CONSERVATION 825 



tridges, which had become adapted to the intensively farmed regions of 

 Europe. 



Of the three general methods used to increase the population of 

 game animals— laws restricting the number killed, artificial stocking and 

 the improvement of the habitat— the latter is the most effective. If the 

 game habitats are destroyed or drastically altered, protective laws and 

 artificial stocking are useless. Protective laws must operate to prevent a 

 population from getting too large as well as too small. Deer populations, 

 in the absence of natural predators but subject to a constant, moderate 

 amount of hunting, may increase to a point where they actually ruin the 

 vegetation of the forest. Hunting should be restricted, of course, when 

 populations are small and increased when they are larger. This requires 

 accurate annual estimates of the population density of the game species. 



Stocking a region artificially with game animals is effective only if 

 they are being introduced into a new region or into one from which 

 they had been killed off. Beavers, for example, had been trapped to ex- 

 termination in Pennsylvania, but restocking with Canadian beavers 

 has been very successfid and it is estimated that there are some fifteen 

 to twenty thousand beavers busy building dams in Pennsylvania. These 

 are now an important factor in flood control in that region. The prin- 

 ciples of population growth make it clear that if game animals of a 

 certain species are already present, artificially stocking that region with 

 additional members of the species will be futile. Stocking a region with 

 a completely new species must be done cautiously, or the species may 

 succeed so well as to become a pest and upset the biotic community, 

 as has happened with rabbits in Australia and the English sparrow in 

 the United States. 



The management of the fish in a pond may be directed toward 

 providing sport for hook and line fishermen or toward raising a crop of 

 food fish and draining the pond at regular intervals to harvest the crop. 

 To provide the best sport fishing it has been found that a lake or pond 

 should be stocked with a combination of the sport fish and its natural 

 prey; stocking a pond with large-mouth bass plus bluegills gives seven 

 to ten times more bass in three years than does stocking with bass alone. 

 Stocking with fish must be done with care, for if a lake that already has 

 about as many fish occupying a certain ecologic niche as possible is 

 stocked with more of the same kind, there will be a decrease in the rate 

 of growth and the average size of the fish. It has been found that sport 

 fishing with hook and line is not likely to overfish a lake; the lake is 

 more likely to be underfished and the resulting crowding leads to a 

 decrease in the average size of the fish population. 



The building of dams raises intricate ecologic problems, for dams 

 may be intended for power, for flood control, for the prevention of soil 

 erosion, for irrigation or for the creation of recreational areas. Since 

 no one dam can satisfactorily accomplish all of these objectives, the pri- 

 mary objective must be clearly delineated and the secondary results 

 must be understood. A contrast of two proposals for dealing with the 

 same watershed (Table 16) shows that the multiple dam plan costs 



