Engineer species transform ecosystems due to their own growth, constitute an integral part of altered environments, and provide new habitats for other species, thus affecting biodiversity and the ecosystem. On rocky shores inside Antofagasta Bay (Northern Chile), the alien ascidian Pyura praeputialis, an engineer species, creates broad belts and dense 3-dimensional matrices that modify the intertidal habitat structure. In all, 116 species of macro-invertebrates and algae inhabit this habitat, compared with the 66 species inhabiting adjacent intertidal rocky shores which lack P. praeputialis. Of the 145 species recorded at the seascape scale (encompassing both mid-intertidal habitat), 55% were found exclusively in intertidal P. praeputialis matrices. Along the coastal gradient, patterns in β-diversity emerge due to the addition of a new set of species to the community inhabiting the P. praeputialis matrices and, to a lesser extent, from spatial turnover. We found differences in the shape of the species frequency distribution between the communities inhabiting the engineered and non-engineered mid-intertidal habitats. However, within the same habitat type, there was no difference in the species frequency distribution between functional groups. Occurrence of macro-algae was not affected by habitat type, but occurence of macro-invertebrates increased significantly in P. praeputialis matrices. P. praeputialis increases species richness at local and seascape scales by providing a novel mid-intertidal habitat which is used by mobile and vagile macro-invertebrates that otherwise would remain excluded from this intertidal level.KEY WORDS: Pyura praeputialis matrices · Ecosystem engineer · Intertidal seascapes · α-and β-diversity · Species frequency distribution · Northern ChileResale or republication not permitted without written consent of the publisher

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