Environmental Gradients and Variation in the Strength of Bird Predation on Oak Herbivores

大多数生态学家现在同意，捕食者和资源与非生物环境相互作用，影响大多数草食动物的生态。最近的兴趣在于确定这两个因素对食草动物的相对影响，以及这些因素在空间和时间上的变化。这项研究比较了三种相互竞争的理论，以解释为什么草食动物群落的捕食压力在空间上有所不同。我们认为，A)叶的质量，而不是植物的生产力，决定了食草动物对植物的生产力，以及B)食草动物对捕食者的脆弱性不同。因此，c)食草动物的捕食压力随着叶片质量的增加而增加，较不容易被捕食的猎物将积累在高质量的植物上。我们计划通过在考韦塔长期生态研究地点探索鸟类对红橡树节肢动物的捕食强度沿海拔梯度的变化来检验这些预测。最近的研究表明，一些食草动物的密度随着海拔的升高而增加，而另一些则下降。同时，叶片品质随海拔升高而增加，而植物生产力则不随海拔升高而增加。结合鸟类捕食实验的结果，这些结果表明，鸟类对自由生活的草食动物的影响随着考韦塔海拔的升高而增加，导致较不脆弱的猎物(采叶者)在优质植物上积累。这项研究的目标是：1)验证在Coweeta观察到的草食动物随海拔高度的明显变化；2)评估鸟类捕食对海拔梯度上节肢动物群落的相对影响；3)评估叶化学变化对这一海拔梯度上橡树草食动物的影响。这些目标将通过现场采样、排除试验和实验室喂养试验相结合来实现。我们的研究将为景观水平上捕食压力的变化如何影响草食动物的种群和群落提供基本信息。这项研究将有助于本科生和研究生的培养。考韦塔的外联活动将确保K-16岁的学生和普通公众都能参加研究活动。所有收集到的数据将通过Coweeta网页向公众和科学界开放。在研究期间收集的节肢动物将成为考维塔收藏的一部分，由佐治亚州国家博物馆主办，并向公众开放。此外，收集的数据将有助于了解食虫鸟类在森林中的作用以及鸟类数量减少对森林生态过程的潜在影响。

Most ecologists now agree that predators and resources interact with the abiotic environment to influence the ecology of most herbivores. Recent interest lies in determining the relative effects of these two factors on herbivores, and how these factors vary in space and time. This study compares three competing theories to explain why predation pressure on herbivore communities varies in space. We propose that A) foliage quality, not plant productivity, determines the productivity of herbivores on plants, and B) herbivores vary in their vulnerability to predators. Consequently, C) predation pressure on herbivores increases with increasing foliar quality, and prey that are less vulnerable to predation will accumulate on high-quality plants. We plan to test these predictions by exploring variation in the strength of bird predation on arthropods of red oak along a gradient in elevation at the Coweeta Long-term Ecological Research site. Recent work has shown that some herbivores increase in density with elevation, while others decline. At the same time, foliar quality increases with elevation, while plant productivity does not. Together with results from bird predation experiments, these results suggest that the impact of birds on free-living herbivores increases with elevation at Coweeta, leading to the accumulation of less vulnerable prey (leaf miners) on high-quality plants. The goals of this study are: 1) to validate apparent changes in herbivore with elevation that have been observed at Coweeta; 2) to assess the relative impact of bird predation on arthropod communities along the gradient in elevation; and 3) to assess the effects of changes in foliar chemistry on oak herbivores over this elevational gradient. These goals will be addressed using a combination of field sampling, exclusion experiments, and laboratory feeding trials. Our study will provide fundamental information on how changes in predation pressure at the landscape level influence populations and communities of herbivores. The study will contribute to the training of undergraduate and graduate students. Outreach activities at Coweeta will ensure that research activities are available to K-16 students and to the general public. All data collected will be available to the public and the scientific community via the Coweeta web page. Arthropods collected during the study will form part of the Coweeta collections, hosted by the State Museum of Georgia, and open to the general public. In addition, the data collected will contribute to an understanding of the role of insectivorous birds in forests and the potential impacts of avian declines on ecological processes in forests.