POSSIBLE LIMITS OF SPACE, FOOD, OR FAVORABLE WEATHER 



FIG. 16-10 Inter-relations of 

 various density-stabilizing and 

 density-limiting factors ("en- 

 vironmental resistance") in the 

 regulation of population size at 

 various possible levels. Consider- 

 able variation occurs between 

 species in the relative importanc 

 and position of the different 

 factors. Note that the limits of 

 population growth set by space, 

 food, or favorable weather may 

 occur for different species at 

 I. 2, 3, 4, or 5. 



but SO does the number of roses available to them. 

 The favorable period normally ends long before the 

 thrips have time to saturate the niche. Summer 

 drought brings high mortality and a decline to the 

 low densities of the species characteristic of late sum- 

 mer and winter. The rise of the thrip population is a 

 race against time, the increase in density greatest in 

 those years when the favorable period lasts longest ; 

 but it never reaches the point where competition be- 

 comes important. Annual variations in maximum 

 densities in this species are almost entirely the result 

 of density-independent climatic factors (Davidson 

 and Andre wartha 1948). What is true for thrips may 

 apply also to many other kinds of insects and in- 

 vertebrates having annual life cycles ; that is to say, 

 climatic factors appear more important than biotic 

 ones in determining the yearly size of the popula- 

 tions (Uvarov 1931, Bodenheimer 1938, Thompson 

 1939, Andrewartha and Birch 1954). 



The various stabilizing and limiting factors act in 

 an intercompensatory manner. All stabilizing factors, 

 for instance, are in temporary abeyance following 

 catastrophes of weather, drought, floods, or other 

 factors until there is recovery of normal population 

 levels again (Nicholson 1954a). 



One of the most thorough studies of stabilizing 

 factors has been made on the muskrat (Errington 

 1946, 1951). This species is subject to such density- 

 dependent mortality factors as intraspecies competi- 

 tion or fighting ; predation, especially by mink and 

 foxes ; emigrations from overcrowded habitats ; and 

 epizootics. Overpopulations may be reduced by one 

 of these factors singly, or by two or more working 

 simultaneously. If fighting or predation keeps the 

 population at a low level, disease is unimportant; but 

 if fighting or predation is negligible some one par- 

 ticular year, then disease may reduce the numbers of 

 animals. Emigration to other areas occurs when a 

 marsh becomes overcrowded or drought reduces the 

 carrying capacity. If freezes, violent storms or floods, 

 drought, or trapping reduces the population exces- 

 sively, there is compensation by increased breeding 

 activity, and for a time all other regulating factors 

 are held in abeyance. The fur yield of a muskrat 

 marsh cannot, therefore, be increased simply by de- 

 stroying the predators, for other controlling factors 

 become proportionately more effective. Trapping for 

 fur, if not excessive, is economically profitable and 

 can be carried on year after year, if the animals elimi- 

 nated through trapping are restricted to the numbers 



232 Ecological processes and dynamics 



