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

该项目研究物种灭绝的原因。 该研究的重点是了解一组相关啮齿动物物种（称为 aplodontids 或“山河狸”）的多样化和衰退的细节。 这组相关物种（称为谱系或进化枝）可以用作模型来了解物种多样化和灭绝的一般过程。 2500万年前，二齿象种类非常丰富且多样化，但此后已衰落为单一物种，目前正受到太平洋西北部沿海森林变化的威胁。 这项研究将重点了解该哺乳动物群体成员在其 3500 万年历史中的生态。各个物种的生态作用将通过对骨骼形态的测量（表明它们如何移动以及如何利用环境）、对牙齿磨损的研究（指示所吃食物的指标）以及对牙釉质同位素组成的研究（指示用水模式和所吃植物的特定类型）来重建。 这些生态数据将被绘制在物种的分支图或“家谱”上，以了解亚倍齿象的生态进化路径。 这个分支图的构建需要对尽可能多的亚倍齿象物种进行深入研究。 重建这条生态进化路径，再加上我们对过去环境变化的现有知识，将使我们能够理解导致这个群体濒临灭绝的生态变化。由于如此多的物种和更大的群体受到栖息地改变和气候变化的威胁，因此有必要了解是什么导致生物体可能灭绝。 了解导致这一类生物体衰退的因素将使我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机制。该项目使用的化石数据早在人类开始影响世界动物区系变化之前就已存在。 早在 400 万年前，远早于人类来到北美之前，阿倍齿象谱系就已经达到了目前的低水平多样性。 因此，它提供了由于“自然”（非人类）原因导致的谱系衰退的案例研究。 这将有助于认识作为自然进化过程一部分而衰落的谱系与因人类相对较新的影响而濒临灭绝的谱系之间的区别。 了解如何识别和减轻人为造成的灭绝对于我们在人类活动日益增加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.