Paleosymmetry: tipping points and the dynamics of range contractions

气候、社会和陆地生态系统等复杂系统可能突然从一种状态转变为另一种状态。这些突然转变的开始被称为阈值或临界点。人们对寻找方法来测量和预测这些临界点非常感兴趣，以便预测一个临界点何时发生，或者系统是否处于临界点的过程中。理解引爆点的本质以及如何识别它们的一种方法是研究过去发生的那些。在最近对古生态学家的50个优先问题的汇编中，有两个问题涉及研究过去的临界点的必要性。这项提议的主要目的之一是首次将花粉颗粒对称性的变化作为气候和植被变化临界点的衡量标准。第二个目标是了解气候和火灾在加拿大西北部的黑白云杉和魁北克的白松的北部分布边界撤退中的共同作用。******气候的大变化会导致过去植物群落组成的重大变化，导致物种范围界限的变化和植被带的转移，如北方林木线的退缩。但是，即使在气候引起的丰度或分布的变化在化石记录中很明显之前，这个物种一定受到了一些压力。如果过去的压力——古压力——可以被测量，那么我们就可以确定气候变化第一次开始影响人口和社区的时间。换句话说，我们将有一个工具来确定气候和社区组成即将发生变化的临界点。我建议使用花粉化石对称性的变化作为古应力的指标，以推断过去的临界点。******一个人的成长和发育取决于其基因蓝图和环境之间的相互作用。许多生物体及其器官表现出两侧对称，即一半是另一半的镜像。随着个体的成长和发展，对称结构的偏离（例如，一只手臂比另一只手臂长）可能是由许多因素引起的，包括环境压力。这些偏离对称的现象被称为“波动不对称”（FA）。FA的变化可以作为临界点的指示。我将研究在过去已知的植被和气候发生重大变化的情况下，双边对称（看起来像米奇老鼠的豆子）的针叶树种化石花粉粒的FA变化，例如加拿大西北部的林木线退缩，魁北克白松的北部分布界限向南移动。这种用FA识别引爆点的新应用有可能应用于各种环境中的各种生物，不仅在气候中，而且在其他环境变量中都有引爆点的信号。

Complex systems such as climate, societies, and terrestrial ecosystems may change abruptly from one state to another. The start of these abrupt transitions is termed the threshold or tipping point. There is enormous interest in finding ways by which we can measure and anticipate these tipping points in order to predict when one will occur or if the system is in the process of tipping. One approach to understanding the nature of tipping points and how they might be identified is to study those that occurred in the past. In a recent compilation of 50 priority questions for paleoecologists, two questions concerned the need to study past tipping points. One of the primary aims of this proposal is to apply, for the first time, changes in the symmetry of pollen grains as a measure of tipping points in climate and vegetation change. The second aim is to understand the joint roles of climate and fire in the retreat of the northern distributional limit of black and white spruce in NW Canada and white pine in Quebec.******Large changes in climate can drive major changes in past plant community composition, lead to changes in the range limits of species, and shifting of vegetation zones, such as the retreat of northern tree line. But even before climate-induced changes in abundances or distributions are evident in fossil records, there must have been some stress imposed on the species. If that past stress – paleostress – could be measured, then we could determine when climate change first started to impact populations and communities. In other words, we would have a tool to identify the tipping point for impending shifts in climate and community composition. I propose to use changes in the symmetry of fossil pollen as an index of paleostress in order to infer past tipping points.******The growth and development of an individual depends on the interaction between its genetic blueprint and its environment. Many organisms and their organs exhibit bilateral symmetry, i.e. one half is a mirror image of the other half. As individuals grow and develop, deviations from symmetrical structures (one arm longer than another, for example) may be induced by a number of factors, including environmental stress. These deviations from symmetry are termed “fluctuating asymmetry” (FA). Changes in FA may serve as indications of a tipping point. I will study changes in FA of fossil pollen grains of conifer species that are bilaterally symmetrical (look like Mickey Mouse beanies) in past scenarios where vegetation and climate are known to have changed significantly, such as tree line retreat in NW Canada, and southward displacement of the northern distributional limit of white pine in Quebec. This novel application of FA for the identification of tipping points has the potential to be applied to diverse organisms in a wide variety of environments, and signal tipping points not only in climate, but in other environmental variables.