An Analysis of the Predatory Interaction Between Paramecium and Didinium

SUMMARY (1) The interaction between the predator Didinium nasutum and its prey Paramecium aurelia was studied in the laboratory. The Paramecium was fed the bacterium Aerobacter aerogenes. (2) The addition of Methyl Cellulose to the culture medium prolonged the coexistence of the species by reducing the rate of ciliate movement and, indirectly, by decreasing the net energy profit of the cells. (3) In the Methyl Cellulose regime, the outcome of the interaction depended upon the concentration of the bacterial nutrient Cerophyl in the medium. The Cerophyl concentration regulates the amount of bacterial food available to the Paramecium and, therefore, can be related to the degree of Paramecium starvation. (4) The efficiency of Didinium as a predator is directly related to the nutritional adequacy of the Paramecium. As the Cerophyl concentration in the medium is reduced and the prey become more starved, the predatory ability of Didinium declines. (5) The type of dynamic behaviour exhibited by the Didinium-Paramecium system at a given Cerophyl concentration is related to the predatory efficiency of Didinium. (6) At low Cerophyl concentrations, the predator and prey coexisted at a numerically stable equilibrium. At intermediate Cerophyl concentrations, although the species coexisted over time, the population densities oscillated with an amplitude related to the Cerophyl level. At high Cerophyl concentrations, the species were unable to coexist. (7) The data was interpreted in terms of the graphical predation theory. The predator and prey zero isoclines were constructed and their points of intersection noted. The comparison of the observed dynamics with those predicted by the graphical model indicated that the theory described the outcome of the Didinium-Paramecium interaction if it was modified to account for time-lags in the response of the predator population to changes in the density of the prey.