Data from: Variation in the impact of non-native seaweeds along gradients of habitat degradation: a meta-analysis and an experimental test

Biological invasions are acknowledged among the main drivers of global changes in biodiversity. Despite compelling evidence of species interactions being strongly regulated by environmental conditions, there is a dearth of studies investigating how the effects of non-native species vary among areas exposed to different anthropogenic pressures. Focusing on marine macroalgae, we performed a meta-analysis to test whether and how the direction and magnitude of their effects on resident communities and species varies in relation to cumulative anthropogenic impact levels. The relationship between human impact levels and non-native species impact intensity emerged only for a reduced subset of the response variables examined. Yet, there was a trend for the effects of non-native species on community biomass and abundance and on species abundance to become less negative at heavily impacted sites. By contrast, the magnitude of negative effects of seaweed on community evenness tended to increase with human impact levels. The hypothesis of decreasing severity of invader’ impacts along a gradient of habitat degradation was also tested experimentally at a regional scale by comparing the effects of the removal of non-native alga, Caulerpa cylindracea, on resident assemblages among rocky reefs exposed to different anthropogenic pressures. Assemblages at urban and pristine site did not differ when invaded, but did so when C. cylindracea was removed. Our results suggest that, despite the generally weak relationship between human impacts levels and non-native species impacts, more negative impacts can be expected in less stressful environments (i.e. less degraded or pristine sites), where competitive interactions are presumably the driving force structuring resident communities. Implementing strategies for controlling the establishment of non-native seaweeds should be, thus, considered a priority for preserving biodiversity in relatively pristine areas. On the other hand, control of invaders at degraded sites could be warranted to lessen their role as propagule sources.

生物入侵（Biological invasions）已被公认为全球生物多样性变化的核心驱动因素之一。尽管已有确凿证据表明物种相互作用强烈受环境条件调控，但目前仍鲜有研究探讨外来物种的影响如何在受不同人为压力的区域间产生差异。本研究以海洋大型海藻（marine macroalgae）为研究对象，开展元分析（meta-analysis），旨在检验外来物种对本地群落及物种的影响方向与强度，是否随累积人为影响水平发生变化，以及具体的变化规律。

研究发现，人为影响水平与外来物种影响强度之间的相关性，仅在部分被检测的响应变量中得以显现。不过，在受人为干扰严重的区域，外来物种对群落生物量、丰度以及物种丰度的负面影响呈现出减弱的趋势。与之相对，大型海藻对群落均匀度的负面影响程度，则往往随人为影响水平升高而加剧。

本研究还在区域尺度开展了实验，以验证“外来入侵物种对栖息地的影响程度随栖息地退化梯度降低”这一假说：通过对比移除外来藻类杉叶蕨藻（Caulerpa cylindracea）后，不同受人为压力的岩礁上本地群落的响应差异。入侵状态下，城市区域与原始未受干扰区域的本地群落并无显著差异；但在移除杉叶蕨藻后，二者群落结构出现明显分化。

本研究结果表明，尽管人为影响水平与外来物种影响之间的整体相关性较弱，但在低压力环境（即退化程度更低或原始未受干扰的区域）中，外来物种的负面影响往往更强——这类区域中，种间竞争作用大概率是调控本地群落结构的核心驱动力。因此，在相对原始未受干扰的区域，制定管控外来大型海藻定植的策略，应被视为保护生物多样性的优先事项。另一方面，在退化生境中开展外来入侵物种管控工作，则可有效降低其作为繁殖体源的作用。