Propagule pressure and functional diversity: interactive effects on a macroalgal invasion process

Invasive species are considered an increasing global threat to marine ecosystems. Understanding which factors can accelerate or reduce invasion success is one of the main goals of invasion ecology. The present study investigated the interactive effects of propagule pressure and native functional diversity on the invasion success of the invasive alga Sargassum muticum (Yendo) Fensholt over a 1 yr invasion process (i.e. settlement, recruitment and colonisation). The invasibility of macroalgal assemblages with different functional diversity was assessed under high and low propagule pressure. Synthetic assemblages resembling natural marine macroalgal assemblages were built and used as model systems. Recruitment discs were used to record early settlement in the assemblages. Recruitment and colonisation success were monitored in the field 2 and 10 mo after exposure to propagule rain in the laboratory. The availability of resources (space and photosynthetically active radiation [PAR]) was also monitored for each assemblage and used as predictive variables in the analyses. The effects of propagule pressure and functional diversity varied among the invasion stages. Space availability did not play a significant role in the recruitment success of the invader. However, PAR was a key resource in its invasion success. Overall, the invasion success of S. muticum was influenced differently by different traits of the recipient assemblages. Hence, invasion success may be better explained by the interaction of environmental factors that co-vary with species diversity and identity at different invasion stages.