Role of interspecific interactions in biological invasions: new evidence from the Mediterranean Sea

Despite the field of invasion ecology has rapidly grown, the mechanisms behind the invasion success of alien species and the biotic resistance to invasions still need a thorough understanding. Comprehending the response of resident assemblages to biological invasions and the possible impacts produced by alien species introduction is, therefore, crucial to expand the current knowledge of the invasion process in the recipient systems. In this thesis different aspects of biological invasion process have been critically examined emphasizing the importance and complexity of the interactions between alien species and native biota, in the Mediterranean marine coastal ecosystems. Here, five different invaders were used as model organisms: two seaweed species, Caulerpa cylindracea and Caulerpa taxifolia var. distichophylla, the crab Percnon gibbesi and two rabbitfish species, Siganus rivulatus and Siganus luridus. Firstly, in Chapter 1 a general overview of the invasion processes was provided. The following two chapters were focused on the interactions between the generalist native herbivore, Paracentrotus lividus, and the two co-occurring alien seaweeds C. cylindracea and C. taxifolia var. distichophylla. Specifically, in Chapter 2 the feeding behaviour of P. lividus was investigated in order to predict the potential vulnerability of the two alien seaweeds (alone and intermingled) to sea urchin grazing and the possible biological control exerted by the native herbivore on the two seaweed populations. The results from feeding choice experiments showed that P. lividus is able to incorporate in its diet the two co-occuring alien seaweeds suggesting a potential capacity of the sea urchin to control their expansion in the invaded area. Chapter 3 aimed to investigate the potential physiological stress induced on P. lividus by the consumption of the two alien seaweeds, C. cylindracea and C. taxifolia var. distichophylla. Based on the presence of chemically distinct toxic metabolites in the two invaders, a synergistic effect of toxic metabolites on the sea urchin righting behaviour was predicted, when the two algae were consumed simultaneously. The findings showed a significant increase of P. lividus righting time as a consequence of the simultaneous consumption of the two alien seaweeds, suggesting that synergistic interactions among the two invaders can enhance their likelihood to survive. Reduction in sea urchin performance may have negative implications for their overall grazing activity and, ultimately, for their ability to control the expansion of alien seaweeds in the invaded area. More generally, it is well-known that native-alien species interactions may influence the invasion process favoring or hindering the establishment and spread of alien species. As predicted by theory, the success of alien species can be restricted by interspecific interactions, such as competition and predation, from resident species. Therefore, pristine communities, with high species richness and diversity, are less prone to invasions than those already degraded by human activities. In this regard, Marine Protected Areas (MPAs), supporting a greater number of species than unprotected areas, are expected to limit the successful establishment of undesirable invaders. In Chapter 4 it was tested if the high native diversity observed within MPAs can provide biotic resistance to the invasion of the alien crab P. gibbesi compared to fished areas. Results revealed a negative relationship between the diversity of native communities and the invasiveness of the alien crab. Specifically, it was shown that MPAs, as protected systems with high native species richness and diversity, may hinder the spread of the high invasive crab P. gibbesi. Finally, evidence from the Mediterranean Sea showed that the invasion of herbivorous fishes may induce phase shifts in the recipient communities. In the last chapter (Chapter 5) the relationship between the biomass of two alien herbivorous fishes (S. luridus and S. rivulatus) and the cover of barren was investigated in sublittoral rocky reefs across the Aegean Sea. Results showed that the biomass of the two alien fishes was positively correlated to the barren cover reinforcing the idea that these invaders are able to induce shifts from diverse and productive algal forest to the low-complex barren state, affecting the structure and composition of the shallow rocky reefs in the Mediterranean Sea. All findings of this thesis have interesting implications for the potential mechanisms of invasion process and contribute to the increase of knowledge on the invasion dynamics, reinforcing the idea that biotic interactions are among the main drivers affecting alien species invasion process.