THE BIOTIC ENVIRONMENT OF ORGANISMS 567 



Some indirect restraints. The indirect restraints that may be placed 

 upon an organism by its biotic environment are variable and fluctuating 

 but nonetheless important. Ordinarily, for instance, the martin of the 

 Canadian coniferous forests is not molested to any important extent by 

 the Canada lynx that feeds extensively upon the varying hare of this 

 region. But the hare population is subject to periodic epidemics that 

 tremendously reduce its numbers, and at these times the martin not 

 only finds severe competition from the lynx but is actually preyed upon 

 by it. 



Another example is the relationship that appears to exist between 

 the bob white quail and the cotton rat of our southeastern coastal plain. 

 Here the rats, when they are very numerous, may become an enemy of 

 the quail through feeding upon the latter's eggs and young. The rats are 

 the usual and preferred food of a number of hawks and owls that do not 

 ordinarily molest the quail to any appreciable extent. When, however, the 

 rat population is for any reason markedly reduced, the hawks and owls 

 that had depended upon them for a food supply turn to the quail and 

 may become an important check upon the quail population. It would 

 thus appear that the rat population, if it is either too great or too small, 

 affects the quail adversely; but in some intermediate and average size 

 it is beneficial in forming a "buffer population" that protects the quail 

 from injurious predation by the hawks and owls. If this relationship is 

 real — and there is much evidence to substantiate it — the various parasites 

 that live upon the cotton rats and those that live upon the hawks and 

 owls must also play an indirect but often an actual part in the quail's 

 environment. 



Highly probable or partly substantiated instances of such indirect 

 relationships are very common, and there is little doubt that any form 

 of wildlife, if it were sufficiently studied, would show the same sorts 

 of indirect dependencies upon other organisms. The complexity of these 

 relationships, however, and the continual fluctuations in their inten- 

 sity make them exceedingly difficult to demonstrate or to evaluate 

 quantitatively. 



One of the important features of all biotic resistance is that it is very 

 variable and that it tends to become increasingly severe whenever the 

 organism's reproductive potential is temporarily freed from restraints 

 that ordinarily hold it in check. If, for instance, any species encounters 

 a period of particularly favorable physical conditions, the resultant drop 

 in environmental resistance permits a prompt increase in population size. 

 Sooner or later average or adverse physical conditions will return, but 

 not because of any influence exerted by the rise in population. The biotic 

 resistance, on the other hand, tends to increase with and as a consequence 

 of the increase in the organism's population size. For this increase in- 



