LEOPOLD: THE CONSERVATION OF WILDLIFE 803 



Whatever the level of a local game population, it reproduces annually and 

 creates a "surplus," which in one way or another will be dispersed prior to the 

 next breeding season. The surplus may be shot by hunters, or it may be taken 

 by predators, or disease, or accidents. The important thing is that it will disap- 

 pear. The annual increase in wild populations, which may be as low as 10 per 

 cent in bears or as high as 300 per cent in quail, is never saved in a fully stocked 

 habitat, but tends to be vulnerable to many kinds of losses, down to the level 

 of "carrying capacity." Below that level, losses are few. These principles were 

 first stated by Errington (1934, 1936, 1943) based on studies of bobwhites in 

 Wisconsin and muskrats in Iowa. They have been verified by many subsequent 

 investigations of other species in a variety of habitats and give support to the 

 idea that management should strive to raise the carrying capacities of local 

 environments as the cheapest and surest way to increase game. 



But these general concepts served merely to orient thinking without defining 

 the specific nature of the relationships between vertebrate organisms and their 

 environments. Recent research has sought to refine our understanding of "car- 

 rying capacity" and of the reaction of individual animals to their surroundings 

 and to each other. 



Considerable attention has been focused, for example, on the question of 

 game nutrition. Even a few years ago, anything a bird or mammal ate was 

 considered "food" and the only measure of importance applied to items of 

 diet was quantitative. But it was noted that some types of food supported higher 

 populations than others, and this led to investigations in qualitative nutrition. 

 Among the various sorts of winter browse eaten by deer, for example, those 

 species that seem best to maintain the animals have proved to be high in protein. 

 On an adequate protein diet, deer remain strong and vigorous through the win- 

 ter, the does bear many healthy fawns, and young adults breed at an early age. 

 Conversely, on low protein the deer weaken, become subject to high losses from 

 predators, disease, or outright starvation, and they raise few fawns (Longhurst 

 et al., 1952). Thus food quality has a great deal to do with carrying capacity 

 of deer ranges by regulating both the rate of increase and the extent of loss in 

 the herds. Parallel studies of production and loss in quail populations suggest 

 that there may be similar striking effects of changing quality in the diet. 



Another phase of ecology that is being much studied today is the matter of 

 competition between members of a population and how competition serves to 

 regulate population levels. Besides competing for food and for the best areas 

 of cover, members of a dense population seem to affect each other in some subtle 

 way that lowers reproductive rate. Thus in populations of bobwhite quail, ring- 

 necked pheasant, mule deer, and brown rats, the rate of fecundity per individual 

 female has been found to be inversely proportional to the density of the popu- 

 lation. The most precise measure of this phenomenon has been made in rat 

 populations in the city of Baltimore (Emlen et al., 1948; Davis, 1951). Following 

 artificial reduction of the rats to a low level, there was a marked increase in the size 

 and frequency of litters produced by the surviving females. As the population 

 again approached carrying capacity, fecundity decreased until a stable population 

 was restored, in which death rate and birth rate balanced . The obvious implication 

 in management is that a heavy artificial kill, as by hunting, is compensated by an 

 increased birth rate — a vital point in determining desirable rates of harvest. 



