Impacts of Quercus alba foliage quality on herbivorous insect attack and bird predation

Background/Question/Methods As plant productivity or foliage food quality increase, herbivore populations and predator impacts on herbivores are expected to increase as well. We tested this prediction in a well-studied oak-herbivore-bird system. We used light manipulation to produce saplings with different foliage characteristics followed by bird predator exclusion in a common light environment to control for the effects of light on herbivore and predator distribution. We measured insect herbivore abundance and herbivore damage on trees at the end of the year. Results/Conclusions Light treatments produced plants with strikingly different foliage quality, and herbivores responded to this quality but in a manner opposite to our predictions: herbivores were more abundant on high-tannin, low-nitrogen sun trees than presumably higher-quality shade trees, which were low in tannins and high in nitrogen. Herbivore damage reflected this pattern with greater leaf damage to sun trees, but the effect was not due to compensatory feeding on low-quality leaves. Bird exclusion did not affect herbivore abundances. Although leaf damage was greater on exclosure trees, birds did not interact with leaf quality. Our results may support the plant vigor hypothesis, in which herbivores prefer more vigorously growing plants such as sun trees in this experiment. Although bird exclusion did not affect herbivore abundance, it did increase per capita feeding, suggesting that the presence of birds on control trees may have altered insect feeding behavior.