tioii. and disease. The effect of density-dependent 

 factors has been much studied from a mathematical 

 viewpoint, but the present approach will be largely 

 introductory and non-mathematical. 



Competition 



The definition and basic principles of compe- 

 tition have already been considered. We are here 

 concerned with how competition helps to stabilize a 

 population at a particular level. In this respect com- 

 petition is primarily for space, cover, and food. 



Every terrestrial green plant requires a volume of 

 soil for its root system and a volume of air in which 



it can display its foliage to receive solar radiation. In 

 a dense forest the individual tree grows tall because 

 of competition with its neighbors. Trees unable to 

 keep up with this competition become overtopped by 

 other trees and, lacking sunlight, die. Sessile marine 

 animals, such as corals, mussels, and barnacles, may 

 crowd into close physical contact, even growing on 

 top of one another, but there is undoubtedly a limit 

 to the number that can survive and carry on normal 

 activities in an area of restricted size. 



Competition for space is well demonstrated in 

 those species that defend territories ; birds, for in- 

 stance. With increase in number of birds in an area 

 there is, at first, some accommodation as the size of 

 territories varies inversely with the size of the popula- 



% 



FIG. 

 white 



100 150 200 250 

 APRIL DENSITY 



■2 Per cent yearly increase In population siie of 

 relation to April densities (Errlngton 1945). 



400 600 800 1000 1200 1400 

 MIDSUMMER DENSITY 



ss rates of bobwhite from r 

 to midsummer densities (Err 



ngton 1945). 



Regulation of population size 22 



