Competition among plants within populations affects plant size, nutrient status and allocation to defenses. Herbivory places additional stress on plant allocation of resources. When resources are limited due to intraspecific competition, induced defenses may reduce costs of defense responses and trade-offs between allocation to growth or defense. We hypothesized that increased intraspecific competition would result in a decrease in plant size and leaf tissue nutrient quality, and that both intraspecific competition and leaf damage severity would affect inducibility of leaf defensive traits. We tested these hypotheses in common milkweed (Asclepias syriaca) using greenhouse experiments that manipulated plant density and damage severity treatments. We measured a suite of leaf traits generally related with herbivore performance, including size; nitrogen, carbon, lignin, and fiber concentrations; and latex production. Increased density decreased plant size and leaf nutrient quality, but increased lignin levels. Damage severity increased leaf lignin levels and latex production. There were no density–damage severity interactions. We additionally addressed the question of whether plants respond differently to simulated or natural herbivory and hypothesized that insect herbivores and mechanical plant tissue removal would similarly affect induced defensive responses. Leaf fiber and lignin increased in response to damage, but the response was greater on plants subjected to simulated, compared with caterpillar herbivory. Other plant traits responded similarly to either damage type. Our findings suggest that intraspecific competition has the potential to generate feedbacks among plants and herbivores as plants respond to herbivory.

种群内植物之间的竞争影响植物的大小，营养状况以及防御措施的分配。草食给植物的资源分配带来了额外的压力。当资源由于种内竞争而受到限制时，诱导防御可能会降低防御反应的成本以及分配给增长或防御的权衡取舍。我们假设种内竞争加剧会导致植物大小和叶片组织营养素质量下降，种内竞争和叶片损伤严重性都会影响叶片防御性状的诱导。我们在常见的马利筋草（Asclepias syriaca）中检验了这些假设），通过控制植物密度和危害严重程度处理的温室实验。我们测量了一系列与草食动物的表现相关的叶片性状，包括大小；氮，碳，木质素和纤维浓度；和乳胶生产。密度的增加降低了植物的大小和叶片的养分质量，但木质素水平却提高了。危害严重程度增加了叶片木质素水平和乳胶产量。没有密度-破坏严重性相互作用。我们还解决了植物对模拟或天然草食动物的反应是否不同的问题，并假设昆虫草食动物和机械植物组织的去除会类似地影响诱导的防御反应。叶片纤维和木质素对伤害的响应增加，但是在模拟作物上的响应更大。与毛毛虫草食相比。其他植物性状对任一损伤类型的反应相似。我们的发现表明，随着植物对草食动物的反应，种内竞争有可能在植物和草食动物之间产生反馈。