It is a fundamental principle of ecology that diversity leads to stability. Most ecosystems are very diverse. Even a climax forest, consisting largely of a single tree species, maintains considerable diversity. This diversity occurs both within the one species of climax tree, and among the many other, less prominent species that invariably inhabit a climax forest.

Diversity was also the rule among ancient farmers, and it still is among modern subsistence farmers. Most subsistence farmers in the tropics grow a mixture of crop species in one field, and each species is a mixture of varieties. The ancient Aztecs grew an extraordinarily successful mixture of maize, squash, and beans, and the cultivation of this mixture is still quite a common practice in modern Mexico.

Commercial agriculture, however, stresses crop uniformity. An important reason for this is the cost of labour. Mixed cropping is labour-intensive, and it is usually not amenable to the major labour-saving practices, such as mechanical harvesting and selective herbicides. This difference is revealed in the degree of economic development of a country. In a non-industrial country, as many as 80% of the population may be engaged in agriculture while, in an industrial country, this figure may be as low as 2%.

Crop uniformity carries a cost, however, and the cost is an ecological one. It is a reduced ecological stability. This loss of stability results mainly from an increased liability to damaging epidemics and infestations. There has also been a loss of genetic flexibility, which is the ability to respond to selection pressures, and which must now be discussed.

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