54 FRESH-WATER BIOLOGY 



as to cover several animal communities. The association which 

 would result from an indiscriminate distribution is first eliminated. 

 Then from the total number of collections, the number of collec- 

 tions containing each species, and the number of collections con- 

 taining both species, he derives a coefficient of association by very 

 simple calculations. 



Each animal prefers certain food. The food relations of pond 

 animals are shown in Fig. 6. For purposes of illustration one may 

 suppose the existence of a community composed of the species 

 named only. 



Any marked change of conditions will disturb the balance in an 

 animal community. Assuming that because of some unfavorable 

 conditions in a pond during their breeding period the black bass 

 decrease markedly, the pickerel, which devours yornig bass, must 

 feed more exclusively on insects. The decreased number of black 

 bass would reheve the drain upon the crayfishes, which are eaten 

 by the bass; crayfishes would accordingly increase and prey more 

 heavily upon the aquatic insects. This combined attack of pick- 

 erel and crayfishes would cause insects to decrease and the number 

 of pickerel would fall away on account of the decreased food supply. 

 Meanwhile the bullheads, which are general feeders and which eat 

 aquatic insects, might feed more extensively upon mollusks because 

 of the decrease of the former, but would probably decrease also 

 because of the falhng off of their main article of diet. It may 

 reasonably be assumed that the black bass would recover its num- 

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

 of its yoimg. A further study of the diagram shows that a balance 

 between the numbers of the various groups of the community 

 might soon result. Under certain circumstances, such as the ex- 

 tinction 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 commu- 

 nity 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. 



The causes of fluctuations of numbers of organisms are numer- 

 ous. Cold winters often destroy aquatic vertebrates. Large rain- 



