RESUMEN
Birds, bats and ants are recognised as significant arthropod predators. However, empirical studies reveal inconsistent trends in their relative roles in top-down control across strata. Here, we describe the differences between forest strata in the separate effects of birds, bats and ants on arthropod densities and their cascading effects on plant damage. We implemented a factorial design to exclude vertebrates and ants in both the canopy and understorey. Additionally, we separately excluded birds and bats from the understorey using diurnal and nocturnal exclosures. At the end of the experiments, we collected all arthropods and assessed herbivory damage. Arthropods responded similarly to predator exclusion across forest strata, with a density increase of 81% on trees without vertebrates and 53% without both vertebrates and ants. Additionally, bird exclusion alone led to an 89% increase in arthropod density, while bat exclusion resulted in a 63% increase. Herbivory increased by 42% when vertebrates were excluded and by 35% when both vertebrates and ants were excluded. Bird exclusion alone increased herbivory damage by 28%, while the exclusion of bats showed a detectable but non-significant increase (by 22%). In contrast, ant exclusion had no significant effect on arthropod density or herbivory damage across strata. Our results reveal that the effects of birds and bats on arthropod density and herbivory damage are similar between the forest canopy and understorey in this temperate forest. In addition, ants were not found to be significant predators in our system. Furthermore, birds, bats and ants appeared to exhibit antagonistic relationships in influencing arthropod density. These findings highlight, unprecedentedly, the equal importance of birds and bats in maintaining ecological balance across different strata of a temperate forest.
RESUMEN
Polyphagous insect herbivores feed on multiple host-plant species and face a highly variable chemical landscape. Comparative studies of polyphagous herbivore metabolism across a range of plants is an ideal approach for exploring how intra- and interspecific chemical variation shapes species interactions. We used polyphagous caterpillars of Lymantria mathura (Erebidae, Lepidoptera) to explore mechanisms that may contribute to its ability to feed on various hosts. We focused on intraspecific variation in polyphenol metabolism, the fates of individual polyphenols, and the role of previous feeding experience on polyphenol metabolism and leaf consumption. We collected the caterpillars from Acer amoenum (Sapindaceae), Carpinus cordata (Betulaceae), and Quercus crispula (Fagaceae). We first fed the larvae with the leaves of their original host and characterized the polyphenol profiles in leaves and frass. We then transferred a subset of larvae to a different host species and quantified how host shifting affected their leaf consumption and polyphenol metabolism. There was high intraspecific variation in frass composition, even among caterpillars fed with one host. While polyphenols had various fates when ingested by the caterpillars, most of them were passively excreted. When we transferred the caterpillars to a new host, their previous experience influenced how they metabolized polyphenols. The one-host larvae metabolized a larger quantity of ingested polyphenols than two-host caterpillars. Some of these metabolites could have been sequestered, others were probably activated in the gut. One-host caterpillars retained more of the ingested leaf biomass than transferred caterpillars. The pronounced intraspecific variation in polyphenol metabolism, an ability to excrete ingested metabolites and potential dietary habituation are factors that may contribute to the ability of L. mathura to feed across multiple hosts. Further comparative studies can help identify if these mechanisms are related to differential host-choice and response to host-plant traits in specialist and generalist insect herbivores.