The number and distribution of non-indigenous species in coastal habitats is increasing. Our ability to prioritise the management of this threat is limited by our understanding of their impacts. We investigated the density dependent effects of the non-indigenous solitary ascidian Pyura doppelgangera on native mussels and rocky shore communities in northern New Zealand. Minimal recruitment of P. doppelgangera was recorded during a 1.5-year experiment. Mussels showed no sign of overgrowth or spatial competition with P. doppelgangera, and their physiological condition was not impacted. We found marginal effects of the ascidian on community development, associated with small increases in diversity. We concluded that P. doppelgangera is not an aggressive competitor nor a threat to native communities, as previously thought, and that it has a very limited natural recruitment and spread potential. Reports from local Māori and a literature review suggest that P. doppelgangera has been present in the area for longer than previously thought, raising questions about its ‘introduction’ status and its current designation as a pest.

沿海生境中非本地物种的数量和分布正在增加。我们对威胁的优先级进行管理的能力受到我们对威胁影响的理解的限制。我们调查了新西兰北部非本土孤海Pyura doppelgangera对当地贻贝和多岩石的海岸社区的密度依赖性影响。在1.5年的实验过程中，仅记录了最小程度的P. doppelgangera募集。贻贝没有显示出与P. doppelgangera过度生长或空间竞争的迹象，并且它们的生理状况也没有受到影响。我们发现海鞘对社区发展的边际影响与多样性的少量增加有关。我们得出结论，P。doppelgangera正如先前所认为的那样，它既不是侵略性竞争者，也不是对土著社区的威胁，它具有非常有限的自然招募和传播潜力。当地毛利人的报告和文献综述表明，该地区存在的P. doppelgangera比以前想象的要长，这引起了人们对其“引种”状态及其当前被指定为害虫的疑问。