Biological interactions and resilience of seagrass ecosystems

Resumen

Our world is subjected to a panoply of drivers of change. In this context, the understanding on how our biosphere resists, absorbs or is altered by the changes, appears as a hot question in ecology. In this respect, two ecological concepts appear as essential, resilience and biotic interactions. Resilience is related to how ecosystems persist under stress or suffering disturbances. Interactions among species are to a large part responsible for the delivery of ecosystem functions, and form the architecture of biodiversity. Moreover, a substantial part of ecosystem resilience is founded on species interactions. This thesis is an attempt to shed some light on these issues through the deep exploration of specific case studies in seagrass ecosystems, in particular how seagrasses respond to external drivers (or how resilient they are), how these responses affect species interactions and which mechanisms allow coexistence of species linked by positive and negative interactions. Our approach is based upon field observations and field manipulative experiments. Chapter 1 shows how an increase of organic matter in sediment weakens the mutualism between the bivalve Loripes lucinalis and the seagrass Cymodocea nodosa. The mechanism implied is the effect of this increase (and, probably, the resulting anoxia) on seagrass root morphology (plant trait), which results in a lower provision of habitat for the bivalves, whose abundance decreases. The weakening of the mutualism can potentially decrease the resilience of these ecosystems to eutrophication and, therefore, compromise their persistence. Chapter 2 describes a facilitative cascade in which the seagrass C. nodosa favors the abundance of the pen shell Pinna nobilis, which positively affects the sea urchin Paracentrotus lividus, which in turn consumes the seagrass. We suggest that the persistence of this three-species assemblage rests on the very local impact of sea urchins on the seagrass, likely driven by behavioural and denso-dependent processes. Chapter 3 and 4 show that fast-growing species such as C. nodosa are highly resilient to stress or disturbances when affecting only the aboveground parts, recovering fast (within two weeks) from a single event of disturbance. C. nodosa shows several mechanisms of tolerance, such as compensatory growth, reallocation of internal resources and enhancement of the formation of new modules, when coping to repeated defoliation simulating herbivory. However, when the belowground parts are lost by disturbances, recovery is highly delayed up to two years and is dependent on the characteristics of the disturbance such as size and timing. Overall, this research has contributed to increase our understanding on how ecosystems respond to changes and how species interactions are maintained and disrupted. We have shown that environmental changes can alter the functioning of seagrass ecosystems at least in two directions. Firstly, by altering fundamental biological interactions, such as the seagrass-lucinid mutualism and, secondly, by affecting the resilience of ecosystems dominated by a foundation species, which promote species coexistence. Advances in the two complementary and interlinked directions will be crucial to better manage and preserve ecosystems and prevent their potential collapse under the increasing human-induced change the world is submitted to.