ORIGINAL PAPER Functional diversity and climate change: effects on the invasibility of macroalgal assemblages F. Vaz-Pinto • C. Olabarria • I. Gestoso • E. Cacabelos • M. Incera • F. Arenas Received: 18 November 2011 / Accepted: 3 January 2013 / Published online: 13 January 2013 Ó Springer Science+Business Media Dordrecht 2013 Abstract Climate-driven and biodiversity effects on the structure and functioning of ecosystems are increasingly studied as multiple stressors, which subsequently may influence species invasions. We used a mesocosm experiment to test how increases in temperature and CO 2 partial pressure (pCO 2 ) interact with functional diversity of resident macroalgal assemblages and affect the invasion success of the non-indigenous macroalga Sargassum muticum. Early settlement of S. muticum germlings was assessed in the laboratory under common environmental conditions across three monocultures and a polyculture of functional groups of native macroalgae, which had previously grown for 3 weeks under crossed treat- ments of temperature and pCO 2 . Functional diversity was a key driver shaping early settlement of the invader, with significant identity and richness effects: higher settlement occurred in low-diversity and low- stature assemblages, even after accounting for treat- ment biomass. Overall, early survivorship of settled germlings responded to an interaction of temperature and pCO 2 treatments, with survivorship enhanced in one treatment (high pCO 2 at ambient Temperature) after 3 days, and reduced in another (ambient pCO 2 at high Temperature) after 10 days, although size was enhanced in this same treatment. After 6 months in the field, legacy effects of laboratory treatments remained, with S. muticum reaching higher cover in most assemblages previously subjected to ambient pCO 2 , but ephemeral green algae appearing disproportion- ately after elevated-pCO 2 treatment. These results caution that invasion outcomes may change at multi- ple points in the life cycle under higher-CO 2 , higher- temperature conditions, in addition to supporting a role for intact, functionally diverse assemblages in limiting invader colonization. Keywords Multiple stressors Á Invasibility Á Functional diversity Á Sargassum muticum Á pCO 2 Á Temperature F. Vaz-Pinto (&) Á E. Cacabelos Á F. Arenas Laboratory of Coastal Biodiversity, CIIMAR, Centro Interdisciplinar de Investigac ¸a ˜o Marinha e Ambiental, Universidade do Porto, Rua dos Bragas 289, 4050-123 Porto, Portugal e-mail: f_vazpinto@yahoo.com F. Vaz-Pinto ICBAS, Instituto de Cie ˆncias Biome ´dicas Abel Salazar, Universidade do Porto, Rua de Jorge Viterbo Ferreira, 4050-313 Porto, Portugal C. Olabarria Á I. Gestoso Departamento de Ecologı ´a y Biologı ´a Animal, Facultad de Ciencias del Mar, Universidad de Vigo, 36310 Vigo, Spain Present Address: E. Cacabelos Centro Tecnolo ´gico del Mar, Fundacio ´n CETMAR, c/Eduardo Cabello s/n, 36208 Vigo, Spain M. Incera Centro Tecnolo ´gico del Mar, Fundacio ´n CETMAR, c/Eduardo Cabello s/n, 36208 Vigo, Spain 123 Biol Invasions (2013) 15:1833–1846 DOI 10.1007/s10530-013-0412-4