Safe harbours for the intruders of the sea: Greater abundance of invasive species on artificial structures and management implications

• Artificial structures (AS) and natural reefs (NRs) have similar richness of invasives. • AS support significantly higher abundance of invasive species than NRs. • Inadequate study designs and scarce data create persistent knowledge gaps. • Anticipating invasions through planning and monitoring is the best strategy. Artificial structures, both coastal (e.g. piers, seawalls) and offshore (e.g. platforms), have been hypothesised to support greater abundance and diversity of invasive species, including introduced species that have established and spread or have the potential to do so, compared with natural reefs. Despite this widespread assumption, no systematic evaluation has been conducted, leaving a critical knowledge gap regarding the ecological consequences of expanding artificial infrastructure. To address this gap, a systematic review and meta-analysis – comparing abundance, richness and cover of invasive species between artificial structures and natural reefs – were conducted. The influence of moderators on effect sizes – including distance to ports, shores and natural reefs, artificial structure category, size, age, material and invasive species traits – was also assessed. Artificial structures, on average, harboured more abundant – but not more diverse – populations of invasive species. Structural and spatial variables further influenced outcomes, with effect sizes generally small despite being statistically significant. Artificial structures located farther from ports and shore exhibited greater differences from natural reefs, and those constructed from concrete or metal supported higher invasive species abundance. Contributing factors likely include proximity to invasion points (e.g. harbours), structural characteristics of the infrastructure, and competitive traits of invasive species. Given the rapid global expansion of artificial structures, particularly in coastal environments, risk-reduction strategies are essential. This study provides management-relevant recommendations to support risk-aware planning, ecological design and adaptive management of marine artificial infrastructure, thereby mitigating invasion risk and guiding sustainable policy and practice.