CAS Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla 666303, People's Republic of China.
Center of Conservation Biology, Core Botanical Gardens, Chinese Academy of Sciences, Mengla 666303, People's Republic of China.
Proc Biol Sci. 2021 Jan 27;288(1943):20202352. doi: 10.1098/rspb.2020.2352. Epub 2021 Jan 20.
Conspecific negative distance- and density-dependence is often assumed to be one of the most important mechanisms controlling forest community assembly and species diversity globally. Plant pathogens, and insect and mammalian herbivores, are the most common natural enemy types that have been implicated in this phenomenon, but their general effects at different plant life stages are still unclear. Here, we conduct a meta-analysis of studies that involved robust manipulative experiments, using fungicides, insecticides and exclosures, to assess the contributions of different natural enemy types to distance- and density-dependent effects at seed and seedling stages. We found that distance- and density-dependent mortality caused by natural enemies was most likely at the seedling stage and was greater at higher mean annual temperatures. Conspecific negative distance- and density-dependence at the seedling stage is significantly weakened when fungicides were applied. By contrast, negative conspecific distance- and density-dependence is not a general pattern at the seed stage. High seed mass reduced distance- and density-dependent mortality at the seed stage. Seed studies excluding only large mammals found significant negative conspecific distance-dependent mortality, but exclusion of all mammals resulted in a non-significant effect of conspecifics. Our study suggests that plant pathogens are a major cause of distance- and density-dependent mortality at the seedling stage, while the impacts of herbivores on seedlings have been understudied. At the seed stage, large and small mammals, respectively, weaken and enhance negative conspecific distance-dependent mortality. Future research should identify specific agents of mortality, investigate the interactions among different enemy types and assess how global change may affect natural enemies and thus influence the strength of conspecific distance- and density-dependence.
种内负距离和密度依赖关系通常被认为是控制全球森林群落组装和物种多样性的最重要机制之一。植物病原体、昆虫和哺乳动物食草动物是最常见的被认为与这种现象有关的天敌类型，但它们在不同植物生活阶段的一般影响仍不清楚。在这里，我们对涉及稳健的操纵实验的研究进行了荟萃分析，使用杀菌剂、杀虫剂和围栏，以评估不同天敌类型对种子和幼苗阶段距离和密度依赖效应的贡献。我们发现，天敌引起的距离和密度依赖死亡率最有可能发生在幼苗阶段，并且在年平均温度较高时更大。在幼苗阶段应用杀菌剂时，种内负距离和密度依赖关系显著减弱。相比之下，种内负距离和密度依赖关系在种子阶段并不是一个普遍的模式。高种子质量降低了种子阶段的距离和密度依赖死亡率。排除大型哺乳动物的种子研究发现了显著的负种间距离依赖死亡率，但排除所有哺乳动物的研究结果则是非显著的。我们的研究表明，植物病原体是幼苗阶段距离和密度依赖死亡率的主要原因，而食草动物对幼苗的影响则研究不足。在种子阶段，大型和小型哺乳动物分别减弱和增强了负种间距离依赖死亡率。未来的研究应该确定具体的致死因素，研究不同天敌类型之间的相互作用，并评估全球变化如何影响天敌，从而影响种内距离和密度依赖关系的强度。
