208 THE STUDY OF PLANT COMMUNITIES ' Chapter VIII 



When man has tampered with the balance among the species of 

 a community by eliminations or introductions, he has not always 

 considered the possible effects upon the community as a whole. If 

 large carnivores are destroyed, herbivores increase, and, if their 

 reproductive capacity is great, they may soon become so abundant 

 that their grazing destroys the community or changes it radically. 

 If a predator is introduced whose prey is some native species that 

 is a pest, the predators may eliminate the pest and then become 

 pests themselves. 10 



Only a few examples are necessary to illustrate these points. The 

 Indian mongoose was introduced into Haiti, Jamaica, and other 

 West Indian islands to rid them of rats and snakes. This the mon- 

 goose did most effectively, but its numbers increased, and, with its 

 natural prey disappearing, it turned to robbing birds' nests of eggs 

 and young. Now it is practically impossible to raise poultry there. 

 The gypsy moth was accidentally introduced into Massachusetts 

 when it escaped from cultures being reared to test its silk-produc- 

 ing ability. It is now a serious pest of fruit and shade trees in most 

 of the eastern United States although much money and effort have 

 been expended to control it. On the other hand, introductions of 

 about sixty foreign predators or parasites of the gypsy moth have 

 resulted in the establishment of a dozen or more that are aiding in 

 its partial control. The destruction of coyotes in some western 

 states has resulted in such marked increase of rabbits that their 

 winter feeding on tree seedlings modifies vegetational develop- 

 ment (see Fig. 95). 



On game reserves where predators have been eliminated and no 

 hunting is permitted, the population of herbivores, such as deer, 

 usually increases rapidly. When the number of deer exceeds the 

 natural carrying capacity of the region, a shortage of food results 

 during unfavorable seasons. Then, especially in winter, many ani- 

 mals die unless they are fed bv man. As a result of supplementary 

 feeding, the population is still larger the next season, and the prob- 

 lem is not solved. Controlled hunting is now permitted on several 

 such reserves where the population capacity has been determined. 

 The effects on the vegetation of such overcrowding are very con- 

 spicuous. All young woody plants protruding above snow are 

 eaten off, and the lower limbs of young trees, even conifers, are 



