What are the four system that …

1. There are four principal biological systems of the Earth - (1) fisheries (2) forests (3) grasslands (4) croplands, as pointed by Mr Lester R Brown.
2. Unfortunately, the human claims on these systems as reaching points, where their productivity is getting impaired.
3. The protein hungry world is over-fishing every day. In poor countries, local forests are being .destroyed everyday to get firewood for cooking.
4. The growing use of dung for burning deprives the soil of an important natural fertilizer.
5. The forests precede mankind and deserts follow. The world's tropical forests are eroding at the rate of forty to fifty millions acres per year. Also, grasslands and croplands are being converted into deserts and wastelands 

Mr Lester R. Brown in his thought-provoking book, The Global Economic Prospect, points out that the earth’s principal biological systems, i.e., fisheries, forests, grasslands, and croplands form the foundation of the global economic system. In addition to supplying our food, these four systems provide almost all the raw materials for industry except minerals and petroleum-derived synthetics. It is because of this that fisheries collapse, forests disappear, grasslands are converted into barren wastelands and croplands deteriorate. In a protein-conscious and protein-hungry world, over-fishing is common. In poor countries, local forests are being demolished in order to procure firewood for cooking. Since tropical forests house various species of life, they face extinction as a result of its destruction.