Paleosymmetry: tipping points and the dynamics of range contractions

点击翻译按钮获取中文摘要

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.