Harmful Effects 



321 



Perhaps the most generally harmful effect of increasing numbers 

 among animal populations is the competition for food. In the photo- 

 graph, Fig. 9.5, a "deer-line" is distinctly visible on the vegetation at 

 a height of about 2 m. This denudation of the trees has been caused 

 by the browsing of an excessive number of deer. With the shortage 

 of food the growth of the deer population will be curtailed, and, if 

 all the edible vegetation throughout the area is removed, the deer 

 will be in danger of starvation. In extreme instances the vegetation 

 may be permanently injured with the result that the area will sub- 

 sequently not support as many deer as previously. 



The effect of competition for food on the growth of bluegill sunlish 

 is well demonstrated by records of farm fish ponds. In one test a 

 pond was stocked in March with fish averaging 5.7 g. When weighed 

 again in June of the same year, the average size of the fish had in- 

 creased to 76 g. During this month each pair of sunfish produced an 

 average of 4000 young. Since the young were not removed either 

 artificially or by a predatory species of fish, the new generation en- 

 tered into direct competition with the adults for food. When the 

 pond was drained in November of the same year, the average weight 

 of the parent group had decreased from 76 g to 54 g, showing that 



Swingle and Smilli, 1942 



Fig. 9.6. Effect of overcrowding on the growth of bluegill sunfish. Both fish are 

 1 year old: upper specimen from a pond stocked with 3750 fish per hectare (1500 

 per acre); lower specimen from a pond stocked with 450,000 fish per hectare 



(180,000 per acre). 



