Plant pathogens and tropical tree diversity

地球上至少有一半的动植物物种生活在热带森林中，维持这种非凡的多样性的生态过程引起了相当大的兴趣。解释热带树木的高度多样性是一个特别的挑战，因为大多数植物物种使用非常相似的资源：只有一个或几个高度竞争的物种可能会统治森林，而不是经常观察到的每公顷数百个物种。如果我们能够解释热带树木的高度多样性，那么热带森林中其他分类群的高度多样性可能会随之而来。最近，越来越多的证据表明，由于种子和幼苗的死亡率与密度密切相关，热带森林中的植物多样性很高。换句话说，种子和幼苗在靠近母树和靠近同一物种的其他幼苗时更容易死亡。这使得当地的稀有植物物种处于优势地位，阻止任何一个物种达到高丰度。人们普遍怀疑真菌样植物病原体是造成幼苗密度依赖的原因。然而，它们在维持热带森林高度植物多样性方面的作用的证据主要是间接的，它们对热带森林群落更广泛结构的影响尚未得到充分探讨。我们将在中美洲伯利兹的研究地点检验植物病原体保持高度植物多样性的假设。我们将解决五个关键的突出问题：(i)病原体介导的密度依赖有多普遍？病原体要在热带森林群落中发挥重要的结构作用，它们的影响必须是广泛的。使用选择性杀菌剂的排除实验将揭示病原体是否驱动密度依赖性死亡率。（ii）病原体是专家还是通才？当天敌表现出较高的寄主特异性时，密度依赖性具有最强的多样性增强作用。分子和实验方法将用于评估从单个热带树种幼苗中分离的病原体种类和菌株的特异性。（iii）病原体是否介导植物物种间的间接相互作用？我们将评估病原体在植物物种对之间共享的程度，并调查共享寄生对整个社区的影响。病原体如何影响宿主密度与死亡率之间的关系？我们将确定密度和死亡率之间的关系是否具有一种可能促进植物多样性的形式，我们将确定密度和与母树的距离在影响死亡率方面的相对重要性。病原体造成的死亡是否会增加多样性？我们将通过比较杀菌剂处理和未处理土壤的种子雨多样性和新苗多样性，对病原体促进植物多样性假说进行首次实验验证。

At least half of all the earth's species of plants and animals live in tropical forests, and there is considerable interest in the ecological processes maintaining this extraordinary diversity. Explaining the high diversity of tropical trees is a particular challenge, because most plant species use very similar resources: just one or a few highly competitive species might be expected to dominate the forest, rather than the hundreds of species per hectare that are often observed. If we can explain the high diversity of tropical trees, then the high diversity of other taxa in tropical forests may follow. Recently, evidence has been growing that high plant diversity in tropical forests arises because of strong density-dependent seed and seedling mortality. In other words, seeds and seedlings are more likely to die when close to their parent tree and close to other seedlings of the same species. This puts locally rare plant species at an advantage, preventing any one species from reaching high abundance. It is widely suspected that fungus-like plant pathogens are responsible for seedling density-dependence. However, the evidence for their role in maintaining high plant diversity in tropical forests is largely circumstantial, and their effects on the wider structure of tropical forest communities have not been explored fully . We will test the hypothesis that plant pathogens maintain high plant diversity at our study site in Belize, Central America. We will address five key outstanding questions: (i) How widespread is pathogen mediated density-dependence? For pathogens to play an important structuring role in tropical forest communities, their effects must be widespread. Exclusion experiments using selective fungicides will reveal whether pathogens are driving density-dependent mortality. (ii) Are pathogens specialist or generalist? Density dependence is expected to have the strongest diversity-enhancing role when natural enemies show high host specificity. Molecular and experimental approaches will be used to assess the specificity of pathogen species and strains isolated from seedlings of individual tropical tree species. (iii) Do pathogens mediate indirect interactions among plant species? We will assess the extent to which pathogens are shared between pairs of plant species, and investigate the community-wide impacts of shared parasitism. (v) How do pathogens affect the relationship between host density and mortality? We will determine whether the the relationship between density and mortality is of a form likely to promote plant diversity, and we will determine the relative importance of density and distance from parent tree in influencing mortality. (iv) Does mortality from pathogens increase diversity? We will carry out the first experimental test of the hypothesis that pathogens promote plant diversity by comparing the diversity of the seed rain with the diversity of new seedlings in fungicide-treated and untreated plots.