The role of trophic interactions between fishes, sea urchins and algae in the northwest Mediterranean rocky infralittoral Bernat Hereu Fina Departament d’Ecologia Universitat de Barcelona 2004 Tesi Doctoral Universitat de Barcelona Facultat de Biologia – Departament d’Ecologia The role of trophic interactions between fishes, sea urchins and algae in the northwestern Mediterranean rocky infralittoral Memòria presentada per Bernat Hereu Fina per a optar al títol de Doctor en Biologia al Departament d’Ecologia, de la Universitat de Barcelona, sota la direcció dels doctors Mikel Zabala Limousin i Enric Sala Gamito Bernat Hereu Fina Barcelona, Febrer de 2004 El director de la Tesi El director de la Tesi Dr. Mikel Zabala Limousin Dr. Enric Sala Gamito Professor titular Assistant professor Facultat de Biologia Scripps Institution Oceanography Universitat de Barcelona University of California Contents Contents....................................................................................................................................................3 Chapter 1- General introduction..............................................................................................................1 The theoretical framework: trophic models.............................................................................................4 Trophic cascades: evidence of “top-down” control................................................................................6 Trophic cascades in marine systems........................................................................................................7 Quantitative models and the role of humans ...........................................................................................8 Box 1: The ‘algae-sea urchin-fish’ triangle in the Mediterranean..........................................................9 An overview of the challenges ahead.....................................................................................................17 Objectives and the structure of the thesis ..............................................................................................19 Box 2: The study area............................................................................................................................28 Chapter 2- Multiple controls of community structure in a sublittoral marine environment............35 Introduction ...........................................................................................................................................38 Material and methods............................................................................................................................40 Results....................................................................................................................................................46 Discussion..............................................................................................................................................62 Appendix................................................................................................................................................70 Chapter 3- The effects of sea urchins and fish in depleting palatable algae on a Mediterranean subtidal community .................................................................................................................................73 Abstract..................................................................................................................................................75 Introduction ...........................................................................................................................................75 Material and methods............................................................................................................................77 Results....................................................................................................................................................82 Discussion..............................................................................................................................................85 Chapter 4- Temporal and spatial variability in settlement of the sea urchin Paracentrotus lividus in the NW Mediterranean ...........................................................................................................................89 Abstract..................................................................................................................................................91 Introduction ...........................................................................................................................................91 Materials and methods ..........................................................................................................................93 Results....................................................................................................................................................98 Discussion............................................................................................................................................102 Chapter 5- The effects of predator abundance and habitat structural complexity on survival of juvenile sea urchins ...............................................................................................................................105 Abstract................................................................................................................................................107 Introduction .........................................................................................................................................107 Material and methods..........................................................................................................................109 Results..................................................................................................................................................111 Discussion............................................................................................................................................118 Chapter 6- Movements patterns of the sea urchin Paracentrotus lividus in the NW Mediterranean .................................................................................................................................................................121 Abstract................................................................................................................................................123 Introduction .........................................................................................................................................123 Methods ...............................................................................................................................................125 Results..................................................................................................................................................129 Discussion............................................................................................................................................133 Chapter 7 – Temporal variability in abundance of the sea urchin Paracentrotus lividus in the northwestern Mediterranean: comparison between a reserve and an unprotected area................137 Introduction .........................................................................................................................................139 Material and Methods..........................................................................................................................140 Results..................................................................................................................................................143 Discussion............................................................................................................................................158 Chapter 8- Effects of removing sea urchins (Paracentrotus lividus): stability of barren states and Cystoseira balearica forest recover in NW Mediterranean.................................................................163 Introduction .........................................................................................................................................165 Material and methods..........................................................................................................................166 Results..................................................................................................................................................172 Discussion............................................................................................................................................177 Chapter 9- General Discussion .............................................................................................................183 Question 1: top-down controls or bottom-up controls?.......................................................................185 Question 2: the relative importance of sea urchins and fish as herbivores.........................................186 Question 3. The importance of floristic composition and functional groups.......................................187 Question 4. What controls the demographic dynamics of sea urchin populations? ............................189 Question 5: Why are the predictions made by trophic models not fulfilled in marine reserves?.........196