Interactions among the Predatory Rotifer Asplanchna and Two Prey, Paramecium and Euglena

Interactions among the predatory rotifer Asplanchna and alternative prey Paramecium and Euglena are examined in a laboratory environment to determine whether the addition of an alternative, less desirable prey stabilizes a simple predator—prey interaction. Rates of increase, carrying capacities, and dynamics of predator resting egg production are determined by both short—term and long—term experiments with partial medium renewal. Isoclines and isoplanes are constructed; relative stability of the Asplanchna—Paramecium and the Asplanchna—Paramecium—Euglena interactions is determined. According to a mathematical analysis of deviations from equilibrium, stability properties change only slightly with addition of Euglena. Although Asplanchna rate of increase is reduced by addition of Euglena, feeding rate on Paramecium remains high, and Asplanchna cannot survive on Euglena alone. The biology of the organisms used in this study affect stability properties of the system more than any other factor.