Predator–prey instability: individual‐level mechanisms for population‐level results

Abstract

1. In previous work we established that increasing temperature led to a destabilization of the population dynamics of the invertebrate carnivore Mesostoma ehrenbergii and its prey Daphnia pulex, which ultimately resulted in the local extinction of Daphnia at higher temperatures. Two mechanisms are proposed to explain the population‐level phenomena: (1) quantitative changes in carnivore vital rates with increasing temperature led to stronger functional and numerical response and (2) qualitative changes in the dynamic allocation of energy to reproduction by the predator with increasing temperature introduces inverse density dependence in the predator’s response. 2. The growth of individual M. ehrenbergii was monitored under various food conditions to determine the effect of two temperatures (18 and 24 °C) and five food levels on rates of growth, prey consumption and reproduction and on reproductive allocation patterns. 3. The first mechanism was supported by both higher consumption rates (stronger functional response) and faster growth rates with earlier age at maturity and shorter generation time (stronger numerical response). 4. Evidence for mechanism two was also provided by an alteration of the reproductive allocation pattern with temperature. Viviparous (subitaneous) eggs were more likely to be produced by this carnivore at low food levels at 24 °C, while at 18 °C, high food levels were required before individuals made this switch. This shift actually introduces inverse density dependence in the predator’s numerical response which is highly destabilizing. 5. Based on the results of this study, the differential effect of M. ehrenbergii on the dynamics and structure of its D. pulex prey populations can be attributed to changes in both physiological rates and reproductive allocation patterns with temperature.