It has been increasingly recognized that rapid adaptive changes of traits can affect population dynamics. While the rapid adaptive changes result from two different mechanisms: rapid contemporary evolution and phenotypic plasticity, their effects on population dynamics are seldom compared explicitly. Here we theoretically examine the impacts of rapid evolution and phenotypic plasticity in predator-prey systems. Specifically we focus on two stabilities in this study: ecological stability (oscillation or equilibrium) and evolutionary stability (the system is invaded by other genotypes or not). Our model is based on rotifer (predator) – green algae (prey) experimental system in a chemostat. Rapid evolution is often defined as a temporal genotypic frequency change from standing genetic variation. To describe rapid evolution we used bi-clonal and multi-clonal models based on Jones & Ellner (2007) and Jones et al. (2009). Inducible defense is a plastic response to increased predation, in which prey changes its phenotype according to predator density. Our model is based on Vos et al. (2004) that formulated the inducible defense.

Results/Conclusions

Our analysis of the models revealed that phenotypic plasticity tends to stabilize population dynamics more strongly than does rapid evolution of bi-clonal model. However, this conclusion turns over when multi-clonal model is used to describe rapid evolution: evolution always leads to stability while plasticity produces oscillation. Thus, depending on the genetic diversity, evolution can stabilize or destabilize the population dynamics compared to phenotypic plasticity. In addition, when the phenotypically plastic genotype of prey, which exhibits a defensive phenotype when exposed to predators, and the two specialist genotypes of prey (undefended and defended ones) compete with each other, the plastic genotype cannot coexist with the specialists unless the system shows limit cycles. Also, the plastic genotype cannot beat the specialists without a forced environmental fluctuation. Thus, plastic genotype is not evolutionary stable especially in stable environments. In conclusion, it is hard to predict the dynamics of rapidly adapting populations without knowing a mechanism by which phenotypic variation is generated. Our study also revealed that an environmental fluctuation is crucial for the evolution and maintenance of plasticity.