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

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.
We used a hierarchical integral projection model (IPM), parameterized with 13 years of field data across 20 populations, encompassing 2435 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 megamatrices to determine coupled dynamics in individuals damaged vs. not damaged by herbivores in roadsides and in Florida scrub with different times since fire.
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.
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 nonlinearities 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.

多数生态系统中自然干扰与人为干扰并存，但二者如何交互作用以塑造种群统计结果，目前仍缺乏充分认知。这类交互作用可能以非加性方式改变种群动态，若仅聚焦单一干扰开展研究，种群统计预测将极具挑战性。因此，明确这类干扰的交互效应，对于评估多数物种在多种胁迫因子下的种群生存力至关重要。
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株濒危草本奥氏蛇鞭菊（Liatris ohlingerae）的13年野外调查数据对模型进行参数化。我们探究了脊椎动物啃食、火灾与人为活动（沙质道路）对种群生命率（vital rates，包括存活、生长、繁殖与种群补充）的交互作用，并最终分析其对种群增长率（λ）的影响，以验证如下假说：干扰交互作用会依据环境背景，放大或削弱种群增长率间的差异。我们构建了巨型矩阵（megamatrices），以分析路边与佛罗里达灌丛中不同火烧后恢复时长的样地内，受啃食与未受啃食个体的耦合动态。
本研究在IPM模型中发现，火灾与啃食、生境类型与啃食之间均存在显著的交互作用，对种群生命率与种群增长率均产生显著影响。我们还发现，不同生境与火烧后恢复时长情境下的λ变异模式存在差异：路边种群的增长率显著高于灌丛种群，且增长率随火烧后恢复时长的增加而下降。啃食与火灾、人为干扰均对λ存在交互效应：啃食会降低由人为干扰导致的λ差异，同时小幅扩大由火烧后恢复时长差异导致的λ差异。
综合与结论：各类干扰的共存，既可能放大也可能削弱其他干扰对种群增长率的影响，进而塑造出既非线性亦非加性的复杂种群动态。这类真实存在的非线性关系，为种群动态的理解与预测带来了挑战。本研究针对一种濒危植物的种群动态，探究了各类干扰源的影响，发现了影响种群增长率的复杂交互作用。我们认为，在IPM模型中整合多种交互作用的胁迫因子，将能够更精准地估算生态过程对物种种群生存力的整体影响。