Resumen de The role of behavioral and feedback mechanisms in mediating herbivory processes in mediterranean seagrass ecosystems
The role of consumer-resource relationship is the basis of the structure and functioning of ecological communities. The traditional lens for studying such interactions have tended to omit the importance of the species behaviour, particularly in the case of plant-herbivore interactions. However, they way the herbivore exploits the plant and the way the plant responds to that pressure is crucial in determining the stability of the interaction. Thus, both herbivore and plant performances, contextualised in the environment where the interaction takes place, can act as drivers of the resilience of the system. This thesis focuses on Posidonia oceanica ecosystems and specifically on the interaction between this seagrass and its two main herbivores – the fish Sarpa salpa and the sea urchin Paracentrotus lividus –. I assess the importance of herbivore behaviour (the aggregation patterns and feeding behaviour of S. salpa and the canopy cover dependence of P. lividus) and the resilience provided feedback mechanisms (direct responses of the plant and indirect ecosystem processes) under changing conditions resulting from global change. Disentangling these components of the interaction allows us to assess its sensitivity to each of the components and to test the response and resilience of the system under different conditions. Chapter 1 shows the importance of body size of Sarpa salpa individuals in their individual feeding activity, shoaling aggregation patterns and shoal feeding strategies, as well as the potential consequences of this on the seagrass P. oceanica. On the one hand, the larger the individuals are, the more feeding activity they show. On the other hand, S. salpa individuals tend to aggregate with conspecifics of the same body size and group size is positively related to the size of the individuals in the group. In addition, feeding strategies increase in complexity with group size and tend to be focused on very specific spots within the meadow. Thus, as individuals grow, they increase their potential impact on the seagrass, both in terms of their consumption capacity and the formation of large shoals capable of concentrating their consumption in very specific areas of the meadow. This distribution of herbivory can lead to spatial heterogeneity with consequences for the functioning of the ecosystem dominated by P. oceanica. In chapter 2, a number of regulatory mechanisms arising from an episode of intense herbivory are found to provide resilience to Posidonia oceanica system once its canopy height is reduced. Four of the mechanisms evaluated function as feedback mechanisms, one of them being actively deployed by the plant (compensatory growth) while the other three (preference for an alternative resource, increased risk of predation and reduced urchin numbers due to competition for the resource and loss of cover) are triggered indirectly and their effectiveness is based on inducing changes in the behaviour of the herbivore Paracentrotus lividus. The results obtained show how P. oceanica is able to invest efforts in recovering part of the lost leaf biomass while the system itself is able to regulate herbivory pressure as long as the environmental conditions are appropriate to the occurrence of these mechanisms. The role of these mechanisms is key to avoid the potential collapse of P. oceanica meadows under the stress caused by an episode of intense herbivory. In chapter 3, the effect of global warming on the Posidonia oceanica – Sarpa salpa interaction is assessed through a combination of gradient approaches in the field and manipulative laboratory experiments. On the one hand, the results show that increasing water temperature significantly increases S. salpa growth rates during its larval stage, reducing its larval period (fewer days in the water column) and limiting its dispersal, while showing no effect on feeding behaviour during its adult phase. On the other hand, warming negatively affects the growth rates of P. oceanica and makes it more palatable towards S. salpa according to the results from the preference experiment. Our study shows that S. salpa could develop faster in warmer environments during its most vulnerable stage, increasing its survival but decreasing its dispersal capacity. At the same time, it could increase its preference for P. oceanica in its adult stage, which, together with the reduction of seagrass growth, could considerably intensify the strength of the interaction under warming conditions. All in all, the results of this study have contributed to confirm the relevance of herbivores’ behaviour in the way they exploit P.oceanica, mainly the feeding strategies of S. salpa, and how feedback mechanisms, provide resilience and allow the ecosystem to be maintained in an vegetated state. All of this gets relevant when contextualised within the process of global change, with a probable strengthening of the plant-herbivore interaction. Understanding the sensitivity of the interaction equilibria to each of its components is crucial to decide where to invest conservation efforts in these ecosystems and to be able to anticipate how changes in the contextual conditions may alter the final balance of the interaction.
