EQUILIBRIUM 



71 



decrease of the former (see chap, i, p. 15), but would probably decrease 

 also because of the falling-off of their main article of diet. We may 

 thus reasonably assume that the black bass would recover its numbers 

 because of the decrease of pickerel and bullheads, the enemies of its 

 young. A further study of the diagrams shows that a balance between 

 the numbers of the various groups of the community would soon result. 



Diagram 4. — Showing the life histories of the animals of the pond community 

 in the form of circles. The heavy, vertical, black lines represent the animals which are 

 dependent upon the most elementary food substances. A represents dead animal 

 matter; B, the protozoa, rotifers, and Entomostraca, the smallest animal food. The 

 black lines come into contact with different numbers of life cycles, but are indirectly 

 connected with all so that any change in the position or rate of movement (meaning 

 number or rate of reproduction and growth) of the rod must effect the entire com- 

 munity; compare with Diagram 3. 



Diagram 5. — Showing the food relations in the brook community. A repre- 

 sents algae which grow upon the stones. B represents the floating animal bodies and 

 other organic matter. The latter are of small importance because of their small 

 number and the swift current. 



Under other circumstances, such as the extinction of the black bass, the 

 resulting condition would be entirely different from the original one, 

 but a balance between supply and demand would nevertheless finally 

 be established. The community is said to have equilibrated when such a 

 condition is reached; that is, a new equilibrium is established which 

 may or may not be like the old. 



