Biotic Potential and Environmental Resistance 335 



food supply or of breeding sites, accumulation of metabolites, and the 

 like— will appear. The combined effect of these factors tending to 

 curtail population growth is called environmental resistance. The 

 capacity to increase resides within the species, but the degree to 

 which it is realized is determined by the environment, including the 

 changes in the environment brought about by the species itself, and 

 aspects of the environment consisting of other members of the species. 

 Thus the actual rate of the increase of a population is determined by 

 the balance struck between biotic potential and environmental 

 resistance. 



Certain features of the environment are largely or entirely unaf- 

 fected by changes in the density of the population. These are density- 

 independent factors. For example, an increase in the abundance of 

 a species in a marine area does not affect the temperature or the 

 salinity of the water, but changes in these factors, harmful or other- 

 wise, are brought about by agents unrelated to the density of the 

 population. Other changes in the environment are directly related 

 to the abundance of the animals or plants concerned; these are 

 density-dependent factors. Scarcity of such necessities as food, oxy- 

 gen, or breeding sites may become increasingly acute as a population 

 of animals grows; lack of nutrients, excessive pH values, or other 

 inimical condition may be brought about by the growth of a plant 

 population. In addition, the susceptibility of organisms to disease 

 as well as the ease of transmission is often increased as the density of 

 the population grows. Generally speaking, the physical features of 

 the environment tend to be density-independent and the biotic in- 

 fluences are often density-dependent factors, but the reverse is some- 

 times true, and certain factors may change from one category to the 

 other according to circumstances. For example, the predation of a 

 tawny owl ( Strix aluco ) on a population of mice in an English wood- 

 land was found to be a density-independent factor since the owl was 

 observed to eat only 4 to 6 wood mice (Apodemus sylvaticus) each 

 day, regardless of the size of the prey population (Miller, 1951). But 

 in other situations predation will be a density-dependent factor if 

 each predator kills more prey when the prey are abundant. During 

 periods of plenty many predators are known to kill more than they can 

 eat. Increased density of prey may also cause increase in the pred- 

 ator factor by making possible increased reproduction and growth of 

 the predator population as well as by attracting predators from neigh- 

 boring areas. 



The question just discussed of whether the density of a population 

 controls or modifies a certain environmental factor must be carefullv 



