Arthropod species richness, herbivory, and predator/prey ratios along a tree diversity gradient in a central European deciduous forest

The relationship between biodiversity and ecological functions such as herbivory is a topical question in ecology. Species-rich semi-natural forests have become rare in temperate regions with possible impacts on arthropod diversity, herbivory rates and pest outbreaks. In experimental grassland, plant species richness has been shown to correlate with insect diversity and herbivory, but little is known for mature forest stands differing in tree species diversity. The Hainich National Park is the biggest remaining semi-natural deciduous forest in Germany, with tree species diversity ranging from monospecific beech stands to mixed forests with up to nine broad-leaved tree species. We examined herbivory of beech (Fagus sylvatica), Norway maple (Acer platanoides) and sycamore maple (Acer pseudoplatanus) saplings in forest stands with 1, 3, and 5 canopy tree species, and expected insect herbivory to decrease with increased tree species richness. We further hypothesized that arthropod species richness and predator/prey ratios would be higher in more diverse forest stands. For sampling of arthropods, flight interception traps and trap nests for bees and wasps were installed in the canopy and understory. Herbivory was measured for 1040 saplings (~14,000 leaves) by estimation of leaf area loss and by recording galls and mines. Taxa responded in different ways, but we found that arthropod species richness and predator/prey ratios increased while herbivory decreased with tree diversity. These results support our hypothesis that maintaining high tree diversity in semi-natural forests is important for biodiversity and ecosystem functioning.