The effects of agricultural development on the environment, as a means of assisting developing countries in tropical regions. are considered as an ecological aspect of human activity. Good results, with high short-term productivity, were obtained from the development by modern agricultural technology, for example. by increased crop-yields from the use of new varieties and chemicals and by farm cultivation through deforestation. However, in the long term harmful effects accumulated for agricultural land, with the loss of stability of production, and with the onset of 'desertification', due to poor understanding of the ecosystem. The ecological activities of organisms are reviewed from the viewpoint of two aspects: disoperation (an 'opposing' term), such as competition and exploitation, and cooperation (a 'harmonizing' term), such as mutualism and aggregation, which are characteristic to each species of organism in the pioneer phase and in the climax phase of the ecological succession, respectively. The human population, as one species of the biological community in the ecosystem, possesses the two aspects within a population and even in an individual. The pioneer character of the human population promotes the development of civilization in human society, but this character often accompanies the disoperative character, as so-called r-strategic organisms. On the other hand, its climax (so-called K-strategic) character tends to conserve nature against the development of environment. The human being has a special character of elasticity, which is an ability to integrate the two terms in harmony, thus creating culture. The assistance of developing countries by those more developed, would promote peace, when they keep cooperation with each other both at the national level and at the personal level.