Abstract
The intensive foraging of insectivorous birds and bats is well known to reduce the density of arboreal herbivorous arthropods but quantification of collateral leaf damage remains limited for temperate forest canopies.We conducted exclusion experiments with nets in the crowns of young and mature oaks, Quercus robur, in south and central Germany to investigate the extent to which aerial vertebrates reduce herbivory through predation. We repeatedly estimated leaf damage throughout the vegetation period.Exclusion of birds and bats led to a distinct increase in arthropod herbivory, emphasizing the prominent role of vertebrate predators in controlling arthropods. Leaf damage (e.g., number of holes) differed strongly between sites and was 59% higher in south Germany, where species richness of vertebrate predators and relative oak density were lower compared with our other study site in central Germany. The effects of bird and bat exclusion on herbivory were 19% greater on young than on mature trees in south Germany.Our results support previous studies that have demonstrated clear effects of insectivorous vertebrates on leaf damage through the control of herbivorous arthropods. Moreover, our comparative approach on quantification of leaf damage highlights the importance of local attributes such as tree age, forest composition and species richness of vertebrate predators for control of arthropod herbivory.
Highlights
The question of ‘‘Why is the world green?’’ raised by Hairston et al [1] has encouraged many ecologists to investigate whether herbivore population dynamics are limited by the availability of food plants and plant defence mechanisms or rather by top-down control through predators
Our study revealed a strong effect of vertebrates on intensity of arthropod herbivory on Quercus robur in the two environmental settings tested
The exclusion of vertebrate predators led, in all cases, to a greater mean damage of leaf area (%) and larger mean number of holes per leaf, suggesting that birds and bats significantly reduced the number of leaf-chewing arthropods and herbivory (Table 1 and 2)
Summary
The question of ‘‘Why is the world green?’’ raised by Hairston et al [1] has encouraged many ecologists to investigate whether herbivore population dynamics are limited by the availability of food plants and plant defence mechanisms or rather by top-down control through predators. Herbivorous arthropods play a decisive role in ecosystem functioning because, as midtrophic level species, they are influenced by bottom-up and topdown forces. Herbivorous arthropods impact the fitness of many plant species and associated nutritional cycles [2,3]. Numerous studies have demonstrated that leaf quality (e.g. content of secondary metabolites) is a major determinant affecting the distribution and abundance of herbivorous arthropods [3,4,5,6]. Studies focusing on top-down forces have revealed significant impacts of predators on lower trophic levels, such as herbivorous arthropods [7,8,9,10,11]. These, in turn, can drastically reduce leaf area through their feeding activities and can affect the biomass and fitness of trees [6,12,13,14,15,16,17]
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