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 macroalga 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 physical (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

Synergism between two anthropic impacts: Caulerpa racemosa var. cylindracea invasion and seawater nutrient enrichment

“…For example, sediment traits of urban wastewater impacted seabed are described to enhance the competitiveness of C. taxifolia when it is in contact with Mediterranean seagrasses (Chisolm et al 1995, Ceccherelli & Cinelli 1997. Changes in the structure of assemblages following nutrient enrichment and in particular the increase of Womersleyella seta cea, suggest further positive feedback between im pacts, as the presence of algal turfs can favor colonization by C. racemosa , Piazzi et al 2003, Bulleri & Benedetti-Cecchi 2008. Different roles, as passenger or driver of ecosystem modifications (MacDougall & Turkington 2005), have been attributed to C. racemosa ) and the importance of nutrient enrichment in this context could be an interesting topic for further investigation.…”

The seaweed Caulerpa racemosa on Mediterranean rocky reefs: from passenger to driver of ecological change

“…For instance, positive feedbacks through the alteration of nutrient dynamics have been identified as important mechanisms enhancing the persistence of exotic terrestrial plants (Allison andVitousek 2004, Levine et al 2006). Here, C. racemosa, altering the depositional environment, would favor the dominance by stress-tolerant filamentous forms that compose algal turfs (Airoldi 2003, Balata et al 2008; these, in turn, can facilitate the persistence and spread of C. racemosa by providing safe anchoring to its prostrate stolons (Bulleri and Benedetti-Cecchi 2008).…”

“…We predicted that enhanced rates of sedimentation, an important agent of change in coastal environments (Airoldi 2003), would produce either a degradation of extant assemblages or prevent the recovery of canopy-forming species after disruption from other disturbances (e.g., sea storms), ultimately leading to the domination by algal turfs and C. racemosa (Bulleri and Benedetti-Cecchi 2008). In contrast, if C. racemosa is the main driver of change, we predicted that its removal would trigger a shift in the dominance from algal turfs to erect or canopy-forming species at degraded sites.…”

Photosynthesis and daily metabolic carbon balance of the invasive <i>Caulerpa racemosa</i> var. <i>cylindracea</i> (Chlorophyta: Caulerpales) along a depth gradient

“…Much more research is necessary to determine which factor(s) are responsible for uncoupling C. racemosa net productivity and biomass. In particular, attention must be paid to the multiple environmental (abiotic and biotic) factors that can influence vertical patterns of algal abundance (Piazzi et al, 2001;Ruitton et al, 2005Ruitton et al, , 2006Bulleri and Benedetti-Cecchi, 2008;Cebrian and Ballesteros, 2009;Klein andVerlaque, 2008, 2009;Tomas et al, 2011), as well as the annual carbon balance of the algae.…”