Change agents: sea urchin regulation and state shifts in Mediterranean macrophyte systems = Agents de canvi: el control de les garotes i els canvis d'estat en els ecosistemes de macròfits de la Mediterrània
[eng] Macrophyte systems are globally vulnerable to overgrazing, often shifting precipitously to functionally poorer alternative stable states triggered by herbivore population outbreaks. Sea urchin herbivory can cause shifts in shallow macrophyte systems in the Mediterranean (i.e. macroalgal commun...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2016 |
| País: | España |
| Institución: | Universidad de Barcelona |
| Repositorio: | Dipòsit Digital de la UB |
| OAI Identifier: | oai:diposit.ub.edu:2445/104085 |
| Acceso en línea: | https://hdl.handle.net/2445/104085 http://hdl.handle.net/10803/397785 |
| Access Level: | acceso abierto |
| Palabra clave: | Macròfits Hàbitat (Ecologia) Eriçons de mar Macrophytes Habitat (Ecology) Sea urchins |
| Sumario: | [eng] Macrophyte systems are globally vulnerable to overgrazing, often shifting precipitously to functionally poorer alternative stable states triggered by herbivore population outbreaks. Sea urchin herbivory can cause shifts in shallow macrophyte systems in the Mediterranean (i.e. macroalgal communities and seagrass meadows) changing the structure of these communities to completely bare extensions; rocky macroalgal systems are particularly prone to barren formation. These sudden transitions are inherently surprising making it very difficult to manage the rich diversity and productivity these ecosystems support. This thesis attempts to address the uncertainty underlying these regime transitions by first identifying the location of critical thresholds in response to sea urchin stressors and then predicting how these thresholds vary under different environmental conditions. Specifically, it describes how inherently poor nutrient regions are more prone to catastrophic shifts related to two synergistic mechanisms, i) compensatory feeding by urchins in low nutrient regions and ii) higher rates of algal growth in high nutrient areas. Additionally, it identifies the principal regulatory processes determining variations in sea urchin numbers (main stressor) in the complex seascapes of mixed macroalgal communities and seagrass meadows that dominate shallow Mediterranean shores. Particularly, this thesis shows that habitat can determine when in the sea urchin’s life history this regulation occurs. While predation is the most important agent of regulation in both macroalgal and seagrass meadows, populations are also highly influenced by migration and settlement processes. Finally, it describes the importance of identifying hotspots of functional predation (main regulating agent) across a wide stretch of coastline to implement management actions and preserve the sustainability of macrophyte ecosystems. In parallel, this thesis also tests the effectiveness of a widely-employed but rarely tested assay to measure rates of predation that was central to this work. The results of this assessment show that marking sea urchins in the field by piercing is a very reliable tool to measure predation rates, neither affecting the mortality of the marked organism nor enhancing its detectability by predators. Overall, the results presented here will enhance the predictability of catastrophic transitions in Mediterranean macrophyte systems and could potentially be extrapolated to other ecosystems in temperate seas. |
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