Data from: Interactions among predators and plant specificity protects herbivores from top predators

The worldwide loss of top predators from natural and agricultural systems has heightened the need to understand how important they are in controlling herbivore abundance. The effect of top predators on herbivore species is likely to depend on 1) the importance of the consumption of intermediate predators by top predators (intra-guild predation; IGP), but also on 2) plant specificity by herbivores, because specialists may defend themselves better (enemy-free space; EFS). Insectivorous birds, as top predators, are generally known to effectively control herbivorous insects, despite also consuming intermediate predators such as spiders, but how this effect varies among herbivore species in relation to the cascading effects of IGP and EFS is not known. To explore this, we excluded birds from natural fynbos vegetation in South Africa using large netted cages and recorded changes in abundance relative to control plots for 199 plant-dwelling intermediate predator and 341 herbivore morpho-species that varied in their estimated plant specificity. We found a strong negative effect of birds on the total abundance of all intermediate predators, with especially clear effects on spiders (strong IGP). In contrast with previous studies, which document a negative effect of birds on herbivores, we found an overall neutral effect of birds on herbivore abundance, but the effect varied among species: some species were negatively affected by birds, suggesting that they were mainly consumed by birds, whereas others, likely released from spiders by IGP, were positively affected. Some species were also effectively neutrally affected by birds. These tended to be more specialized to plants compared to the other species, which may imply that some plant specialists benefited from protection provided by EFS from both birds and spiders. These results suggest that the response of herbivore species to top predators may depend on cascading effects of interactions among predators and on their degree of plant specificity.

全球范围内自然与农业生态系统中顶级捕食者（top predators）的丧失，使得学界愈发需要明晰它们在调控植食动物种群丰度方面的核心作用。顶级捕食者对植食物种的影响主要取决于两个因素：一是顶级捕食者对中间捕食者的捕食强度（集团内捕食（intra-guild predation, IGP）），二是植食动物的植物宿主特异性——专食性物种往往具备更强的反捕食防御能力，即天敌自由空间（enemy-free space, EFS）。作为顶级捕食者的食虫鸟类，虽普遍被认为可有效管控植食性昆虫，同时也会捕食蜘蛛这类中间捕食者，但目前学界仍不清楚这类效应在不同植食物种间的差异，以及其与IGP和EFS的级联效应之间的关联。为探究这一科学问题，我们通过大型网笼将鸟类排除在南非自然的凡波斯（Fynbos）植被区域之外，并以对照样地为参照，记录了199种栖息于植物上的中间捕食者以及341种植食性形态种的种群数量变化，这些物种的宿主植物特异性程度存在显著差异。研究结果显示，鸟类对所有中间捕食者的总种群丰度存在显著的负调控效应，其中对蜘蛛的抑制效果尤为明显，体现为强烈的IGP作用。与此前研究中记录到的鸟类对植食动物的负调控效应不同，本研究发现鸟类对植食动物总丰度的整体效应为中性，但不同物种间的响应存在显著分化：部分物种受鸟类捕食的负面影响，表明它们是鸟类的主要捕食对象；而另一些物种则因IGP作用从蜘蛛的捕食压力中得以释放，因此呈现正响应。此外还有部分物种对鸟类的响应基本为中性，相较于其他物种，这类物种往往具有更强的植物宿主专一性，这或许意味着部分专食性植食动物可借助EFS获得来自鸟类和蜘蛛的双重保护。综上，本研究结果表明，植食物种对顶级捕食者的响应，可能取决于捕食者间相互作用的级联效应，以及其自身的植物宿主特异性程度。