Interspecific facilitation is a widely recognized process structuring natural communities. Benthic invertebrates are known to facilitate other invertebrates but how the former affect macroalgae has been rarely investigated. In the present study we explore some of the mechanisms underpinning the facilitation of the kelp Undaria pinnatifida by the solitary ascidian Styela clava, two species introduced worldwide. First, we evaluated the relationship between habitat complexity and the recruitment of U. pinnatifida in natural assemblages using the height and density of S. clava as proxies of habitat complexity. Second, we experimentally assessed the effect of habitat complexity using two sizes of mimics of S. clava arranged at two density levels in a crossed-factor design. Finally, we tested whether the biologic effect of S. clava was more important than its physical effect, by comparing U. pinnatifida’s recruitment on live S. clava and on the mimics. We observed a positive effect of height on U. pinnatifida’s recruitment in natural assemblages as well as through the experimental approach using the mimics. In addition, an effect of density and the interaction between density and height was observed in natural assemblages exclusively. We also found a strong positive effect due to biological features, as U. pinnatifida recruited more than ten times more on live S. clava than on the mimics. Our results indicate that a biologic non-trophic effect plays a more important role in this facilitation process than the structure alone.

种间促进是一个被广泛认可的构建自然群落的过程。已知底栖无脊椎动物可以促进其他无脊椎动物，但很少研究前者如何影响大型藻类。在本研究中，我们探讨了支持海带Undaria pinnatifida由孤立的海鞘Styela clava促进的一些机制，这两个物种在世界范围内引入。首先，我们使用S. clava的高度和密度作为栖息地复杂性的代理来评估栖息地复杂性与自然组合中U. pinnatifida的补充之间的关系。其次，我们使用两种尺寸的S. clava模拟物实验性地评估了栖息地复杂性的影响在交叉因子设计中以两个密度水平排列。最后，我们通过比较U. pinnatifida在活S. clava和模拟物上的招募来测试S. clava的生物学效应是否比其物理效应更重要。我们观察到高度对U. pinnatifida在自然组合中的募集以及通过使用模拟物的实验方法产生积极影响。此外，仅在自然组合中观察到密度的影响以及密度与高度之间的相互作用。我们还发现了一个强大的积极影响，由于生物学特性，如U.菜招聘了超过十倍以上的活S.鞘比在模仿。我们的结果表明，与单独的结构相比，生物非营养效应在这一促进过程中起着更重要的作用。