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，人们对维持这种非凡多样性的生态过程非常感兴趣。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种物种。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类群的高度多样性可能也会随之而来。最近，越来越多的证据表明，热带森林的植物多样性之所以高，是因为种子和幼苗的死亡率与密度密切相关。换句话说，当种子和幼苗靠近它们的母树和靠近同一物种的其他幼苗时，它们更容易死亡。这使得当地稀有的植物物种处于优势地位，防止任何一个物种达到高丰度。人们普遍怀疑，真菌类植物病原体是负责幼苗密度依赖性。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基本上是间接的，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。我们将测试的假设，植物病原体保持高植物多样性在我们的研究地点在中美洲的伯利兹。我们将解决五个关键的悬而未决的问题：（一）病原体介导的密度依赖性有多普遍？病原体要在热带森林群落中发挥重要的结构作用，其影响必须是广泛的。使用选择性杀真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？密度依赖性，预计有最强的多样性增强的作用时，天敌表现出高主机特异性。分子和实验方法将用于评估从个别热带树种幼苗中分离的病原体物种和菌株的特异性。(iii)病原体介导植物物种间的间接相互作用吗？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。(v)病原体如何影响宿主密度和死亡率之间的关系？我们将确定密度和死亡率之间的关系是否是一种可能促进植物多样性的形式，我们将确定密度和距离母树在影响死亡率的相对重要性。(iv)病原体的死亡会增加多样性吗？我们将进行第一次实验测试的假设，病原体促进植物多样性的种子雨的多样性与新的幼苗在杀菌剂处理和未处理的地块的多样性进行比较。

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.