A Laboratory Study of the Population Dynamics and Productivity of Dugesia Polychroa (Turbellaria: Tricladida)

Abstract

Populations of the flatworm Dugesia polychroa were studied in the laboratory over 42 mo under constant regimes of temperature, food, and space. Three populations completed four reproductive cycles, a fourth achieved an equilibrium size in its third cycle, while a fifth became extinct after its second reproductive period. The populations fluctuated in numbers and biomass during each cycle. As the populations matured, the mean population size (in terms of numbers and biomass) increased, as did the mean number of adults present. There were also increases in the length of the cycles, the reproductive periods, and total number of cocoons laid; there were decreases in the rate of cocoon production, adult fecundity, viability of cocoons, amplitude of the fluctuations in, and the average of the biomass turnover rate. In general, mortality balanced recruitment with the majority of mortality occurring at the time of recruitment. Only the absolute amount of food presented was found to be of importance in influencing the above parameters. The relationship between food availability and average biomass standing stock gave a value of 0.76, which was used to obtain good comparative abundance estimates for a field situation. Modifications of the above parameters were what might be expected from a density dependent organism and the results support field observations on triclads in general, and on D. polychroa in particular, showing that their populations are regulated by intraspecific competition for food.