Increasing and fluctuating resource availability enhances invasional meltdown

Exotic plant invaders can promote others via direct or indirect facilitation, known as “invasional meltdown”. Increased soil nutrients can also promote invaders by increasing their competitive impacts, but how this might affect meltdown is unknown. In a mesocosm experiment, we evaluated how eight exotic plant species and eight Eurasian native species responded individually to increasing densities of the invasive plant <i>Conyza canadensis</i>, while varying the supply and fluctuations of nutrients. We found that increasing density of <i>C. canadensis</i> intensified competitive suppression of natives but intensified facilitation of other exotics. Higher and fluctuating nutrients exacerbated the competitive effects on natives and facilitative effects on exotics. Overall, these results show a pronounced advantage of exotics over native target species with increased relative density of <i>C. canadensis</i> under high nutrient availability and fluctuation. We integrate these results with the observation that exotic species commonly drive increases in soil resources to suggest the Resource-driven Invasional Meltdown and Inhibition of Natives hypothesis (RIMIN) in which biotic acceleration of resource availability promotes other exotic species over native species, leading to invasional meltdown.

外来植物入侵种可通过直接或间接促进作用推动其他入侵种的扩散，这一现象被称为“入侵崩溃（invasional meltdown）”。土壤养分含量升高亦可通过增强入侵种的竞争效应来促进其扩散，但该过程如何影响入侵崩溃仍不明确。本研究通过中宇宙实验（mesocosm），探究了8种外来植物与8种欧亚本土植物分别对入侵植物加拿大一枝黄花（Conyza canadensis）密度升高的响应，同时设置了不同的养分供给水平与养分波动模式。研究结果显示，加拿大一枝黄花密度升高会加剧对本土植物的竞争抑制作用，同时增强对其他外来植物的促进效应。更高的养分含量与养分波动会进一步强化其对本土植物的竞争抑制，以及对其他外来植物的促进作用。综合来看，在高养分供给与养分波动的条件下，随着加拿大一枝黄花相对密度升高，外来植物相较于本土目标物种展现出显著的竞争优势。结合“外来物种通常会提升土壤资源含量”这一观测结果，本研究提出了资源介导的入侵崩溃与本土物种抑制假说（Resource-driven Invasional Meltdown and Inhibition of Natives hypothesis，简称RIMIN）：即生物作用加速的资源可获得性会优先促进其他外来植物的生长，压制本土物种，最终引发入侵崩溃。