Birds and bats reduce herbivory damage in Papua New Guinean highland forests

Insectivorous predators, including birds and bats, play crucial roles in trophic cascades. However, previous research on these cascades has often relied on permanent predator exclosures, which prevent the isolation of specific effects of birds and bats, given their different activity patterns throughout the day. Moreover, limited knowledge exists regarding the variations in individual effects of these predators under different biotic and abiotic conditions, such as changes in elevation. To address these uncertainties, our study aimed to investigate the distinct effects of bats and birds on arthropod densities in foliage and herbivory damage in lowland and highland rainforests of Papua New Guinea (PNG). Predator exclosures were established for one month to exclude diurnal or nocturnal predators across 120 saplings (ca. 2.5-4 m tall) selected from two lowland and two highland forests (i.e., 30 saplings per study site) along the Mt. Wilhelm transect in PNG. Arthropods were collected and measured, and herbivory damage was analysed at the end of the experiment. Birds significantly reduced arthropod densities by 30 %, particularly in arthropods longer than 10 mm, regardless of elevation. Additionally, both birds and bats appeared to mitigate herbivory damage in highland forests, with protected saplings displaying up to 189 % more herbivory. Our results support previous studies that have demonstrated the ability of insectivorous predators to reduce leaf damage through the control of arthropods. Furthermore, our approach highlights the importance and necessity of further research on the role of seasons and elevations in trophic cascades.

Methods
We conducted predator exclosure experiments at four elevational study sites along the Mt. Wilhelm elevational gradient in Papua New Guinea. The experiments were carried out on 120 saplings from 11 tree species. The predator exclosures, constructed from PVC tubes and agricultural nylon netting, were used to individually exclude birds and bats from the saplings. Each exclosure was accompanied by control saplings. After one month of exclusion, we collected all arthropods from the saplings to estimate their densities and sizes and assessed the extent of herbivory damage. We compared the experimental outcomes with the richness and abundance of vertebrate predators (birds and bats) using previously published data from this location.

食虫捕食者（包括鸟类与蝙蝠）在营养级联（trophic cascades）中发挥着关键作用。然而，过往针对此类营养级联的研究多依赖永久性捕食者排除装置（predator exclosures），由于鸟类与蝙蝠的昼夜活动模式存在显著差异，此类装置无法分离二者各自的特异性调控效应。此外，目前学界对这类捕食者在不同生物与非生物因子（如海拔变化）下的个体效应差异仍缺乏深入认知。

为填补上述研究空白，本研究以巴布亚新几内亚（PNG）低地与高地雨林为研究对象，探究蝙蝠与鸟类分别对叶面节肢动物（arthropods）密度以及植食为害程度的独特调控效应。研究沿巴布亚新几内亚威廉峰样带设置2处低地雨林与2处高地雨林样地，共选取120株株高约2.5~4米的幼树（saplings），每个样地设置30株；通过为期1个月的捕食者排除装置，分别排除昼行性与夜行性捕食者。

实验结束后，研究人员采集并测量节肢动物，并分析植食为害情况。结果显示，鸟类可使节肢动物密度显著降低30%，尤其是体长超过10毫米的节肢动物，且该效应不受海拔影响。此外，在高地雨林中，鸟类与蝙蝠均能缓解植食为害：受捕食者保护的幼树的植食为害程度最高较未受保护的对照幼树高出189%。本研究结果佐证了过往研究结论，即食虫捕食者可通过调控节肢动物种群降低叶片损伤。同时，本研究的实验方法也凸显了进一步探究季节与海拔在营养级联中调控作用的重要性与必要性。

## 研究方法
本研究在巴布亚新几内亚威廉峰海拔梯度沿线的4处海拔样地开展捕食者排除实验。实验对象为11个树种的120株幼树。研究采用聚氯乙烯（PVC）管材与农用尼龙网搭建捕食者排除装置，为每株幼树单独设置装置以阻隔其接触鸟类与蝙蝠。每台排除装置均配有对应的对照幼树。经过1个月的排除处理后，研究人员采集幼树上的全部节肢动物以估算其密度与体型大小，并评估植食为害程度。同时，本研究结合该区域已发表的公开数据，将实验结果与脊椎动物捕食者（鸟类与蝙蝠）的物种丰富度和多度进行了对比分析。