Dynamics of Popualations. II. Factors Controlling Population Growth and Population Size in Drosophilia Pseudoobscura and in Drosophila Melanogaster

Four populations of Drosophila pseudoobscura and four of D. melanogaster were studied for the influence of certain environmental factors on the productivity and size of the populations. For each species a 2 x 2 factorial design was used, the variables being temperature (25°C and 19°C) and amount of food (2 and 3 food unites per week). A serial transfer technique was used to maintain the populations. In both species the main factor determining the productivity of the populations is the amount of food; the main factor limiting the population size is their living space. At 19°C the productivity and the size of the populations of D. pseudoobscura is considerably larger than at 25°C, which is a suboptimal temperature for this species. The D. melanogaster populations produce at 25°C more but smaller flies than at 19°C with no significant effect on the biomass produced. At 19°C the average size of the populations of D. melanogaster is slightly larger than at 25°C due to an increased average longevity. In both species, the number of food units given to the populations per unit time does not have a significant effect on the productivity per food unit. The populations receiving three food units per week produce, therefore, approximately 50% more flies than those given two food units per week. The average size of the former populations is, however, only moderately larger, space being considered the limiting factor. The chromosomal polymorphism initially present in the populations of D. pseudoobscura was maintained, indicating that heterozygotes had a higher fitness than the corresponding homozygotes. The average productivity and the average size of the populations of D. melanogaster are considerably greater than those of D. pseudoobscura. The complexity of the factors affecting the carrying capacity of a population is pointed out in the discussion.