Top-down and bottom-up regulations are two major mechanisms controlling structure and stability of ecosystems. Top-down regulation argues that top predators control biodiversity and biomass of lower trophic levels through trophic cascading effect, while bottom-up regulation emphasizes that biodiversity and biomass are determined by the limitation of resources, abiotic stresses or inducible defense such as plant secondary metabolic products. Both regulations have been shown to control biodiversity and stabilize community structure in terrestrial ecosystems. However, their relative contributions have been debated, especially in terrestrial ecosystems where the top-down effect is often thought to be weak. Here we studied a hydro-inundated land-bridge system in subtropical China and tested the relative importance of these two controls in determining the rodent-mediated regeneration of a locally dominant tree species.
Results/Conclusions
Our results showed that both controls operated in terrestrial habitats and that their relative importance switched as habitat size changed. Habitat loss initially removed predators of rodents that released rodent populations and triggered massive seed predation (top-down control), leading to reduced seedling establishment. A further reduction in habitat size led to decrease in rodent population that was supposed to increase seedling survival of the tree species, but the decline in habitat size deteriorated the abiotic environments (bottom-up control) that severely prevented seedling recruitment. As the ongoing global land use change is creating increasing number of small-sized forest fragments, our findings underpin the restoration strategies of seriously fragmented forests.