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

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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.SynthesisLocal anthropogenic stressors that severely alter biotic and abiotic conditions may underpin context‐dependency in the impacts of biological invasions. We used a meta‐analysis and an experimental test to examine the relationship between cumulative human impacts and ecological impact of non‐native seaweeds on resident assemblages. Our results suggest that more negative impacts of non‐native seaweeds on the abundance and biomass of resident assemblages can be expected in less degraded or pristine sites. Possibly, stronger impacts prevail at pristine sites, where assemblages are mainly structured by biotic interactions. Hence, management efforts should be mostly directed to prevent the establishment and spread of non‐native seaweeds in pristine areas. On the other hand, weak, but positive effects of seaweeds at the most degraded sites add to the ongoing debate on the role of non‐native species in rehabilitation plans.