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Deforestation is the state of being clear of trees. Types of Deforestation are discussed below:

1. Bush burning: In the rain forest and savannah of Africa, the common traditional methods of preparing the bush for the farming season is deforestation and bush burning. This involves massive or selective felling of trees, clearing the shrubs and undergrowth in the dry months of October to March/April. The bush is then set on fire. Forest fires and huge billows of smoke are common features during these months.

Agricultural activities, designed and intended to have beneficial results from man’s point of view eventually end up upsetting the natural balance that had existed in the environment. Bush fires destroy useful birds, insects and useful soil flora (vegetation) and fauna (animals). The ash produced from burnt material soaks into the soil when the rains come and this produces a change in the delicate pH of the soil, making it slightly alkaline from the typical acid soils of tropical Africa. One of the greatest disadvantages of bush burning is the removal of vegetation and exposure  of top soil to gully erosion. In the grassland savannah regions, wind erosion occurs.

2. Timber exploitation: The removal of trees from the forest through felling and sawing into timber for local or export markets is a major sub-agricultural activity in the tropics. In Rivers State alone, the rain forest in Degema, Elele, Bori, Bonny and Ahoada Local Government Areas are among the oldest in the world. These forests are capable of supporting a thriving timber business on a sustainable basis. Unfortunately, the heavy de-vegetation activities going on in these forests presently will surely cause the greatest upset in the rain forest ecosystem. Vegetation acts as a protective layer, minimising heat transfer between air and earth. When vegetation is removed, atmospheric heat (especially in the tropics with high ambient or room temperature) is conducted to greater depths below the surface. This causes the drying up of streams and lakes by evaporation and the shrinkage of sources of springs and streams by lowering the water table. Besides, trees act as wind breakers, protecting buildings and causing the moisture-ladened south-eastern winds to drop their moisture as rains.

3. Tillage: Agriculture is concerned with the skin of the earth, the top layer of soil. It is, within that region that the greatest changes can be made in most of the characteristics of the natural environment. Tillage, which is practised mainly by peasant farmers in Africa and Asia is the breaking up of the surface soil to ease sowing of crops. By tillage, the soil structure is altered. Other methods of breaking up the soil surface are ploughing and harrowing. All these methods expose the soil and result in man’s most negative impact on the soil. Tillage of the soil is manually done by local farmers, using simple tools like hoes, cutlasses, machetes and diggers. Ploughing is a more advanced form of tillage, in which hand ploughs pushed by the farmers may be used or pulled by animals (oxen).

In modern agriculture, i.e. since the advent of farm machinery and mechanisation, agriculture has undergone revolutionary advancement resulting in large commercial farms deploying a wide range of sophisticated equipment from tractors to combine harvesters. The technology for producing equipment has also advanced considerably over the years, making them (equipment) more efficient and less expensive. This has also led to greater destruction of the soil ecosystem.

In developed countries of Europe and America, a few farmers can produce enough food to feed a very large population because of the efficiency of agricultural production. Eventually, all these methods alter the structure of the soil and expose it to erosion. They also lead to the exposure and destruction of useful soil flora and fauna, as well as loss of soil nutrients by erosion.

4. Animal breeding and domestication: Human interference with animals or domesticated pets has produced far reaching effects on the environment and ecological systems, if not more than his agricultural activities on vegetation. Eradication of species and selective breeding of others have increased the number of grazing animals (herbivores) at the expense of flesh eating animals(carnivores).

Introduction of exotic animal species into the environment has resulted in an imbalance, which manifests in the over population of the new animal, and extinction or excessive pressure on the population of the native species, with which they compete for food. In the Rivers State basin, for example, the introduction of Heterotis miloticus (‘Ecomog fish’) has resulted in the overpopulation of the fish, because it has few competitors in its feeding habits, which is filter feeding.

5. Chemical application:

(a) Fertilisers: The use of fertiliser came into effect when farmers realised that crops yield declined, when a piece of land is used over and over again for crop production. Farmers started by using natural manure or farmyard manure (animal and plant materials allowed to decompose to form manure). They found that adding this natural manure to the soil improved crop yield, but natural manure was limited in quantity and mineral content for different crop varieties.

Fertilisers became artificially manufactured by industrial fixation of atmospheric nitrogen to produce different types of fertilisers (e.g. NPK, urea, muriate of potash, etc). These fertilisers have become extensively used and have changed agriculture. Crop production has increased and agriculture has become big time and commercial business.

Fertilisers, especially the artificial fertilisers have effects on the soil, streams and lakes into which they are leached. While natural fertilisers tend to maintain the humus of top soil, chemical fertilisers destroy the crumb structure of the soil and change the pH and chemical balance of the soil. When the artificial fertilisers are washed into streams, rivers and lakes, they cause overgrowth of water plants and algae (algae bloom), because of the excessive nitrates and phosphates they have introduced into these aquatic habitats.

This overgrowth of the aquatic microscopic green algae is called eutrophication. The overgrowth means more food than the consumers can eat. The result is death and decay of the excess algae. The aerobic bacteria that facilitate the decay use up oxygen from the surrounding water, leaving the water in short supply of oxygen. This also causes death of aquatic plants and animals. The overgrowth has high biochemical oxygen demand (BOD) and they get this oxygen from the water. By so doing, aquatic animals like fish are deprived of oxygen and die of suffocation. The dead animals, the remains of the partially decomposed algae, form a stinking oxygen deficient mud which is very unhealthy.

(b) Herbicides: Herbicides or weedicides are chemicals, used in agriculture to kill weeds. In a farm, weeds normally compete with crops for nutrients, space and sunlight. Being wild, weeds are tougher and if uncontrolled, will outgrow and suppress the growth of economic crops causing crop failure and famine. Therefore, they have positive and negative effects on the ecosystems. The use of herbicides is important in agriculture because it enables the farmer to control weeds over a wider area of farmland at comparatively lower cost and shorter time than manual weeding by hiring labourers. Manual labour, using hoes and machetes is inefficient. The weeding is not uniform and when tiredness sets in, labourers are known to cut some crops which add up to a substantial amount of crops destroyed in this way.

Herbicide technology has revolutionised agriculture by giving rise to big commercial farms, which apply a wide variety equipment for planting, harvesting and storage of crops. Herbicides and fertiliser application have greatly increased agricultural production, even in developing countries like Nigeria, where food is still produced primarily for domestic consumption.

Herbicides are chemicals that interfere with the soil chemistry like fertilisers do. They are leached into streams, rivers and lakes and pose health hazards to aquatic life and man who may drink the water and suffer sickness or die, thus, upsetting the ecosystem.

(c) Insecticides: Chemicals used to kill or control pests are called pesticides. Pesticides, used to kill insects are called insecticides, i.e. insecticides are pesticides. Insecticides are the most widely used pesticides, both in agriculture and public health and the most hazardous. Insect killers are among the most toxic pollutants and contaminants, principally, because of their long residual effects, i.e. when they drop on any substance, they remain active or potent (effective) for a long time. This means when they are sprayed on an insect, they remain on the insect in poisonous form. The insect is eaten by a secondary consumer, then by a tertiary consumer. The transfer continues up the chain till it finally ends up in man, where it accumulates in the muscles or fatty adipose tissues and cause health hazards.

Pesticides are used in agriculture to kill insect pests. Insects are among the most numerous creatures in nature and the most pestilent. Without pesticides, the energy, money and time put into agricultural activities will come to nought. Insect pests like locust (short-horned grasshoppers) and stem borers (larval form of moths and butterflies) are known to wipe out large fields of maize, rice, cowpea, etc., if left uncontrolled.

The early class of pesticides were manufactured from chemical compounds called chlorinated hydrocarbons and organophosphates. Their good effect is in controlling insect pest in farms and warehouses, where foods are stored and in killing insects like mosquitoes which spread malaria fever. Their bad effects manifest when introduced into the environment. They pollute the air and surface water. They kill beneficial insects like bees that pollinate flowers. An example of this kind of pesticide is DDT (Dichlorodiphenyl-trichloroethane). The adverse effects of this pesticide can be summarised as follows:

(i)  DDT does not break down into simpler harmless substances, i.e. it has a long residual effect and remains poisonous for a long time.
(ii) DDT destroys beneficial insects that boost crop yield by pollinating flowers (e.g, bees).
(iii) DDT is leached into the soil and streams where it kills fish and useful soil animals like earthworms.
(iv) DDT accumulates in tertiary consumers (higher carnivores) in a food chain and affects the breeding of birds of prey. This effect on the bird population is an ecological problem and DDT has been banned in developed countries.

New technology has emerged for producing environment friendly pesticides. This involves the manufacture of pesticides which break down into harmless substances shortly after application. These modern pesticides are produced from compounds called synthesis pyrethroids. They are toxic, but have short residual effect and do not accumulate in the body as harmful substances. They have quick knock down effects on insects and degenerate into harmless substances shortly after use. They can therefore be sprayed on vegetables, which could be eaten raw within 4 to 6 weeks after spraying without fear.