The presence of trait variation in prey or predator populations may affect the stability and the shape, i.e. amplitude and phase, of predator-prey dynamics. However, while previous studies have shown how trait variation can alter the overall amplitude of the predator-prey oscillations, this altered amplitude remained constant over time. This strongly contrasts with empirically observed predator-prey dynamics and recent theoretical work, showing that several mechanisms may lead to so called intermittent predator-prey cycles where the amplitude of the predator-prey dynamics varies temporally. For instance, trait differences that determine the functional responses of two predators may provoke temporal fluctuations in the amplitudes of the predator-prey cycles due to recurrent changes in the relative abundance of the two predator types: a predator with a relatively linear functional response promotes small-amplitude oscillations whereas a predator with a more strongly non-linear functional response stimulates larger amplitudes. We analyzed various models that incorporate trait variation within prey, predators, or both, to identify a general mechanism that gives rise to intermittent cycles in the population dynamics.
Results/Conclusions
We identified three general conditions that are necessary for intermittent cycles to occur. First, the predator-prey system comprises at least two subsystems that exhibit substantial differences in the amplitude of their population dynamics and thus tendency to promote stable or unstable population dynamics. Second, these subsystems recurrently alternate in their dominance, leading to a second “trait” cycle superimposed on the population dynamics. Finally, the time scale of the trait dynamics must be significantly slower than that of the population dynamics. For instance, co-evolution may promote intermittent cycles in predator-prey dynamics by inducing a lag in the predators’ trait adjustment in response to altered trait values of the prey. The resulting temporal variation in the interaction strength between predator and prey is associated with temporal changes in the amplitudes of the population dynamics since e.g., a dominance of defended prey dampens the population dynamics whereas a high abundance of undefended prey enhances it. Our results highlight that intermittent cycles may frequently occur in simple predator-prey systems allowing for trait variation.