Cyclical succession in grazed ecosystems: The importance of interactions between different-sized herbivores and different-sized predators

Body size of vertebrate herbivores is strongly linked to other life history traits, most notably (1) tolerance of low quality forage and (2) vulnerability to predation, which both impact the composition and dynamics of natural communities. However, no study has thus far explored how the combination of these two body-size related traits affects the long-term composition and dynamics of the herbivore and plant communities. We made a simple model of ordinary differential equations and simulated a grassland system with three herbivore species (small, medium, large) and two predator species (small, large) to investigate how the combination of low-quality tolerance and predation-vulnerability structure the herbivore and plant community. We found that facilitation and competition between different-sized herbivores and predation by especially small predators stimulate coexistence among herbivore species. Furthermore, the interaction between different-sized herbivores and predators generated cyclical succession in the plant community, i.e. alternating periods of short vegetation dominated by high-quality plants, with periods of tall vegetation dominated by low-quality plants. Our results suggest that cyclical succession in plant communities is more likely to occur when a predator predominantly preys on small herbivore species. Large predators also play an important role, as their addition relaxed the set of conditions under which cyclical succession occurred. Consequently, our model predictions suggest that a diverse predator community plays an important role in the long-term dynamics and maintenance of diversity in both the herbivore and plant community.