Facilitation of the introduced green alga Caulerpa racemosa by resident algal turfs: experimental evaluation of underlying mechanisms

Abstract

There is convincing evidence showing that resistance to invasion greatly varies among recipient assemblages. We assessed whether variation in the performance of the introduced macro- alga Caulerpa racemosa between algal turfs and surfaces dominated by encrusting corallines (barrens) arises from their physical or biological properties, by comparing the invasion success of this alga between natural assemblages and their mimics. Furthermore, to establish whether greater retention of sediments by algal turfs can favour C. racemosa, sediment deposition in barrens was raised to levels matching those recorded in turfs. The effects of herbivores on patterns of distribution of C. racemosa between habitats were evaluated by removing urchins from some experimental patches. The performance of C. racemosa was greater in turfs than in barrens, regardless of the type of substratum (natural or artificial), suggesting that positive effects of turfs on C. racemosa are phys- ical (e.g. entrapment of fragments of the alga and/or enhancing the anchoring of stolons). Removing urchins or enhancing the deposition of sediments in barrens had no effect on C. racemosa. Our results show that the complexity of biogenic substrata is key to determining the spread of this invader and that facilitation of exotic species by natives could enhance ecosystem invasibility. This study has important implications for the management of C. racemosa, as turfs are progressively replacing canopy-forming species along urbanised coasts. Also, it suggests that current invasion theories based on the assumption that interactions between native and exotic species are exclusively negative may be inadequate to predict future invasion scenarios.