Demography and dispersal at a grass-shrub ecotone: A spatial integral projection model for woody plant encroachment

IF 7.1 1区 环境科学与生态学 Q1 ECOLOGY
Trevor Drees, Brad M. Ochocki, Scott L. Collins, Tom E. X. Miller
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引用次数: 0

Abstract

The encroachment of woody plants into grasslands is a global phenomenon with implications for biodiversity and ecosystem function. Understanding and predicting the pace of expansion and the underlying processes that control it are key challenges in the study and management of woody encroachment. Theory from spatial population biology predicts that the occurrence and speed of expansion should depend sensitively on the nature of conspecific density dependence. If fitness is maximized at the low-density encroachment edge, then shrub expansion should be “pulled” forward. However, encroaching shrubs have been shown to exhibit positive feedbacks, whereby shrub establishment modifies the environment in ways that facilitate further shrub recruitment and survival. In this case there may be a fitness cost to shrubs at low density causing expansion to be “pushed” from behind the leading edge. We studied the spatial dynamics of creosotebush (Larrea tridentata), which has a history of encroachment into Chihuahuan Desert grasslands over the past century. We used demographic data from observational censuses and seedling transplant experiments to test the strength and direction of density dependence in shrub fitness along a gradient of shrub density at the grass–shrub ecotone. We also used seed-drop experiments and wind data to construct a mechanistic seed-dispersal kernel, then connected demography and dispersal data within a spatial integral projection model (SIPM) to predict the dynamics of shrub expansion. Contrary to expectations based on potential for positive feedbacks, the shrub encroachment wave is “pulled” by maximum fitness at the low-density front. However, the predicted pace of expansion was strikingly slow (ca. 8 cm/year), and this prediction was supported by independent resurveys of the ecotone showing little to no change in the spatial extent of shrub cover over 12 years. Encroachment speed was acutely sensitive to seedling recruitment, suggesting that this population may be primed for pulses of expansion under conditions that are favorable for recruitment. Our integration of observations, experiments, and modeling reveals not only that this ecotone is effectively stalled under current conditions but also why that is so and how that may change as the environment changes.

草灌丛交错带的人口分布与扩散:木本植物入侵的空间积分投影模型
木本植物对草原的侵蚀是一个全球性的现象,对生物多样性和生态系统功能具有重要影响。了解和预测森林扩张的速度以及控制扩张的潜在过程是研究和管理森林入侵的关键挑战。空间种群生物学理论预测,种群扩张的发生和速度应敏感地取决于同比密度依赖的性质。如果适应度在低密度侵占边缘最大,则应将灌木扩张“向前拉”。然而,灌木的入侵表现出积极的反馈,即灌木的建立改变了环境,促进了进一步的灌木招募和生存。在这种情况下,低密度的灌木可能会有适应性成本,导致扩张从前缘后面被“推”出去。研究了近百年来入侵奇瓦瓦荒漠草原的木榴灌木(Larrea tridentata)的空间动态特征。利用观察性普查和幼苗移栽试验的人口统计学数据,研究了草-灌木交错带灌木适宜度在密度梯度上的密度依赖强度和方向。利用种子落实验和风数据构建了种子扩散机制核,并利用空间积分投影模型(SIPM)将人口统计学和传播数据联系起来,预测了灌木扩张的动态。与基于正反馈潜力的预期相反,灌木入侵波被低密度前沿的最大适应度“拉动”。然而,预测的扩张速度非常缓慢(约8 cm/年),这一预测得到了过渡带独立调查的支持,调查显示12年来灌木覆盖的空间范围几乎没有变化。侵吞速度对苗期招募极为敏感,表明该种群可能在有利于苗期招募的条件下准备了扩张的脉冲。我们将观察、实验和建模结合起来,不仅揭示了这种过渡带在当前条件下实际上是停滞不前的,而且还揭示了为什么会这样,以及随着环境的变化,这种情况会如何变化。
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来源期刊
Ecological Monographs
Ecological Monographs 环境科学-生态学
CiteScore
12.20
自引率
0.00%
发文量
61
审稿时长
3 months
期刊介绍: The vision for Ecological Monographs is that it should be the place for publishing integrative, synthetic papers that elaborate new directions for the field of ecology. Original Research Papers published in Ecological Monographs will continue to document complex observational, experimental, or theoretical studies that by their very integrated nature defy dissolution into shorter publications focused on a single topic or message. Reviews will be comprehensive and synthetic papers that establish new benchmarks in the field, define directions for future research, contribute to fundamental understanding of ecological principles, and derive principles for ecological management in its broadest sense (including, but not limited to: conservation, mitigation, restoration, and pro-active protection of the environment). Reviews should reflect the full development of a topic and encompass relevant natural history, observational and experimental data, analyses, models, and theory. Reviews published in Ecological Monographs should further blur the boundaries between “basic” and “applied” ecology. Concepts and Synthesis papers will conceptually advance the field of ecology. These papers are expected to go well beyond works being reviewed and include discussion of new directions, new syntheses, and resolutions of old questions. In this world of rapid scientific advancement and never-ending environmental change, there needs to be room for the thoughtful integration of scientific ideas, data, and concepts that feeds the mind and guides the development of the maturing science of ecology. Ecological Monographs provides that room, with an expansive view to a sustainable future.
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