Are extrafloral nectaries efficient against herbivores? Herbivory and plant defenses in contrasting tropical species

Aims Plants have limited resources for defenses and species that invest in biotic defenses might exhibit leaves that invest less in other types of defenses.We have investigated whether plants that have few mechanical defenses,but have extrafloral nectaries(EFNs)patrolled by ants,are less prone to herbivory,compared with plants without EFNs that have tougher leaves.Methods Data from the literature were extracted to examine the reported levels of herbivory in plants with or without EFNs.In a savanna vegetation in southern Brazil,field data were collected in leaves from six tropical species and herbivory and specific leaf area(SLA)levels were measured.We further evaluated differences in herbivory and SLA among species and between plants with or without EFNs.In order to test the relationship between herbivory and leaf toughness we regressed average herbivory and average SLA per plant.Important Findings Plants exhibited variable levels of leaf damage,but plants without ant defenses experienced the highest levels of leaf area loss to herbivory.Levels of mechanical defenses were also variable among the plant species.Plants without EFNs were tougher,exhibiting lower values of SLA.Although plants without EFNs had more sclerophyllous leaves,this mechanical defense was not sufficient to impair and/or reduce herbivore feeding,suggesting that the biotic defenses performed by patrolling ants might be more effective than investment in mechanical defenses associated with leaf palatability.

植物用于防御的资源有限,而那些具有生物防御的物种,其叶片可能会较少地用于其他类型的防御。我们比较了机械防御能力较弱,但具有花外蜜腺的植物,与没有花外蜜腺但叶片更有韧性的植物是否更不容易被取食。我们通过分析文献数据,调查有无花外蜜腺的植物被取食的情况。在巴西南部的热带稀树草原植被中,我们采集了6种热带植物的叶片数据,并测定了被取食情况和比叶面积。我们进一步评估了有无花外蜜腺的物种之间以及植物之间被取食情况和比叶面积的差异。为了检验动物取食与叶片韧性之间的关系,我们对单株平均被取食情况和平均叶片韧性进行了回归。研究结果表明,植物表现出不同程度的叶片损伤,但没有蚂蚁防御的植物因被取食造成的叶面积损失最大。不同植物的机械防御水平也不相同。无花外蜜腺的植株更坚韧,比叶面积值更小。虽然无花外蜜腺的植物有更多的坚硬叶片,但这种机械防御不足以损害和/或减少食草动物的取食,这表明由蚂蚁形成的生物防御可能比用于与叶片适口性相关的机械防御更有效。