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1. This study involves an examination of two- and three-trophic-level food chains at two temperatures (18 and 25 °C) in order to determine how the addition of a carnivore to a predator–prey system can alter the dynamics of populations and how this effect may be temperature mediated. The system consisted of phytoplankton, Daphnia pulex and the flatworm Mesostoma ehrenbergii.

2. Although the plant–herbivore system is inherently unstable at 25 °C, the addition of the carnivore led to a further destabilization of the Daphnia–algal dynamics at the higher temperature. No destabilization effect of the carnivore was noted at 18 °C. At the lower temperature, all populations persisted and the carnivore induced changes only in the age structure of the Daphnia populations rather than in overall biomass.

3. The differential effects of the carnivore at two temperatures can be attributed to shifts in the life history, physiological rates and the reproductive strategy employed by Mesostoma.

4. Previous theoretical work has predicted that the addition of a third trophic level to an unstable predator–prey system should stabilize dynamics. Our results indicate that the effect of a carnivore on plant–herbivore dynamics can be significantly affected by ambient temperature.