Green world maintained by adaptation

An ecological theory (HSS hypothesis) predicts that carnivores maintain the terrestrial ecosystem with abundant plants (green world) by regulating herbivore abundance. However, a weak density dependence of herbivores will make the equilibrium unstable and results in population oscillations with a large amplitude. Here, we study a possibility that the dynamics can be stabilized if defence trait by herbivores and offence trait by carnivores change in an adaptive manner. When the cost constraints on adaptation are strong in both the herbivores and the carnivores, the equilibrium is more likely to be stable if the herbivore adapts more quickly than the carnivore. When the constraints on the adaptation are asymmetric between species, the equilibrium is likely to be unstable. We conclude that the green world may be maintained by fast and costly adaptation by the herbivore through mechanisms such as phenotypic plasticity and behavioural change. Plant defence which is poisonous and prickly has been proposed as one of explanations, however, world can be green through adaptation in higher trophic levels even without plant's defence.