70 AQUATIC CONDITIONS 



because of the increasing unsuitability of the ponds as fish breeding- 

 places. The oxygen content decreases, particularly on the bottom. 

 The distribution of the fish present in these ponds, and whose breeding 

 habits were known, was found to be correlated with the distribution 

 of the bottom upon which they breed. This becomes less and less in 

 amount as the ponds grow older. 



3. EQUILIBRIUM 



Each animal prefers certain food. The food relations of pond 

 animals are shown in Diagram 3, below. For purposes of illustration 

 let us suppose the existence of a community composed of the species 

 named only. 



Black bass adults 

 Black bass young 



DIAGRAM 3. Showing food relations of aquatic animals. Arrows point from the 

 organisms eaten to those doing the eating. For explanation see text. 



Any marked fluctuation of conditions is sufficient to disturb the 

 balance of an animal community (see chap, i, p. 18). Let us assume 

 that because of some unfavorable conditions in a pond during their 

 breeding period the black bass (79) decreased markedly. The pickerel, 

 which devours young bass, must feed more exclusively upon insects. 

 The decreased number of black bass would relieve the drain upon the 

 crayfishes, which are eaten by bass, crayfishes would accordingly increase 

 and prey more heavily upon the aquatic insects. This combined attack 

 of pickerel and crayfishes would cause insects to decrease and the number 

 of pickerel would fall away because of the decreased food supply. Mean- 

 while the bullheads, which are general feeders and which devour aquatic 

 insects, might feed more extensively upon mollusks because of the 



