The cause of the decline of North Africa is popularly attributed to climatic changes, the theory being that the area became hotter and drier and the people were forced to abandon a thriving civilization. However, some geologists, after carefully weighing the evidence, have challenged the conclusion that the climate has changed in any important way since Roman times. Although climatological factors may also have contributed, it was man who destroyed the balance of nature responsible for maintaining soil fertility and moisture. Why is it that under certain circumstances, the plant cover is unable to regenerate itself, thus setting the stage for serious erosion (腐蚀)? To answer this question, let us take an extreme example, the destruction of tropical rain forests. In general, the equatorial (赤道附近的) soil is poor: forests can exist in these regions only because they are part of a balanced cycle. All that the forest produces is returned to the forest. The organic matter that falls from the trees constitutes the humus (腐殖土) that the forest requires. When people cut down all the trees to make room for crops, the soil is laid bare and, deprived of shade, heats up. An increase in soil temperature increases the rate of decomposition (分解) of organic matter. Organic nitrogen (氮) is converted into soluble ammonia (氨) and nitrates (硝酸盐), which the rains quickly leach (沥滤，溶滤)away. It has been calculated that a rise in temperature from 77 to 78.8 may increase the loss of nitrogen by fif to twenty pounds per acre per year. With the fertility of soil destroyed, an irreversible change from forest to desert can be initiated. What do people usually think contributed to the decline of North Africa?