Vertebrates, but not ants, protect rainforest from herbivorous insects across elevations in Papua New Guinea

Abstract

AbstractAimThe effects of insectivorous predators on herbivorous prey should have a cascading effect releasing herbivory pressure and favouring plant's biomass. However, it remains unclear whether different types of predators regulate herbivores to the same degree across seasons, and how their interactions affect lower trophic levels across elevations where predator communities differ significantly. Therefore, we investigated the impact of excluding flying vertebrate predators and ants (individually and in combination) on arthropods and herbivory across tropical seasons along a rainforest gradient spanning 3500 m a.s.l.LocationPapua New Guinea.TaxonMulti‐taxon.MethodsWe excluded predators from 560 saplings in two 6‐month‐long predator exclusion experiments, controlling for seasonality. Saplings were spread across eight sites, evenly spaced at 500 m elevational increments from 200 to 3700 m a.s.l. We measured change in the abundance of arthropods and herbivory damage and analysed them by linear and generalized linear mixed models.ResultsExclusion of flying vertebrate predators, but not ants, led to a significant increase in both arthropod density and herbivory damage. The density of arthropods increased significantly by 37% when flying vertebrates were excluded and by 33% when both flying vertebrates and ants were excluded. Both season and elevation significantly influenced this effect. Leaf damage increased significantly by 50% in exclosures of flying vertebrates alone and by 36% in combined exclosures of flying vertebrates and ants. In contrast, the exclusion of ants alone had no significant effect on arthropod density or leaf damage, which increased by 12% and 9%, respectively, although the effect decreased with increasing elevation.Main conclusionsThe overall effect of flying vertebrate predators on arthropod density and leaf damage remains consistently strong along the whole elevational gradient. In contrast, ant‐driven trophic cascades were detected only in lowland. Disappearance of insectivorous flying vertebrates could lead to substantial negative consequences for plants.