7° 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. 
Mussels 
Bullheads 
Sphaeridae 
Ge 
a Small aquatic insects 
& Large aquatic insects 
Entomostraca 
LOZO}OIg 
a 
0 aR 
3 
oO oO 
Protozoa 
Decaying Vegetation 
Amphipods 
Black bass adults ee 
Pickerel 
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 
