480 ECOLOGY AND WILD LIFE CONSERVATION 



tiny mites, and protozoan parasites infest even the tiniest of many celled 

 organisms ; even hydra has a species of ciliated protozoan which clings 

 to its body apparently living off its secretions. The old verse had more 

 than a spark of truth in it. 



Photo by Lovell 



Fig. 30.4. Cowbird eggs in the nest of a wood thrush. Here, eight spotted cowbird 

 eggs have replaced all but two of the eggs of the wood thrush. This parasitism prob- 

 ably arose because the cowbird followed bison herds and did not stay in one place long 



enough to nest like other birds. 



The Balance of Nature 



One of the most important concepts that has been developed by 

 ecologists is the balance of nature. A common question asked by the 

 biological illiterate is: "What good is that predator or that parasite?" 

 Perhaps they help keep some species of fast-reproducing animal in check 

 and so prevent it from occupying more than a fair share of the earth's 

 surface. The American Indian lived with nature, took only what he 

 needed, and rarely upset the natural conditions. Modern man, however, 

 has upset this balance in nearly every part of the world. Modern 

 civilization destroys not only the animals and plants but their habitats 

 as well, often with disastrous results to wild life and to man himself. 

 Great floods have inundated our cities since the protective cover of the 

 forests has been removed. The giant reservoirs created by hydroelec- 

 tric dams are rapidly filling up with silt because irresponsible farming 



