UMR1202, BIOGECO, INRA, 33610, Cestas, France.
Oecologia. 2012 Feb;168(2):415-24. doi: 10.1007/s00442-011-2089-7. Epub 2011 Aug 3.
According to the associational resistance hypothesis, neighbouring plants are expected to influence both the insect herbivore communities and their natural enemies. However, this has rarely been tested for the effects of canopy trees on herbivory of seedlings. One possible mechanism responsible for associational resistance is the indirect impact of natural enemies on insect herbivory, such as insectivorous birds. But it remains unclear to what extent such trophic cascades are influenced by the composition of plant associations (i.e. identity of 'associated' plants). Here, we compared the effect of bird exclusion on insect leaf damage for seedlings of three broadleaved tree species in three different forest habitats. Exclusion of insectivorous birds affected insect herbivory in a species-specific manner: leaf damage increased on Betula pendula seedlings whereas bird exclusion had no effect for two oaks (Quercus robur and Q. ilex). Forest habitat influenced both the extent of insect herbivory and the effect of bird exclusion. Broadleaved seedlings had lower overall leaf damage within pine plantations than within broadleaved stands, consistent with the resource concentration hypothesis. The indirect effect of bird exclusion on leaf damage was only significant in pine plantations, but not in exotic and native broadleaved woodlands. Our results support the enemies hypothesis, which predicts that the effects of insectivorous birds on insect herbivory on seedlings are greater beneath non-congeneric canopy trees. Although bird species richness and abundance were greater in broadleaved woodlands, birds were unable to regulate insect herbivory on seedlings in forests of more closely related tree species.
根据关联抗性假说，相邻植物预计会影响昆虫食草动物群落及其天敌。然而，对于树冠树木对幼苗食草的影响，这很少得到验证。一种可能的关联抗性机制是天敌对昆虫食草的间接影响，例如食虫鸟类。但这种营养级联在多大程度上受到植物群落组成（即“相关”植物的身份）的影响尚不清楚。在这里，我们比较了鸟类排除对三种不同森林生境中三种阔叶树种幼苗昆虫叶片损伤的影响。食虫鸟类的排除以物种特异性的方式影响昆虫食草性：桦树幼苗的叶片损伤增加，而栎树（欧洲山毛榉和麻栎）的鸟类排除则没有效果。森林生境既影响昆虫食草性的程度，也影响鸟类排除的效果。与阔叶林相比，松树林中阔叶幼苗的总叶片损伤较低，这与资源集中假说一致。鸟类排除对叶片损伤的间接影响仅在松林显著，而在外来和本地阔叶林地则不显著。我们的结果支持了天敌假说，该假说预测，食虫鸟类对幼苗昆虫食草的影响在非同源树冠树木下更大。尽管阔叶林地的鸟类物种丰富度和数量较大，但鸟类无法在亲缘关系更密切的树种的森林中调节幼苗的昆虫食草性。
