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

地球上至少有50%的动植物物种生活在热带雨林中，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。生态学家有兴趣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。对热带森林植物多样性高的一个主要解释是詹森-康奈尔效应。这一理论表明，诸如取食植物的昆虫和真菌疾病等害虫可以帮助维持热带生物多样性，如果(1)它们专门针对特定的植物物种，(2)它们造成“密度依赖”的死亡率（即，它们在当地种子和幼苗丰富的地方杀死更多的种子和幼苗）。这种有害生物的压力起到了负反馈机制的作用，使当地稀有植物物种处于有利地位，并阻止任何一种物种达到高丰度。最近的研究表明，这种来自昆虫和真菌的密度依赖形式在维持热带植物多样性方面起着关键作用。我们现在想要发现这个过程在不同的气候条件下是如何变化的。湿润的热带森林比干燥的森林有更多的植物种类，我们将测试这样一个理论，即这些地方更强烈的密度依赖害虫压力是这些差异背后的一个因素。我们还将调查未来的气候变化（降雨增加或减少）是否有可能改变Janzen-Connell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，我们将利用从干燥（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测试我们的假设。我们将在田间和受控的苗圃条件下进行实验，控制种子和幼苗的密度，真菌病原体和植物食性昆虫的存在，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以及在多大程度上改变热带植物多样性。

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.