1] Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK [2] Ecosystem Management Group, Institute of Terrestrial Ecosystems, ETH Zürich, Universitätstrasse 16, 8092 Zürich, Switzerland.
1] Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK [2] School of Natural Resources and the Environment, University of Arizona, Tucson, Arizona 85721, USA.
Nature. 2014 Feb 6;506(7486):85-8. doi: 10.1038/nature12911. Epub 2014 Jan 22.
Tropical forests are important reservoirs of biodiversity, but the processes that maintain this diversity remain poorly understood. The Janzen-Connell hypothesis suggests that specialized natural enemies such as insect herbivores and fungal pathogens maintain high diversity by elevating mortality when plant species occur at high density (negative density dependence; NDD). NDD has been detected widely in tropical forests, but the prediction that NDD caused by insects and pathogens has a community-wide role in maintaining tropical plant diversity remains untested. We show experimentally that changes in plant diversity and species composition are caused by fungal pathogens and insect herbivores. Effective plant species richness increased across the seed-to-seedling transition, corresponding to large changes in species composition. Treating seeds and young seedlings with fungicides significantly reduced the diversity of the seedling assemblage, consistent with the Janzen-Connell hypothesis. Although suppressing insect herbivores using insecticides did not alter species diversity, it greatly increased seedling recruitment and caused a marked shift in seedling species composition. Overall, seedling recruitment was significantly reduced at high conspecific seed densities and this NDD was greatest for the species that were most abundant as seeds. Suppressing fungi reduced the negative effects of density on recruitment, confirming that the diversity-enhancing effect of fungi is mediated by NDD. Our study provides an overall test of the Janzen-Connell hypothesis and demonstrates the crucial role that insects and pathogens have both in structuring tropical plant communities and in maintaining their remarkable diversity.
热带雨林是生物多样性的重要储存库，但维持这种多样性的过程仍未得到很好的理解。简森-康奈尔假说认为，专门的天敌（如昆虫食草动物和真菌病原体）通过在植物物种高密度时提高死亡率来维持高多样性（负密度依赖性；NDD）。NDD 在热带森林中被广泛检测到，但 NDD 由昆虫和病原体引起的预测对维持热带植物多样性具有群落范围的作用仍未得到检验。我们通过实验表明，真菌病原体和昆虫食草动物会导致植物多样性和物种组成发生变化。有效植物物种丰富度在种子到幼苗的过渡过程中增加，与物种组成的巨大变化相对应。用杀菌剂处理种子和幼苗显著降低了幼苗组合的多样性，这与简森-康奈尔假说一致。虽然使用杀虫剂抑制昆虫食草动物不会改变物种多样性，但它大大增加了幼苗的补充，并导致幼苗物种组成发生明显变化。总的来说，在高同种种子密度下，幼苗的补充明显减少，而这种 NDD 对作为种子最丰富的物种最大。抑制真菌减少了密度对补充的负面影响，证实了真菌对多样性的增强作用是通过 NDD 介导的。我们的研究总体上检验了简森-康奈尔假说，并证明了昆虫和病原体在构建热带植物群落和维持其显著多样性方面都具有关键作用。
