Answer:
There are four principal biological systems of the Earth -
(1) fisheries
(2) forests
(3) grasslands
(4) croplands,
as pointed by Mr Lester R Brown. Unfortunately, the human claims on these systems as reaching points, where their productivity is getting impaired. The protein hungry world is over-fishing every day.
In poor countries, local forests are being .destroyed everyday to get firewood for cooking. The growing use of dung for burning deprives the soil of an important natural fertilizer. 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
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