Unravelling facilitation among introduced species, a mechanistic approach.
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.