• Caulerpa racemosa
• Disturbance
• Invasive macroalgae
• Meta-analysis

In the past few decades, the expansion of anthropogenic activities along coastal areas has produced devastating effects on shallow water systems worldwide. Here, canopy-forming macroalgae, which represent one of the most valuable marine habitats, have shown a marked reduction in their abundance, being increasingly replaced by stress-tolerant species such as those that compose algal turfs. The loss of native species often occurs concomitantly to the colonization of habitat by introduced macroalgae. Episodes of invasion are reported with increasing frequency and are mainly recorded along urbanised coasts, where human activities may facilitate the spread of invaders. Evidence suggests that invasive macroalgae are often not the primary cause of the loss of native canopy-forming species, but more likely a consequence of the decline of native species in degraded systems. Once established, however, invasive macroalgae may prevent the re-establishment of native assemblages, becoming drivers of further ecological change. The introduction of invasive macroalgae can, thus, alter the structure and functioning of natural ecosystems, with potential repercussions on their ability to provide goods and services to humans. Hence, assessing the impacts of invasive macroalgae on native assemblages has been recognised as a research priority in recent years.
The first part of this thesis presents a meta-analysis to provide a quantitative synthesis of the effects of invasive macroalgae on native consumers and primary producers (benthonic or epiphytic). The advantage of a meta-analysis over a review is that of providing a statistically rigorous method to quantitatively compare impacts across disparate studies and, thereby, assess if generalities of impacts exist over and beyond the large variability that characterize ecological experiments. Overall, invasive macroalgae appeared to have moderate to negative effects on consumers and benthic primary producers, whilst no significant effects were observed on epiphytes. Variables that were most affected were fitness and abundance of individual species rather than assemblage attributes. Results were, however, more variable and often weaker when analyses were done on individual species of invasive macroalgae. Although it is possible that different species of invasive macroalgae have different effects on native species, this result might be a consequence of the small number of studies available for each invader.
In accordance with previous meta-analyses, our results showed that there are major gaps in the actual knowledge of the effects of invasive macroalgae on native assemblages, highlighting the need of a greater effort into research on biological invasions. Major shortcomings in the current literature that need to be addressed in future research are (1) the limited number of experimental studies, which are necessary to establish cause-effect relationships, and the lack of proper procedural controls in several of them, (2) a general trend of poor reporting of useful information, such as standard errors, sample sizes or abundance of the invader, (3) the low number of introduced species considered and environmental conditions examined, which can influence species interactions.
The second part of the thesis presents an experimental field study aiming to investigate the mechanisms of colonization by the invasive macroalga Caulerpa racemosa and, particularly, whether the presence of native-canopy forming macroalgae can prevent colonization and/or the spread of this invader. Along the coast of Tuscany, C. racemosa is widespread on shallow subtidal rocky reefs, where it is replacing native canopies of Cystoseira spp. It has been observed, however, that Caulerpa has not established under native canopies which dominate pristine locations, such as the Island of Capraia, although Caulerpa is present outside the canopies. These might prevent the establishment of Caulerpa by occupying the primary substratum available for colonization and/or through shading or sweeping the substratum, preventing the settlement and growth of drifting fragments of Caulerpa. According to this model, Caulerpa would be able to grow and spread where canopies are partially or totally removed, but not where these are intact. The size of cleared patches would also likely influence the spread of Caulerpa, as larger patches provide more space available for propagules to settle and a greater un-shaded or abraded area. To test these hypotheses we experimentally manipulated Cystoseira canopies in a three-factorial experimental design. Cystoseira stands were left untouched or disturbed (factor 1), either by partially or totally removing the algae. Each of these treatments, was repeated in small and large plots (factor 2) to test the influence of the patches on the establishment of the invader. Caulerpa (factor 3) was transplanted in half of the plots of each size, while the other half was initially left free from the invader (i.e. if the invader was growing, we did not remove it).
Results suggest that the disturbance on Cystoseira canopies causes a shift to assemblages dominated by turf-forming species, but is not, by itself, sufficient to allow the settlement of the invader in a new pristine area. In plots where the invader was not initially present, there were, in fact, no effects of the treatments on Cystoseria on the cover of Caulerpa. Hence, Caulerpa was not able to take over space from native canopies, suggesting that the invader is probably not the primary cause of loss of Cystoseira stands. Once established (i.e. in invaded plots), however, Caulerpa was able to grow where canopies were disturbed, independently of the size of the plot. This suggests that intact canopies might act as a barrier against invasion, preventing the settlement of propagules and/or reducing the expansion of stolons of Caulerpa, while disturbance might favour the transition from canopy- to turf-dominated assemblages, facilitating the spread of invasive species.
Even if invasive macroalgae are not the primary cause of the loss of native canopies and degradation of habitat, the consequences of invasions might develop over long time scales and, among them, there are the homogenization of habitats and loss of biodiversity. Moreover, the effects of introduced macroalgae might increase in the future, in combination with other human stressors and climatic changes. Hence, further effort must be put into research for understanding the mechanisms leading to the spread of introduced macroalgae and the consequences for resident assemblages. This must be done taking into account the extreme variability of the effects of different invasive species and of the response of different recipient systems. Thus manipulative experiments must be done under contrasting environmental conditions and in combination with other stressors, which can be important effect-modifiers of the impacts of invasive macroalgae on extant assemblages.