Natural enemies, climate, and the maintenance of tropical tree diversity

地球上至少50%的动植物物种分布在热带雨林中，但这种丰富的生物多样性正受到森林砍伐和气候变化的威胁。生态学家感兴趣的是了解为什么这些栖息地如此多样化，以及它们的多样性在未来将如何变化。对热带森林中高度植物生物多样性的一个主要解释是詹森-康奈尔效应。这一理论表明，食草性昆虫和真菌疾病等害虫在以下情况下有助于维持热带生物多样性：(1)它们专门针对特定的植物物种；(2)它们会导致“密度相关”的死亡(即，在当地种子和幼苗丰富的地方，它们杀死更多的种子和幼苗)。这种害虫压力起到了负反馈机制的作用，使当地稀有植物物种处于优势地位，并阻止任何一个物种达到高丰度。最近的研究表明，昆虫和真菌的这种密度依赖形式在维持热带植物多样性方面发挥着关键作用。我们现在想要发现这个过程在不同的气候制度下是如何变化的。湿润的热带森林比干燥的森林有更多的植物物种，我们将测试这一理论，即这些地方更强烈的依赖密度的害虫压力是这些差异背后的一个因素。我们还将调查未来气候变化(降雨量增加或减少)是否可能改变詹岑-康奈尔效应的强度，从而改变植物多样性。我们的工作将在巴拿马进行，我们将利用从干旱(太平洋)海岸到潮湿(大西洋)海岸的降雨量和土壤湿度的陡峭梯度来检验我们的假设。我们将在田间和受控苗圃条件下进行实验，控制种子和幼苗的密度以及真菌病原体和植食性昆虫的存在，我们将分析长期数据并建立数学模型，以探索气候变化是否会改变热带植物的多样性，以及改变的程度。

At least 50% of earth's plant and animal species is found in tropical rainforests, but this rich biodiversity is under threat from deforestation and climate change. Ecologists are interested in understanding why these habitats are so diverse, and how their diversity will change in the future. One leading explanation for high plant biodiversity in tropical forests is the Janzen-Connell Effect. This theory suggests that pests such as plant-feeding insects and fungal diseases can help maintain tropical biodiversity if (1) they specialise on particular plant species, and (2) they cause 'density-dependent' mortality (i.e., they kill more seeds and seedlings where these are locally abundant). This pest pressure acts as a negative feedback mechanism, putting locally rare plant species at an advantage and preventing any one species from reaching high abundance. Recent research shows that this form of density-dependence from both insects and fungi plays a key role in the maintenance of plant diversity in the tropics. We now want to discover how this process changes under different climatic regimes. Wetter tropical forests have more plant species than drier forest, and we will test the theory that more intense density-dependent pest pressure in these places is a factor behind these differences. We will also investigate whether future changes to the climate (higher or lower rainfall) are likely to alter the strength of the Janzen-Connell Effect, and consequently plant diversity. Our work will take place in Panama, where we will take advantage of a steep gradient in rainfall and soil humidity from the dry (Pacific) coast to the humid (Atlantic) coast to test our hypotheses. We will carry our experiments in the field and in controlled nursery conditions that manipulate the density of seeds and seedlings and the presence of fungal pathogens and plant-feeding insects, and we will analyse long-term data and build mathematical models to explore whether and to what extent climate change will alter tropical plant diversity.