Balance of nature
Balance of nature, concept of nature as a network of relationships and interdependencies between animals and plants, all of which support and control each other in a stable and unchanging equilibrium. The concept has been greatly modified since it was first suggested in the latter half of the 19th and early 20th centuries. It is now recognized that although a degree of balance does exist, it is a highly dynamic and unstable state. The main reason for this is that animals and plants depend not only on each other but on such external factors as climate and availability of food. Where food supplies are plentiful and the range of forms of life is large, a community may achieve a fairly high degree of stability. But where the “food web” is simple, as in the northern tundras, there may be insufficient control mechanisms to prevent the periodic explosions of population that cause, for example, the self-destructive mass migration of lemmings. Many natural communities that were once stable have become unstable or have been destroyed by industry, agriculture, and disposal of sewage. Pesticides may kill beneficial insect parasites and thus lead to an uncontrollable increase in the pest population in succeeding years. Pesticides may also become concentrated in the bodies of predators at the top of the food chain. The death of hawks and owls from pesticides allows the population of rodent pests to increase. A small alteration in the balance of nature may produce unexpected consequences in some other part of the community.