Colonization success of marine non-indigenous species: Monitoring and manipulative approaches

Non-indigenous species (NIS) are considered a global threat to ecosystems with both ecological and economic impacts. Several factors have powerful influences on the bioinvasion process, including physical-chemical constraints and alien-native species interactions. In the marine environment, humans can mediate biological invasions through several vectors like navigation, aquaculture, and artificial canals and ports are considered hotspot areas of NIS introduction. In the Mediterranean Sea, more than seven hundred multicellular NIS have already been recorded, but this number can be underestimated because monitoring surveys for NIS have not yet been carried out on regular basis. This Ph.D. thesis shows the results of a three-year project focused on the assessment of: fouling community composition in areas affected by different vectors of NIS introduction: shipping (by commercial, passenger and recreational vessels) and aquaculture (Aim 1); the “biotic resistance” hypothesis in fouling communities, i.e. the comparative success of NIS colonization in pristine communities (Aim 2); and the control by predators on NIS in fouling assemblages (Aim 3). A three-year monitoring in target sites (aim 1) was done to: i) identify how each port or aquaculture site is interested by NIS colonization, ii) evaluate changes in early stages of fouling community along three years (2018-2020) and iii) detect new NIS at early stages of colonization. The monitoring programme started in July 2018 in the Gulf of La Spezia (Ligurian Sea, Italy), following a standard protocol with PVC panels. A total of 79 taxa were identified, including 11 NIS. In comparison with previous surveys, new NIS arrivals were observed in the Gulf of La Spezia. This first application of the protocol in the Mediterranean Sea demonstrates both pros and cons. The second aim of this project featured the testing of the “biotic resistance” hypothesis in fouling communities. The capability of fouling communities growing in a Ligurian Natural Park to act as “natural barrier” against NIS introduction and settlement was investigated through a transplant experiment performed in 2019. The same experimental design was repeated in 2020 in the province of Leghorn. Differences between conditions (transplanted and not-transplanted communities) were statistically significant for the variables “NIS/native species ratio” and “NIS number”. Moreover, the multivariate analyses on the community structure highlighted the importance of some NIS and native species in the differentiation of conditions. Our results indicate a signal of biotic resistance, but further studies should be conducted to clarify species interactions in harbor communities. The third aim of this Ph.D. project addressed the effect of predators on the population control of fouling NIS grown on experimental artificial substrates. The caging experiment of predator exclusion was set up in a site inside the Gulf of La Spezia in August. Twenty-one PVC panels were employed in this experiment. The panels were divided in three experimental conditions (seven per condition): “control panels”, “half-caged panels” and “caged panels”, in order to assess the predation effect on the fouling assemblages growing onto the panels. Predation effect was observed for two investigated variables and analysing the whole fouling assemblages through multivariate analyses. This Ph.D. project has studied, by means of standard monitoring methodology, robust and valuable information on invasion biology of some components of the fouling communities of urbanized coastal sites. Furthermore, it has been conducted manipulative ecological experiments, which can shed new light on some mechanisms underlying the process of invasion, that is still underway in the Mediterranean Sea. Many questions are still open and need further efforts to be properly understood, but the results here showed could pave the way for future research.