Each climate zone on the planet has ecological characteristics. Bangladesh is located in the tropical (sub-tropical) humid climate zone. Japan, the USA, and European countries are in the temperate climate zone. There are significant differences in temperature, rainfall (allocation, quantity), quantity of biomass production, type of vegetation, soil and other aspects between temperate and tropical ecosystems. The agricultural system is very much influenced by ecological factors. If an agricultural system is not suitable to an ecosystem, it will not sustain its productivity and very often disturbs the whole ecological balance of the area. A suitable agricultural system is therefore essential.

However, this rule is apparently ignored in current agricultural development efforts being carried out in many developing countries within the tropical zone. It was believed that agricultural development would be achieved by introducing technological innovations from industrialized countries. Based on this idea, the `Green Revolution` was started and has been implemented for three decades. Through the Green Revolution, which are very unique and have sustained for many generations, have eroded rapidly. Instead, so-called modern agriculture, which is just copy of the agricultural system in industrialized countries, has been extended intensively in developing countries.

A question in my mind since I first came to tropical countries (India, Bangladesh, etc.) in 1982, has been why is agricultural production per unit of land in tropical countries so much lower than that of temperate countries? For instance, the rice production in Japan is about 7,000 kg/ ha on a average while it is about 2,000 kg/ ha in Bangladesh. The situation of other crops is more or less the same. This has been a very big question because, as we have seen, in terms of biomass production, the most productive place in nature is the tropical rain forest. The potential of the tropical forest for biomass production is about two times that of the temperate forest.

Then why such contradictory results? Let's examine in more detail, the characteristics of the tropical humid climate.

1.4.1 Tropical climate

High temperature and strong sunlight
Tropical zones have very hot claimates. High temperatures are created by strong sunlight and relatively longer lighting period compared with the temperate zone.

Extreme Rainfall
Rainfall shows typical tropical characteristics - strong, intensive, large in quantity and seasonal (rainy and dry season). These characteristics are extreme. In Japan, the actual annual rainfall is about 1,500 mm on average and the allocation is throughout the year. Rainfall occurs once or twice every week and it is a gentle rain. Thus the loss through run off is relatively little. In Bangladesh, actual annual rainfall is about 2,000 mm on average, but the allocation is only during the rainy season (June - October). There is almost no rainfall during the dry season (December - March). Rain is very strong and intensive. Thus the quantity of loss through run off is relatively high in the rainy season. Because of this extreme pattern of rainfall, the quantity of effective rainfall in Japan is much higher than that of Bangladesh.

1.4.2 Allocation of Nutrients in the Tropical Forest

Nutrients in the forest are primarily reserved in two places. One is living tissue (leaves, branches, stems, etc.) which, except for roots, is located mostly above the soil. The other is in organic matter (fallen leaves, humus, etc.) in the soil. The allocation of nutrients is quite different between the tropical and temperate forests.

As the diagram of bellow shows, the ratio is 50:50 in the temperate forest. 50% of total nutrients are stored in living tissue (above soil) and the other half is stored in the soil as organic matter. In the tropical forest, it is 20:80 to 10:90. That is, 80-90% of the total nutrients are stored in living tissue and only 10-20% is stored in the soil as reserve. The difference occurs because of different speed of decomposition (including mineralization) between the temperate and tropical zones as outlined in the chart.

High temperature and humidity in the tropical zone provide optimum conditions for decomposition, which consequently occurs very quickly. This results in two thing. Minerals are available for plants sooner. On the other hand, organic matter cannot in soil is less than that in the temperate forest.

1.4.3 The Multi-Story Structure of the Natural Forest

As we have seen, the tropical climate is extreme and the organic content of soil is relatively little. What kind of system is suitable for dealing with such conditions? Nature shows us an ideal mechanism in the natural forest - the multi-story vegetative cover. The multi-story structure can regulate the extreme conditions and utilize natural energies and resources properly.

As the diagram show, the structure of the forest consists of:

1. big trees with large canopies which cover the whole forest
2. moderate trees under the canopies of the larger trees
3. small trees and shade living plants under these
4. the soil is covered by grass and litter

The strong sunlight is mostly used by the leaves of trees and never strikes the soil surface directly. The beating effect of the strong rain is absorbed by the top canopy, moderate and small trees, and never hits the soil surface directly. Thus rainfall slowly soaks into the litter, soil and roots of plants of the forest to maximum effect. In these ways, the natural forest uses strong sun energy and heavy rainfall properly.

High biomass production of tropical forest results from the maximum utilization of sun energy and water, and quick decomposition which release minerals for plants quickly.

1.4.4 Problems of Agriculture in the Tropical Ecosystem

The extreme climate and the quick decomposition which works positively in the forest does not work the same way in agriculture. Rather, it sometimes works negatively. Conventional agricultural practice starts by cutting and clearing the forest. In this way, 80-90% of the total nutrients are taken away from the land and the soil that is left lacks organic matter, fertility, water holding capacity and other beneficial qualities of soil. Furthermore, strong sunlight can then hit the soil surface directly which degrades the soil structure causing it to become hard. Strong rainfall beats the soil surface and as the thin top soil has little capacity to hold rain water, soil erosion occurs. When the strong sunlight and rainfall are not used properly, they become causes of soil erosion,flood, drought and other natural disasters.

1.4.5 Conclusion

As we seen, the characteristics of the tropical ecosystem are extreme, but very delicately balanced. It is very urgent to build up a suitable agricultural system which can utilize natural energies and resources properly, is strong against natural disasters, and not destructive to the ecological balance in the area. Of course, this should not be a copy of a so-called modern system. If we succeed at building up a suitable agricultural system in the tropical zone, the productivity of this agriculture would be more than that of temperate modern agriculture. Nature shows us that the tropical zone has more potential than the temperate.

Agriculture is artificial but within nature. Agriculture does not exist beyond the rule of nature. Human history tells us that so many civilizations raised and disappeared because they failed in dealing with nature. "Civilization crosses the plant, leaving desert behind." Things happened in this way in the past and present. Deforestation and desertification are now very serous environmental problems in tropical countries. The main cause is agriculture practices which are not suitable and destructive to the ecosystem. We should understand that incorrect agricultural practice destroys the ecological basis which is the human basis. This easily happens in the tropical ecosystem.