Environmental resistance; 



Biotic 

 potential 



Stabilized ) 

 population level 



Population growth curve: 

 dt 



-TIME- 



FIG. 16-4 Relation between blotic potential and environmental 

 resistance in determining the population level attained by a 



tion and amount of competition involved. With 7 

 pairs of bay-breasted warblers on a 10-hectare plot 

 (25 acres), the average size of territories was 3157 

 m^ (0.78 acre). On another plot with 18 pairs the 

 territories averaged 1740 m- (0.43 acre) ; on a plot 

 with 25 pairs, 1497 m- (0.37 acre) ; and on a plot 

 with 42 pairs, only 1174 m^ (0.29 acre) (Kendeigh 

 1947). With decrease in size of territories, however, 

 comes intensification of competitive singing, scolding, 

 chasing, and fighting. On a 6-hectare (15 acre) area 

 there were no instances of destruction of nests, eggs, 

 or young in the six years through which the popula- 

 tion of male house wrens did not exceed 1 1 , but dur- 

 ing the 13 years when such acts of destruction did 

 occur, the male population had ranged from 11 to 16 

 (Kendeigh 1941b). A pair of birds requires a spe- 

 cific minimum territory for successful nesting. When 

 an area becomes saturated with territories compressed 

 to this limited size, disturbances occur in nesting and 

 other individuals attempting to invade the area are 

 forced to go elsewhere. Thus the population density 

 becomes limited by the space available. On the other 

 hand, with species possessing only undefended home 

 ranges, competition for space is of less critical im- 

 portance in regulating population size. 



FIG. 16-5 Competition fo 

 tudlnal section through i 

 1950). 



space by barnacles; a median longi- 

 hummock (after Barnes and Powell 



Related to the competition for space is the com- 

 petition for the most favorable portions of the niche, 

 those offering maximum food and protection. In Hol- 

 land, three species of tits {Parus major, P. coeruleus, 

 P. ater) prefer mixed woods to pine woods. In years 

 when they are scarce, the species are mostly confined 

 to the mixed woods : when populations increase in 

 size, they do so first in the mixed woods until the 

 birds become intolerant of further crowding. Then 

 they spill over into pine woods to nest but never be- 

 come as abundant (Kluijver and Tinbergen 1953). 



Food supply is an important determinant of the 

 carrying capacity of any area. When large numbers of 

 animals are present there is, of course, less food avail- 

 able to any one than when there are few individuals 

 present. Competition for food therefore becomes in- 

 tense in large populations. Population size is as lim- 

 ited as food is available sufficient to supply the mini- 

 mum needs of the individuals already present. 



For fish, the number of individuals may continue 

 to increase in the presence of a limited food supply 

 but each individual becomes stunted in size. There is 

 a tendency for the biomass of a species to be regu- 

 lated by the food supply, with size or weight of indi- 

 viduals varying inversely with number. Thinning the 

 population artificially usually results in increased 

 growth of remaining individuals (Parker 1958). 

 Body size in Daphnia (Frank et al. 1957) and in 

 several species of mammals also appears to be to a 

 certain extent density-dependent in that smaller-sized 

 individuals are characteristic of larger populations 

 (Scheffer 1955). 



The role of competition in regulating population 

 size is thus directly effective by causing mortalities 

 through fighting, nest destruction, and loss of food 

 supplies. It also results, as will be seen in the discus- 

 sion that follows, in lowered rates of reproduction, 

 increased predation, dispersal into other regions, and 

 decreased health and vigor. 



Reproductivity 



A study of the reproductive rate of a species 

 in relation to population density requires separate 

 studies of the number of eggs or young produced, 

 called fecundity or natality, and the number reaching 

 sexual maturity or survival. 



In cultures of Paramecium, a decrease in the vol- 

 ume of culture fluid for the same initial number of 

 individuals, or an increase in the initial number of 

 individuals for a given volume of fluid, decreases the 

 rate at which cell fission occurs (Myers 1927). 

 Birth rate and growth rate in cultures of Daphnia 

 magna vary inversely with the density of population 

 even when a surplus of food is present (Pratt 1943, 

 Frank et al. 1957). 



222 Ecological processes and dynamics 



