Predation by natural enemies is important for regulating herbivore abundance and herbivory. Theory predicts that complex habitats support more natural enemies, which exert top-down control over arthropods and therefore can reduce herbivory. However, it is unclear if theory developed in other more natural systems similarly apply to predation by vertebrate and invertebrate natural enemies across urban habitats of varying complexity. We used plasticine caterpillar models to assess risk of predation by birds and insects, collected leaf-feeding arthropods, and measured herbivory in willow oak trees (Quercus phellos) in two seasons to determine how predation influenced herbivory across urban forest fragments, street trees planted near forest fragments, and downtown street trees. Predation attempts by birds and abundance of chewing folivores were greater on trees growing in urban forest fragments than downtown street trees. Bird predation attempts and herbivory levels were inconsistent for near-forest trees. Predation attempts by arthropods did not statistically vary among the three urban tree habitats. Contrary to expectations based on theory, chewing folivore abundance and herbivory were generally highest on trees growing in urban forests, the most complex habitat we studied, and the habitat where risk of bird predation was greatest. We suggest that urban forest fragments provide better habitat than other urban landscapes for both urban birds and chewing folivores by having greater habitat complexity. Therefore, basal resources, such as availability of suitable habitat, mediate top-down effects on herbivores in cities.

中文翻译：

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与森林树木相比，城市森林碎片中鸟类被捕食和落叶昆虫丰富的风险更大

天敌的捕食对调节草食动物的丰度和食草性很重要。理论预测，复杂的栖息地会支持更多的天敌，从而对节肢动物进行自上而下的控制，因此可以减少草食动物。但是，尚不清楚其他更自然系统中发展的理论是否同样适用于脊椎动物和无脊椎动物天敌在复杂程度各异的城市栖息地的捕食。我们使用了橡皮泥毛毛虫模型来评估鸟类和昆虫捕食的风险，收集了食叶节肢动物，并测量了柳栎树中的食草性（栎类）分为两个季节，以确定掠食如何影响城市森林碎片，森林碎片附近种植的林木和市区林木中的食草动物。在城市森林碎片中生长的树木比市中心的街道树木对鸟类的捕食尝试和大量的咀嚼叶肉更多。近森林树木的鸟类捕食尝试和食草水平不一致。在三个城市树木生境中，节肢动物的捕食尝试没有统计学差异。与基于理论的期望相反，在城市森林中生长的树木，我们研究的最复杂的栖息地以及鸟类捕食风险最大的栖息地中，咀嚼叶肉的丰度和食草性通常最高。我们建议，城市森林碎片通过增加栖息地的复杂性，比其他城市景观为都市鸟类和咀嚼叶子提供更好的栖息地。因此，基础资源（例如合适栖息地的可用性）介导了对城市食草动物的自上而下的影响。