Perturbed Neutrality - beyond null models of biodiversity

生态学家长期以来一直对维持生物多样性的机制感到困惑--这种机制允许如此多的自然物种与竞争资源的其他物种共存。许多生物学家认为，物种通过以不同的方式开发环境而共存，丰度和稀有度的模式反映了不同物种在群落中扮演的不同角色。这一观点得到了查尔斯·达尔文的支持，他在1859年写道：当我们看到一个被缠绕的银行所覆盖的植物和灌木丛时，我们忍不住把它们的比例数量和种类归因于我们所说的偶然。但是，这是多么错误的观点！然而，斯蒂芬·P·哈贝尔(Stephen P.Hubbell)最近的一项理论断言了相反的观点：生态群落中的个体表现得就像他们是完全相同的，一个物种只是因为纯粹随机的偶然事件而变得“稀有”或“常见”。这一理论在描述许多自然系统中稀有和丰富的模式方面取得了显著的成功。然而，即使是哈贝尔本人也不会声称，支撑他的理论的“生态等价性假设”是对物种如何相互作用的忠实描述。问题是：为什么自然系统的行为就好像它们的组成部分在生态上是对等的，而事实并非如此？这个项目将通过研究不同物种的个体在生态上不等同的模型来解决这个问题。虽然以前也研究过类似的模型，但它们非常复杂，无法用数学方法求解，这使得很难将它们与数据进行比较。相比之下，哈贝尔的模型相对简单，其性质可以非常详细地研究。然而，通过研究仅与Hubbell模型略有不同的模型，我们可以使用Hubbell模型的精确解来计算我们模型的预测与他的预测有何不同。这种借助于精确可解模型来研究不可解模型的方法，是应用数学中的一种行之有效的做法。我们将调查竞争能力和物种对不同栖息地偏好的差异如何影响生物多样性的格局。这将向我们展示，这些过程需要多么强大，才能产生与哈贝尔模型明显不同的预测。我们将使用我们的模型来分析热带森林和珊瑚礁等高度多样化系统的数据。通过这种方式，我们将衡量和区分使这些系统如此多样化的不同过程。

Ecologists have long puzzled over the mechanisms that maintain biological diversity - that permit so many natural species to live alongside others with which they compete for resources. Many biologists would maintain that species coexist by exploiting their environment in different ways, and patterns of abundance and rarity reflect the distinct roles played by different species in the community. This view was supported by Charles Darwin, who wrote in 1859 `When we look at the plants and bushes clothing an entangled bank, we are tempted to attribute their proportional numbers and kinds to what we call chance. But how false a view is this!' However, a recent theory by Stephen P. Hubbell asserts the opposite: that individuals in ecological communities behave as if they were exactly the same, and that a species is only `rare' or `common' because of purely random, chance events. This theory has been remarkably successful at describing the patterns of rarity and abundance in many natural systems. However, not even Hubbell himself would claim that the `assumption of ecological equivalence' that underpins his theory is a faithful description of how species interact. The question is: why do natural systems behave as if their constituents are ecologically equivalent, when this is not true? This project will address this question by studying models where individuals of different species are not ecologically equivalent. While similar models have been studied before, they are so complicated that they cannot be solved mathematically, which makes it difficult to compare them to data. By contrast, Hubbell's model is relatively simple, and its properties can be studied in great detail. However, by studying models that are only slightly different from Hubbell's, we can use the exact solution to Hubbell's model to calculate how the predictions of our models differ from his. This method of piggy-backing on an exactly solvable model to study an unsolvable one is a well-established practice in applied mathematics. We shall investigate how differences in competitive ability and in species' preferences for different habitat contribute to patterns of biodiversity. This will show us how strong these processes would need to be to produce noticeably different predictions from Hubbell's model. We shall use our models to analyse data for highly diverse systems such as tropical forests and coral reefs. In this way, we shall measure and distinguish between the different processes that make these systems so diverse.