Online extremist communities operate within a wider information ecosystem shaped by real-world events, news coverage, and cross-community interaction. We adopt a systems perspective to examine these influences using seven years of data from two ideologically distinct extremist forums (Stormfront and Incels) and a mainstream reference community (r/News). We ask three questions: how extremist violence impacts community behaviour; whether news coverage of political entities predicts shifts in conversation dynamics; and whether linguistic diffusion occurs between mainstream and extremist spaces and across extremist ideologies. Methodologically, we combine counterfactual synthesis to estimate event-level impacts with vector autoregression and Granger causality analyses to model ongoing relationships among news signals, behavioural outcomes, and cross-community language change. Across analyses, our results indicate that Stormfront and r/News appear to be more reactive to external stimuli, while Incels demonstrates less cross-community linguistic influence and less responsiveness to news and violent events. These findings underscore that extremist communities are not homogeneous, but differ in how tightly they are coupled to the surrounding information ecosystem.

翻译：线上极端主义社群在由现实事件、新闻报道及跨社群互动所塑造的更广泛信息生态系统中运作。本文采用系统视角，利用两个意识形态相异的极端主义论坛（Stormfront与Incels）及一个主流参照社群（r/News）的七年数据，考察这些影响因素。我们提出三个研究问题：极端主义暴力如何影响社群行为；政治实体的新闻报道是否预示对话动态的转变；以及语言扩散是否发生在主流与极端主义空间之间以及不同极端主义意识形态之间。在方法论上，我们结合反事实合成法以估计事件级别的影响，并采用向量自回归与格兰杰因果分析来建模新闻信号、行为结果及跨社群语言变化之间的持续关系。综合分析表明，Stormfront与r/News对外部刺激表现出更强的反应性，而Incels则显示出较弱的跨社群语言影响及对新闻和暴力事件的反应性。这些发现强调，极端主义社群并非同质，其与周围信息生态系统的耦合紧密度存在差异。