634 General and Applied Biology 



for that food. The result will be either the migration of certain of 

 them in order to get suitable foods or the death of a certain number of 

 the competitors. Since all animals require foods, it is easily seen that 

 this struggle for them is one of the greatest ecologic factors in the animal 

 kingdom. This migration in search of food may upset the natural 

 balance of the new community in which the migrants locate. 



Competition between plants of different species or even between 

 plants of the same species rather closely resembles the struggle for 

 existence in the animal world. Apparently nature sanctions this natural 

 phenomenon in order to permit the fit to survive and exterminate the 

 unfit. Such a struggle for foods, light, moisture, space, or position 

 naturally will affect all of them in a minor or major way with its result- 

 ing ecologic effects. 



(b) Competition Between Sexes: In the process of propagating the 

 race, certain animals may travel long distances for the opposite sex. In 

 other instances the competition of several members of one sex for a 

 limited number of animals of the opposite sex may lead to dispersal or 

 extermination. Since the urge to continue the individual as well as the 

 race is a strong one, it can readily be seen that such a factor might be 

 a very great one in determining the distribution of a particular species. 



(c) Dependence of Certain Plants on Insects for Pollination: Certain 

 plants require insects to carry pollen from the male reproductive organs 

 to the female. In some instances a specific insect is required if extensive 

 pollination is to occur. Bees are quite essential for this purpose in 

 clovers. If bees are absent, the clover will bear a minimum of seed 

 and hence will present an entirely different ecologic picture than if bees 

 were present in sufficient numbers. Hives of bees are frequently to be 

 seen in orchards and in clover fields for this purpose. Of course, such 

 sources of nectar for making honey are also items not to be overlooked. 

 Other plants do not depend upon insects for their pollination, so that 

 the problem is quite different from the one presented above. 



(d) Distribution Affected by Mutual Help, Such as Symbiosis and 

 Commensalism: Sometimes organisms are distributed in certain areas 

 because of the help which they give or receive from organisms of a dif- 

 ferent species. If this help were not available, there would be an entirely 

 different distribution of the species in question. Symbiosis pertains to the 

 rather intimate association of two different species of organisms with a 

 mutual benefit to both. For instance, the termites ("white ants") are 

 able to digest wood because they harbor in their digestive tracts certain 



