EQUILIBRIUM 



167 



without the maladjustment being felt as far as in an aquatic com- 

 munity of corresponding magnitude. 



To illustrate the character of land communities in the matter of food 

 supply and equilibrium we have chosen a number of prairie animals and 

 constructed them into an arbitrary community. This community is 

 graphically represented in Diagram 6. The arrows point from the 

 animal eaten to the animal doing the devouring, many such relations 

 being shown on the basis of actual published records. 



Diagram 6. — Showing the food relations of land animals. Circles and ellipses 

 inclose groups of organisms which are commonly eaten by the same animals, and 

 groups eating similar food. Arrows point from the animals eaten to those doing the 

 eating. For explanation see text. 



From the diagram we note that wolves destroy the bison. If for any 

 reason the wolves increased, they would destroy so many bison that the 

 bison would decrease because wolves were abundant. The greater 

 destruction of mice in summer by the numerous wolves would cause a 

 decrease of mice. Finally, wolves would decrease because of lack of big 

 game in winter and mice in summer. This would give the bison and 

 mice an opportunity to recover their former number and the whole 

 chain of changes would be duplicated and a general equilibrium be 



