Habitat colonisation by amphipods: comparison between native and alien algae.
Invasion of algal habitat by the alien alga Caulerpa racemosa results in changes to habitat structure, part of which may be due to accumulation of large amounts of detritus. In turn, such changes lead to alterations in the assemblage structure of associated invertebrates. The present study was aimed at testing the hypotheses that (i) invasion by C. racemosa will influence the assemblage structure of amphipods associated with vegetated habitats, since colonisation by the invertebrates will differ between native algal habitat and habitat invaded by the alien alga, and (ii) differences in colonisation are mainly due to accumulation of detritus by the alien alga. To test these hypotheses, field experiments on habitat colonisation were undertaken at two sites within each of two Mediterranean localities (Malta and Spain). Using mesh bags, 3 different habitats were introduced at each site: habitat with the native alga Halopteris scoparia, habitat with C. racemosa, and an artificial habitat resembling algal habitat. Additionally, empty mesh bags were deployed as controls. Mesh bags containing plant material were defaunated prior to deployment. For each of the three habitat types, three supplement treatments were used: (i) no detritus, (ii) low detritus, and (ii) high detritus. The bags were placed on a rocky substratum at a water depth of 4.5-5 m for a period of 14 days, after which they were collected and the amphipod assemblage was assessed. The results showed that, overall, amphipod species richness and abundance were higher in bags containing H. scoparia but some amphipods exhibited a broad capability to colonize new habitat such as C. racemosa. Accumulation of detritus by the alien alga may play an important role but small-scale physical or biological habitat features can affect its ecological role.