174 AGGREGATION, COMPETITION AND CYCLES 



with maxima of the sharptailed grouse in southern Manitoba. He 

 states that the maxima for this grouse are preceded and accompanied 

 by grasshopper outbreaks or maxima, the enhanced food supply being 

 the primary cause of the rise. Furthermore, the beginning of a grass- 

 hopper maximum is nearly always preceded or followed by abnormally 

 dry seasons, and the increase may be stopped by excessive rains. The 

 decline in the grouse cycle is attributed mainly to heavy rains and 

 cold, which reduce the amount of food taken to the nest by a third, 

 owing to the inactivity of insects, and thus results in starving the 

 young birds. Disease is regarded as the probable major factor in the 

 reduction of the adults, but they are also decimated by the goshawk, 

 a single predator destroying as many as 50 grouse in a winter. 

 Griddle's explanation ignores possible direct physiological effects of 

 the climatic factors in limiting populations, but nevertheless gives a 

 broad explanation of the observed facts. 



The view of Uvarov (1931) likewise takes food and predators into 

 account along with climate: "The theory of stable equilibrium is based 

 on the assumption that the numbers of an organism depend mainly on 

 the numbers of their enemies and on the quantity of food, i.e., on 

 factors which in their turn are dependent on other organisms. No one 

 will deny the controlling value of these factors, but the evidence col- 

 lected in this section, as well as in the whole of this paper, should go 

 far towards proving that the key to the problem of balance in nature 

 is to be looked for in the influence of climatic factors on living organ- 

 isms. These factors cause a regular elimination of an enormous per- 

 centage of individuals under so-called normal conditions, which in 

 fact are such that insects survive them, not because they are perfectly 

 adapted to them, but only owing to their often fantastically high re- 

 productive abilities. Any temporary deviations in the climatic factors, 

 however slight they may be, affect the percentage of survival, either 

 directly, or indirectly (through natural enemies and food-plants), and 

 thus influence abundance." 



Nicholson (1933) has recognized the importance of climate and 

 enemies, but also points out the limitations of climate in rather strong 

 terms, as follows: "We will suppose tliat the animals in a certain 

 population would increase one hundredfold in each generation if un- 

 checked, and also that, on the average, climate destroys 98 per cent 

 of the animals. It is clear that the number of animals would be 

 doubled in each successive generation if no other factors operated. 

 Climate could never check this progressive increase, for it would con- 

 tinue to destroy only 98 i)er cent, its action being uninfluenced by 

 the density of the animals. If, however, there is some other factor. 



