Data from: A demographic ménage à trois: interactions between disturbances both amplify and dampen population dynamics of an endemic plant

1.Natural and anthropogenic disturbances co-occur in most systems, but how they interact to shape demographic outcomes remains poorly understood. Such interactions may alter dynamics of populations in non-additive ways, making demographic predictions challenging when focusing on only one disturbance. Thus, understanding the interactive effects of such disturbances is critically important to determine the population viability of most species under a diversity of stressors.

2.We used a hierarchical Integral Projection Model (IPM), parameterized with 13 years of field data across 20 populations, encompassing 2,435 individuals of an endangered herb, Liatris ohlingerae. We examined interactive effects of vertebrate herbivory, fire, and anthropogenic activities (sand roads) on vital rates (e.g. survival, growth, reproduction, recruitment), and ultimately on population growth rates (λ), to test the hypothesis that interactions amplify or dampen differences in λ depending on environmental contexts. We constructed mega-matrices to determine coupled dynamics in individuals damaged vs. not damaged by herbivores in roadsides and in Florida scrub with different times-since-fire.

3.We identified strong interactive effects of fire with herbivory and habitat with herbivory on vital rates and on population growth rates in the IPM model. We also found different patterns of variation in λ between habitat and time-since-fire scenarios; population growth rates were higher in roadside populations compared to scrub populations, and declined with increasing time-since-fire. Herbivory had interactive effects with both fire and human disturbances on λ. Herbivory resulted in decreased differences in λ due to anthropogenic disturbance and slightly increased differences in λ due to time-since-fire.

4.Synthesis. The co-occurrence of various disturbances may both amplify and dampen the effects of other disturbances on population growth rate, thus shaping complex population dynamics that are neither linear nor additive. These realistic non-linearities represent challenges in understanding and projecting of population dynamics. Here, we examined the effects of various sources of disturbance on the population dynamics of an endangered plant species, finding complex interactions affecting population growth rates. We argue that integration of multiple, interacting stressors in IPMs will allow more accurate estimation of the overall effects of ecological processes on species viability.

1. 多数生态系统中自然扰动与人为扰动往往同时存在，但二者如何交互作用以塑造种群统计结局，目前仍未得到充分阐明。这类交互作用可能以非加和方式改变种群动态，仅聚焦单一扰动时，种群统计预测将极具挑战性。因此，明晰这类扰动的交互效应，对于评估多种胁迫因子下多数物种的种群生存力至关重要。
2. 本研究采用分层积分投影模型（Integral Projection Model, IPM），以覆盖20个种群、共计2435株濒危草本植物Liatris ohlingerae的13年野外调查数据对模型进行参数化。我们检验了脊椎动物啃食、火烧与人为活动（砂石道路）对生命率（如存活率、生长率、繁殖力、种群补充率）以及最终种群增长率（λ）的交互效应，以验证“扰动交互作用会依据环境背景放大或削弱λ的差异”这一假说。此外，我们构建了巨型矩阵，以解析不同火烧后时间梯度下的路边与佛罗里达灌丛生境中，受与未受植食动物侵害的个体的耦合动态。
3. 本研究在IPM模型中发现，火烧与植食作用、生境与植食作用之间存在显著的交互效应，均对生命率与种群增长率产生影响。我们还发现，不同生境与火烧后时间情境下的λ变异模式存在差异：路边种群的种群增长率高于灌丛种群，且随火烧后时间延长而降低。植食作用与火烧、人为干扰均对λ存在交互效应；植食作用会降低由人为扰动导致的λ差异，同时小幅提升由火烧后时间差异导致的λ差异。
4. 研究总结：多种扰动的共存，既可能放大也可能削弱其他扰动对种群增长率的影响，进而塑造既非线性亦非加和的复杂种群动态。这类符合现实的非线性关系，为种群动态的解析与预测带来了挑战。本研究针对一种濒危植物的种群动态，检验了各类扰动源的效应，发现存在影响种群增长率的复杂交互作用。我们认为，在IPM中整合多交互胁迫因子，将能够更准确地评估生态过程对物种生存力的整体效应。