may be eroded by water.

i) Sheet erosion is the imperceptible removal of thin layers of soil but usually leads to:

(ii) Rill erosion, in which the water cuts channels. The channels deepen as the volume of run-off increases and so become gulleys.

(iii) Gulley erosion. The gulleys so formed reach enormous proportions (Plate 10) so that thousands of hectares of top- soil are carried off. Gulley Erosion is often accentuated by careless ploughing and may follow the tracks made by vehicles, goats, cattle and other farm animals 

(c) Over-grazing. Too large a population of animals on a given area will not make economic use of the food they eat, since its scarcity will make their growth rate too slow. In addition, sheep and goats graze the vegetation very closely, leaving little plant cover on the soil, while their hooves trample and compact the soil into a hard layer. Consequently there is less absorption of rain; the soil dries out quickly and may eventually be blown away.

Methods of reducing erosion

 (a) Terracing. This is cultivation along the line of the contours, in horizontal strips supported by walls, so breaking up the steep downward rush of the surface run-off. Terracing is a useful temporary measure against water erosion, but is a rather costly and difficult method of farming.

(B) Contour ploughing. Ploughing at right angles to the slope, i.e. along the contours instead of up and down the hill, allowss the furrows to trap water rather than channel it away and start gulley erosion.

(C) Strip cropping. This consists of alternate bands of tilled and untilled soil following the contours. Grass and cover crop strips, between strips of ploughed land carrying grain, prevent the soil being washed away from the tilled portions. By alternating the grass and grain each year, the soil is allowed to rebuild its structure while under grass. Strip cropping is also effective against wind erosion if the strips are planted at right angles to the direction of the prevailing wind.

(d) Correct crops for the soil. Steep slopes which should not or cannot be ploughed are covered with pasture crops. The foliage reduces the run-off, and the roots hold the soil in place and preserve its structure. The following Table illustrates this point. and controlling floods. Controlled thinning of forests is also desirable, since thickly afforested areas may prevent too large a proportion of water from ever reaching the ground.


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