Data from: Fragment size affects plant herbivory via predator loss

Fragmentation and resultant changes in patch size are predicted to alter species diversity and community composition, yet the consequences of these differences for species interactions are poorly understood. Theory predicts that predators are more sensitive to fragmentation than their prey, resulting in greater predator loss in small patches. Predator loss, in turn, is predicted to 1) increase herbivory rates overall, and 2) cause herbivores to shift feeding from plants that act as refugia to those that are preferred forage. We tested these predictions in an old-field community using two experiments. The first was a large-scale experiment that included hundreds of arthropod species in fragments of various sizes, and used goldenrod and switchgrass to assess herbivory. Our second experiment manipulated densities of a focal predator species and a focal prey species to determine if changes in densities, rather than other characteristics of fragments, were sufficient to cause the trends observed in the first experiment. We found that predator densities declined in small fragments whereas herbivore densities showed the opposite trend. Total herbivory mirrored herbivore densities by increasing in small patches, and this mean increase was driven by large increases in goldenrod herbivory but declines in switchgrass herbivory. Experimental manipulation of densities confirmed that herbivores preferentially feed on goldenrod, and that predators depress herbivory on goldenrod but have a negligible effect on switchgrass. Our results suggest that fragmentation alters trophic interactions by causing declines in predator densities and increases in herbivore densities, but that feeding preferences of herbivores may generate unequal impacts among plant species.

生境破碎化及其引发的斑块大小变化，预计会改变物种多样性与群落组成，但人们对这类差异对物种种间相互作用的影响尚缺乏充分认知。理论研究表明，捕食者相较于猎物对生境破碎化更为敏感，因此小型斑块内捕食者的种群消亡程度更高。而捕食者的消亡则会带来两个预期结果：其一，整体植食率上升；其二，植食动物的取食偏好会从作为避难所的植物转向其偏好取食的寄主植物。我们依托一处旧农田群落开展两项实验，以验证上述理论预测。第一项为大尺度实验，在不同大小的破碎斑块中纳入了数百种节肢动物，并以一枝黄花（goldenrod）和柳枝稷（switchgrass）为研究对象评估植食水平。第二项实验通过操控目标捕食者与猎物的种群密度，以明确是种群密度变化（而非破碎斑块的其他特征）足以引发第一项实验中观测到的趋势。研究结果显示，小型斑块内捕食者种群密度下降，而植食动物种群密度则呈现相反的上升趋势。总植食水平与植食动物种群密度的变化趋势一致，在小型斑块中上升；而这一整体上升主要由一枝黄花的植食水平大幅升高所驱动，柳枝稷的植食水平则出现下降。种群密度操控实验进一步证实，植食动物优先取食一枝黄花；捕食者可抑制一枝黄花上的植食作用，但对柳枝稷上的植食作用影响可忽略不计。本研究结果表明，生境破碎化通过降低捕食者种群密度、提升植食动物种群密度来改变营养级互作；但植食动物的取食偏好，可能会使不同植物物种受到的影响存在差异。