Testing genotypic variation of an invasive plant species in response to soil disturbance and herbivory

Herbivores, competitors, and predators can inhibit biological invasions (“biotic resistance” sensu Elton 1959), while disturbance typically promotes biological invasions. Although biotic resistance and disturbance are often considered separately in the invasion literature, these two forces may be linked. One mechanism by which disturbance may facilitate biological invasions is by decreasing the effectiveness of biotic resistance. The effects of both disturbance and biotic resistance may vary across invading genotypes, and genetic variation in the invasive propagule pool may increase the likelihood that some genotypes can overcome biotic resistance or take greater advantage of disturbance. We conducted an experimental field trial in which we manipulated soil disturbance (thatch removal and loosening soil) and the presence of insect herbivores and examined their effects on the invasion success of 44 Medicago polymorpha genotypes. As expected, insecticide reduced leaf damage and increased Medicago fecundity, suggesting that insect herbivores in this system provide some biotic resistance. Soil disturbance increased Medicago fecundity, but did not alter the effectiveness of biotic resistance by insect herbivores. We found significant genetic variation in Medicago in response to disturbance, but not in response to insect herbivores. These results suggest that the ability of Medicago to invade particular habitats depends on the amount of insect herbivory, the history of disturbance in the habitat, and how the specific genotypes in the invader pool respond to these factors.

植食者、竞争者与捕食者可抑制生物入侵（即埃尔顿1959年提出的生物抵抗（biotic resistance）），而干扰通常会促进生物入侵。尽管入侵生物学研究中常将生物抵抗与干扰分开探讨，但这两种作用可能存在关联。干扰促进生物入侵的一种机制，是通过降低生物抵抗的有效性实现的。干扰与生物抵抗的作用效果可能因入侵基因型的不同而异，而入侵繁殖体库中的遗传变异，可能提升部分基因型克服生物抵抗或更充分利用干扰的概率。本研究开展了一项野外田间试验，设置了土壤干扰（枯草层移除与土壤翻松）和昆虫植食者存在与否两个处理组，探究二者对44个斑荚苜蓿（Medicago polymorpha）基因型入侵成功的影响。正如预期，杀虫剂降低了叶片受害程度并提升了斑荚苜蓿的繁殖力，表明该系统中的昆虫植食者确实发挥了一定的生物抵抗作用。土壤干扰提升了斑荚苜蓿的繁殖力，但并未改变昆虫植食者所介导的生物抵抗的有效性。我们发现，斑荚苜蓿对干扰的响应存在显著的遗传变异，但对昆虫植食者的响应则无显著遗传差异。上述结果表明，斑荚苜蓿入侵特定生境的能力，取决于该生境中的昆虫植食强度、生境的干扰历史，以及入侵繁殖体库中的特定基因型对这些因素的响应模式。