Terrestrial and water ecosystems are complex and perfectly organised natural “factories” that produce all that is required for life on Earth and to cover man’s basic requirements: food, fibres, water. Some of these functions of the ecosystems are essential to man, such as air and water depuration, climate control, the nutrient cycle, soil fertility. In addition, some ecosystems (beaches, woods, lakes, high mountains, secluded valleys) are our ideal places for recreation, tourism and meditation, so we can say that the ecosystems have permitted our society and economy to develop. 50% of the world’s population are still engaged in farming, forestry and fishing. This proportion becomes 70% if we take the sub-Sahara, Asian and Pacific countries alone. 25% of the world’s countries have economies that still depend, almost entirely, on the sectors above. Farming alone produces 1.3 trillion dollars of food and fibres a year.