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

大多数生态学家现在都同意，掠食者和资源与非生物环境相互作用，影响大多数食草动物的生态。最近的兴趣在于确定这两个因素对食草动物的相对影响，以及这些因素如何在空间和时间上变化。这项研究比较了三种相互竞争的理论，以解释为什么食草动物群落的捕食压力在空间上有所不同。我们认为，A)叶片质量，而不是植物生产力，决定了食草动物对植物的生产力，B)食草动物对捕食者的脆弱性各不相同。因此，C)食草动物的捕食压力随着叶片质量的提高而增加，不易被捕食的猎物将聚集在高质量的植物上。我们计划通过在Coweeta长期生态研究站点沿海拔梯度探索红橡树节肢动物的鸟类捕食强度的变化来验证这些预测。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。与此同时，叶片质量随海拔升高而增加，而植物生产力则不增加。结合鸟类捕食实验的结果，这些结果表明，鸟类对自由生活的食草动物的影响随着海拔的升高而增加，导致不那么脆弱的猎物（采叶者）在优质植物上的积累。本研究的目的是：1)验证Coweeta草食动物随海拔升高的明显变化；2)评估鸟类捕食对海拔梯度节肢动物群落的相对影响；3)在此海拔梯度下，评估叶片化学变化对栎树食草动物的影响。这些目标将通过现场抽样、排除实验和实验室喂养试验相结合来实现。我们的研究将为在景观水平上捕食压力的变化如何影响食草动物的种群和群落提供基础信息。这项研究将有助于培养本科生和研究生。Coweeta的外展活动将确保K-16学生和公众都可以参加研究活动。收集到的所有数据将通过Coweeta网页向公众和科学界提供。在研究期间收集的节肢动物将成为Coweeta收藏品的一部分，由乔治亚州国家博物馆主办，并向公众开放。此外，收集到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.