CYCLES AND NUMBERS 175 



such as a natural enemy, the action of which is governed by the 

 density of animals, the destruction of the remaining 1 per cent, 

 necessary to check an increase, would soon be accomplished. If this 

 example were observed in nature, one would be tempted to conclude 

 that, because climate destroys 98 per cent of the animals while the 

 natural enemy destroys only 1 per cent, the limitation of the popula- 

 tion is mainly due to the influence of climate. However, it is clear 

 that the natural enemy is wholly responsible for control, because 

 climate, by itself, would permit the density of the population to be- 

 come indefinitely great." 



He further concludes that there is a particular or "steady" den- 

 sity at which balance exists for each species, and that competition 

 always tends to cause animals to reach and maintain this density. 

 Climate and animal behavior "cannot themselves determine popula- 

 tion densities, but they may have an important effect upon the values 

 at which competition maintains these densities." It appears that 

 these views fail to take into account the climatic cycle and its signal 

 effects, and there is also grave doubt that the statistical approach is 

 applicable to such an intricate complex of causes and effects. Power- 

 ful as competition is in the community, its action may be largely or 

 almost entirely suspended, in the plant matrix at least, by optimum 

 climatic or edaphic conditions. 



In his studies of the wintering of quail in Iowa in relation to popu- 

 lation, Errington (1934) reaches the following conclusions: ''Food is 

 the first essential constituent of a winter quail territory; cover is the 

 second. The quality, distribution and convenience of food and cover, 

 together with the bobwhite's intolerance of crowding, probably deter- 

 mine in largest measure the carrying capacity of environment for the 

 species. 



"Cover is of value to the bobwhite chiefly as protection or con- 

 cealment in case of attack by enemies. Lack of cover means vulner- 

 ability to predation, whether enemies are few or many. Cover also 

 has a certain value as shelter during periods of wet or cold weather, 

 or during storms, but the necessity of shelter for the bob-white is 

 usually over-rated about as much as escape cover is under-rated." 

 For the southeastern United States, the earlier account by Stoddard 

 (1931) is a mine of information as to causes of dynamic balance in 

 fiuail populations. 



CYCLES AND NUMBERS 



Several important cycles of different character and rank find more 

 or less definite expression in the structure and development of the 

 biome. Chief among these are climatic mass migration and succes- 



