Vertebrates but not ants protect rainforest trees from herbivorous insects along an elevational gradient in Papua New Guinea

Aim The theory on trophic interactions between plants, insect herbivores, and their predators predicts that predators increase plant biomass by feeding on herbivores. However, it remains unclear whether different types of predators regulate herbivores to the same degree, and how the trophic interactions affect lower trophic levels along elevational gradients where predator communities differ significantly. Therefore, we aimed to investigate the impact of vertebrate predators and ants (individually and in combination) on arthropod communities and leaf herbivory along a complete tropical forest gradient. Location Papua New Guinea Taxon Multi-taxon Methods We excluded predators from 560 saplings in two six-month long predator exclusion experiments spanning wet and dry seasons. Saplings were spread across 8 study sites which were evenly spaced at 500 m elevational increments from 200 to 3700m a.s.l.. Results On average the density of arthropods increased significantly by 37% and 33% respectively when vertebrate predators, and both ants and vertebrates predators, were removed. Both season and elevation mediated this effect significantly. At lower trophic levels, both the exclusion of both vertebrates alone, and exclusion of vertebrates plus ants, led to a significant increase in leaf damage by 50% and 36% respectively. In contrast, the exclusion of ants alone had no significant effect on arthropod density or leaf damage, which increased by 12% and 9% respectively. Main conclusions Our results indicate that the relative contribution of birds and bats changes at different elevational sites, while the overall effect of vertebrate predators remains consistent along the whole elevational gradient. This contrasts with ant driven trophic cascades which brought about increased herbivory only at the most productive sites of the elevational gradient, where ant abundance is highest. We conclude that disappearance of insectivorous vertebrate predators can lead to substantial negative consequences for plants.