THE BIOLOGICAL BALANCE SHEET 551 



But here the analogy ends, for no puzzle-maker ever found a way of creating a problem as 

 complex as this one. Reduced to its simplest terms, life for the grouse is a constant tug-of- 

 war between the factors that increase and those that shrink its ranks. On the one hand i^the 

 inherent capacity of the bird to reproduce and to adapt itself to its surroundings. This, as one 

 will remember, may be broken down into such components as the sex ratio, the number of 

 eggs, their hatchability, the livability of the chicks and the physiology and psychology of the 

 species. On the other hand are first, delimiting conditions, principally range quality in 

 terms of food and shelter, and, second, the forces which tend to decimate the grouse crop, 

 such as predation, disease, weather, hunting and accidents. Food is not listed in the latter 

 group since in good grouse coverts it seems to be available in adequate amounts (though 

 possibly not always in suflBcient variety) to meet the basic nutritive requirements of the bird. 

 At least no partridge has been found dead as a result of lack of food. 



Whether or not grouse may be expected to occupy an area depends on the character of the 

 range. If the minimum essentials of food and shelter are lacking the species will not thrive 

 regardless of other factors. In the following discussion it will be assumed that these condi- 

 tions are not seriously delimiting. 



Within narrow limits the factors that tend to increase the grouse crop seem to exert a 

 steady pull. It is, then, the comparative strength or weakness of the influences working in 

 the opposite direction that determines j)rodurtivity. When these influences are weak, the 

 grouse crop may prosper up to the point of saturation. When they are strong, it declines. 



Within broad limits one may discern a general pattern of relationships. In a low popula- 

 tion the birds tend to occupy the most favorable coverts where they have the best food, shel- 

 ter and protection from adverse influences. Survival, particularly during the overwinter per- 

 iod, is high. But as the population increases the birds are forced to spread out into habitats 

 where, because food and shelter are farther apart, they are more exposed to their enemies. 

 More hunters take to the field, predator pressure may increase, and disease, with more birds 

 to work on, has a better chance to spread. Those individuals in the poorer habitats are. 

 likewise, more subject to accident. In other words, in itself, an increase in the numbers of 

 grouse develops a greater resistance on the part of the environment to further expansion. 

 Soon a point is reached where this resistance becomes so strong that the reproductive 

 capacity of the bird cannot keep pace and further increase becomes impossible. Yet at any 

 time the pressure of adverse influences may bring about a decline in abundance. 



But the situation is not as simple as it might seem. Collectively, the forces involved are 

 only as strong as the sum of their individual plus and minus values. Thus, if buffers are 

 plentiful and the grouse not overly abundant, a net gain in the crop may be attained al- 

 though predators may be numerous. On the other hand, even though natural enemies are 

 scarce, a severe rain just at baching time may spell death to so many chicks that no fall 

 surplus can be produced. A single event such as this, however, is only occasionally sufficient 

 to turn the tide. In the main it is the cumulative effect of a number of factors rather than 

 any one of them that causes significant variations in survival. 



The ultimate degree to which grouse abundance ina> be affected depends on the severity 

 and duration of the forces involved. Yet these are relative, constantly conditioned by the end- 

 less variety of circumstances. For instance, when grouse are scarce, a hunting pressure thai 

 may mean little to them when they are abundant, may be the drag that prevents their in- 

 crease. On the other hand, excessive populations appear more susceptible to disease. 



