DISSERTATION RESEARCH: Why Do Clades Lose Diversity? Aplodontoid Rodents as a Case Study in the Decline of Higher Taxa

这个项目研究物种灭绝的原因。这项研究的重点是了解一组相关啮齿动物物种的多样化和衰落的细节，这些啮齿动物被称为山狸或“山狸”。这组相关物种(称为谱系或支系)可以被用作一个模型，以了解物种多样化和灭绝的一般过程。2500万年前，仿齿目动物非常丰富和多样，但后来减少到单一的活物种，现在受到太平洋西北部沿海森林变化的威胁。这项研究将集中于了解这一类哺乳动物在其3500万年的历史中的生态。单个物种的生态作用将通过骨骼形态的测量(表明它们如何移动和如何利用环境)、通过对其牙齿的磨损研究(表明所吃食物的指示器)以及通过对其牙齿釉质的同位素组成的研究(表明水的使用模式和所吃的特定类型的植物)来重建。这些生态数据将被绘制在物种的分支图上，以了解无倍体动物的生态进化路径。构建这个分支图谱需要对尽可能多的非倍体物种进行深入的研究。重建这条生态进化的道路，再加上我们对过去环境变化的现有知识，将使我们能够了解导致这一群体濒临灭绝的生态变化。由于如此多的物种和更大的种群受到栖息地变化和气候变化的威胁，了解是什么导致生物可能灭绝是至关重要的。了解导致这一生物群体衰落的因素将使人们能够认识到人类可能对其他物种产生不利影响的最重要的机制。该项目使用的数据来自远在人类开始影响世界动物群变化之前就存在的化石。早在400万年前，早在人类来到北美之前，非齿目恐龙就已经达到了目前最低的多样性水平。因此，它提供了一个“自然”(非人类)原因导致的世系衰落的案例研究。这将有助于认识到作为自然进化过程的一部分而衰落的谱系与被相对较新的人类影响而被驱使灭绝的谱系之间的区别。了解如何认识和减轻人类造成的物种灭绝，对于我们在人类活动日益增多的情况下继续努力维持健康的环境至关重要。

This project studies why species go extinct. The study focuses on understanding the details of diversification and decline of a group of related rodent species, known as aplodontids or "mountain beavers". This group of related species (called a lineage or clade) can be used as a model to understand the processes of species diversification and extinction in general. Aplodontids were very abundant and diverse 25 million years ago, but have since declined to a single living species, which now is threatened by alteration of coastal forests in the Pacific Northwest. This study will focus on understanding the ecology of members of this group of mammals throughout their 35 million year history. The ecological roles of individual species will be reconstructed from measurements of skeletal morphology (which indicate how they moved and how they used their environment), from study of the wear on their teeth (an indicator of foods eaten), and from studies of the isotopic composition of their tooth enamel (indicative of patterns of water use and of the particular types of plants eaten). These ecological data will be plotted on a cladogram, or "family tree" of species, in order to understand the path of evolution of ecology in aplodontoids. The construction of this cladogram requires intensive study of as many aplodontoid species as possible. Reconstructing this path of ecological evolution, coupled with our existing knowledge of past environmental changes, will enable us to understand the ecological changes that led this group to the brink of extinction. With so many species and larger groups threatened by habitat alteration and climate change, it is essential to understand what makes organisms likely to go extinct. An understanding of factors that led this one group of organisms to decline will enable recognition of the most important mechanisms by which humans may adversely affect other species. This project uses data from fossils that lived long before humans began to influence world faunal change. The aplodontid lineage had already reached its current low level of diversity by 4 million years ago, well before humans even came to North America. Thus, it provides a case study of lineage decline due to "natural" (non-human) causes. This will help in recognizing the difference between a lineage declining as part of the natural evolutionary process and one being driven to extinction by the relatively recent influence of humans. An understanding of how to recognize and mitigate human-caused extinctions is vital to our continuing efforts to maintain a healthy environment in the face of increasing human activity.