Predator pressure, herbivore abundance and plant damage along a subtropical altitudinal gradient

Abstract

Climate change is predicted to not only cause shifts in the latitudinal and altitudinal distribution of species, but also changes in trophic interactions. Studying changes in herbivory and herbivore and predator pressure along altitudinal gradients may assist our understanding of the complex trophic interactions and their responses to future climate change. As part of the IBISCA-Queensland Project, we tested the hypothesis that ant predation pressure influences the abundance of herbivores which in turn influences herbivory in the understorey of subtropical rainforest and that these relationships are modified by altitude. We used the occupancy of ants at tuna baits as a measure of predation pressure, herbivorous beetles as representatives of herbivores, and the extent of damage to mature leaves as a measure of herbivory. Ant predation pressure was the greatest at 300 m above sea level and declined with increasing altitude but only at ground level. In contrast, ant predation pressure on understorey foliage was not related to altitude and was always much lower than that on the ground. Neither altitude nor ant predation pressure at ground or understorey levels significantly influenced the abundance of herbivorous beetles. However, the species richness of herbivorous beetles significantly decreased with increasing altitude, and ground-level ant predation pressure negatively related to beetle species richness, after controlling for the effect of altitude. Levels of herbivory were not related to beetle abundance, whereas it was significantly negatively related to beetle species richness. This was however, opposite to our prediction that increased beetle species richness would increase leaf herbivory. Consequently, we found little evidence for the cascading effects of ants on herbivory in our study system. We suggest that future studies examine other groups of foliagefeeding insects such as lepidopteran larvae and Orthoptera, and the importance of other natural enemies, including parasitoids as well as non-specific predators.