50 100 150 200 



NUMBER OF LARVAE 



FIG. 16-6 Effect of density of larval populations of Drosophilo 

 melonogasier on number pupating, percentage pupating, and 

 wing length of resulting female adults (after Chiang and Hod- 

 son 1950). 



size until the per capita consumption of food was cut 

 by one-fourth as a result of the increased number of 

 animals, and then reproduction stopped altogether. 

 It appears that in times of stress the limited energy 

 resources of animals are diverted from reproduction 

 to individual survival (Strecker and Emlen 1953). 

 In another experiment, food was supplied in excess 

 but space and cover were restricted. Population in- 

 crease was finally limited by litter mortality from 

 cannibalism and desertion. In some of the popula- 

 tions there was a decline in fecundity. This appeared 

 to be the result of a social hierarchy becoming estab- 

 lished so that subordinated individuals failed to get 

 adequate amounts of food, even though a surplus of 

 food was available, and were prevented from complet- 

 ing their mating behavior (Southwick 1955). Re- 

 productivity has also been found to decline in the 

 short-tailed meadow vole in large populations, be- 

 cause of chasing and fighting, when there was a sur- 

 plus of food and water present (Clarke 1955). 



Fertility of eggs appears to be high as they are 

 laid under natural conditions. Egg viability, the 

 capacity to hatch, has been shown in Drosophila cul- 

 tures to be modified by the same factors that affect 

 fecundity, particularly the amount of food available to 

 the adult (Robertson and Sang 1945). 



The survival of young is greatly affected by the 

 number of animals present. When larvae of Dro- 

 sophila are reared at different densities in containers 

 of equal size and with equal amounts of food, the 

 percentage that succeeds in pupating drops in an al- 

 most straight line with increase in density of the 



larvae. However, because of the larger initial num- 

 bers of larvae present, the actual number pupating is 

 greatest at intermediate densities. The size of the 

 adults emerging from the pupae decreases abruptly 

 as density increases. It has been shown experi- 

 mentally that effects produced were due to the ex- 

 haustion of food at progressively earlier growth 

 stages as the population densities increased. The 

 continued growth of some individuals even after the 

 original food was gone was apparently because they 

 devoured dead larvae (Chiang and Hodson 1950). 



The average growth rate of tadpoles in a limited 

 volume of water is inversely proportional to the num- 

 ber of individuals. However, some individuals grow 

 at normal rates at all densities ; the decreasing aver- 

 age growth rate at higher densities is due to the larger 

 number of individuals that become stunted (Rose 

 1960). This effect of overcrowding is produced 

 through water conditioning. 



For the grain weevil there is an optimum inter- 

 mediate density for rate of population increase even 

 though the progeny raised per female decreases pro- 

 gressively as the population increases (Table 16-3). 

 There is a similar relationship among fish (Herring- 

 ton 1947). 



The amount of disturbance of females with suck- 

 ling young in crowded experimental populations of 

 the house mouse decreased the number of litters suc- 

 cessfully weaned. Females abandoned or devoured 

 their young, and when the disturbance factor became 

 sufficiently severe, all successful weaning of litters 

 ceased (Brown 1953). 



The snowshoe rabbit cycle in Minnesota reached 

 a peak of 200 per 100 hectares (500 per square mile) 

 in 1933 and dropped to a low of only 13 per 100 

 hectares (32 per square mile) in February 1938. 

 Mortality percentage of the adults, duration of the 

 reproductive season, proportion of females pregnant, 

 and the average number of embryos per pregnant 

 female remained relatively constant throughout this 

 period. The significant variable that appeared to de- 

 termine the cycle was the mortality of the young after 

 birth. At the peak of the cycle, yearling rabbits con- 

 stituted 60 per cent of the entire spring population, 

 but at the bottom of the cycle in 1937 they constituted 

 only 44 per cent. As the cycle began to swing up- 

 wards again in 1938 and 1939, the proportion of 

 young reached 80 per cent (Green and Evans 1940). 



It is desirable, when considering reproductivity 

 as a density-stabilizing factor, to distinguish clearly 

 between fecundity and success in raising young to 

 maturity. Changes in fecundity are density-depend- 

 ent, at least in some species, but are not usually 

 sufficiently great to be of major importance in stabil- 

 izing the numbers of a species at any definite level 

 (Lack 1954a). However, mortality of the immature 



224 Ecological processes and dynamics 



