Bird Predation, Forest Insects, and Growth of Cottonwoods: Bottom-up and Top-down Influences on a Trophic Cascade in a Threatened Habitat

当捕食者吃掉足够多的食草动物（食草动物）以促进植物生长或改变植物群落的结构时，就会发生营养级联。在海洋和重新引入狼的地方，已经记录了瀑布。虽然人们普遍认为，森林鸟类通过吃这些树木的害虫而使树木受益，但有几个原因可以解释为什么鸟类可能不会推动营养级联。例如，植物可能会通过迅速长出新的叶子来补偿被吃掉的叶子，或者鸟类可能会吃掉与因其他原因（天气，昆虫捕食者，饥饿）而死亡的昆虫相同数量的昆虫。在这项研究中，研究人员将测试鸟类捕食昆虫在某些情况下有利于植物的假设，并将确定影响营养级联强度的条件。研究人员将研究食虫鸟类对美国西南部三角叶杨的影响。将在整棵树周围建造笼子，以将鸟类从对害虫具有高和低遗传易感性的树木中排除。这将是同类研究中第一次测量成年树木的生长反应，也是第一次测量种子产量。 通过在不同季节将鸟类从树木中排除，该研究将确定树木如何应对留鸟（目前全年）或候鸟（仅春季和夏季）的减少。 通过比较鸟类和杀虫剂的效果，研究将记录鸟类在控制昆虫方面的相对有效性。该项目将为一名博士生和至少两名本科生提供培训。

A trophic cascade occurs when a predator eats enough herbivores (plant-eating animals) to improve plant growth or to alter the structure of the plant community. Cascades have been documented in oceans and in places where wolves have been reintroduced. Although it is widely believed that forest birds benefit trees by eating insect pests of these trees, there are several reasons why birds might not drive a trophic cascade. For instance, plants might compensate for eaten leaves by rapidly producing new leaves, or birds might eat the same amount of insects as would have died due to other causes (weather, insect predators, starvation). In this research, investigators will test the hypothesis that predation of insects by birds benefits plants under some circumstances and will identify the conditions that affect the strength of the trophic cascade. The investigators will study the effects of insectivorous birds on cottonwood trees in the Southwestern United States. Cages will be built around whole trees to exclude birds from trees with high and low genetic susceptibility to insect pests. This will be the first study of its kind to measure growth responses of adult trees, and the first to measure seed production. By excluding birds from trees during different seasons, the study will determine how trees would respond to declines in resident birds (present year-round) or migratory birds (spring and summer only). By comparing the effect of birds to that of insecticides, the study will document the relative effectiveness of birds in insect control. The project will provide training for one PhD student and at least two undergraduates.