RESILIENCE OF ROCKY INTERTIDAL BIODIVERSITY TO CLIMATE CHANGE AND INCREASING ENVIRONMENTAL VARIABILITY

The main objective is to understand how natural complex systems respond to multiple perturbations. Natural climate variability shapes the spatio-temporal distribution of rocky intertidal assemblages. My thesis aims to understand how variability in environmental conditions and climate variables, in isolation and in combination, affect coastal primary productivity and the spatio-temporal organization of rocky shore assemblages. To address these challenges, I focused my research on two rocky intertidal systems: epilithic microphytobenthos biofilms and macrobenthic assemblages of algae and invertebrates. In Chapter 2, I assessed the effect of ultraviolet (UV) reduction on the early development of rocky intertidal biofilms under natural field conditions. Despite scarce biofilm colonization, results showed a positive effect of UV radiation on the early stages of biofilm colonization. A necessary condition for environmental variability to be influential is the presence of a nonlinear response of the ecological variables (e.g. biomass) to the environmental drivers (e.g., temperature, sediment deposition). Chapter 3 evaluated how the spatial covariance between warming and sediment accretion modulates the performance of rocky intertidal biofilm. To examine this, I have run a field experiment to empirically determine the response-surface of biofilms to increasing levels of warming and sediment deposition. By creating a constant and variable condition, I evaluated the total variance (TVE) and covariance effects (CE). According to warming-sediment response surfaces (WSRS), biofilm biomass was far lower when high warming and sediment depositions occurred synchronously (positive correlation) than that they occurred asynchronously in space (negative correlation), indicating that negative spatial correlation may reverse the combined impact of high warming and elevated sediment deposition. Simulation results further illustrated that variation in the covariance between warming and sediment deposition accounted up to 140% of the variance effect. These results highlight the negative covariance ultimately reversing the impact of extreme warming and sediment deposition. Biofilm performance was qualitatively predicted by non-linear averaging from a static-response surface. To understand how historical thermal variability impacts a natural system to influence future perturbations, I exposed rocky intertidal biofilms to different levels of fluctuating warming regimes (regular and variable conditions) and compared four components of ecological stability after strong perturbation (Chapter 4). Results indicated that regardless of the level of variability, biofilm pre-exposed to history of warming become less sensitive and more resilient to subsequent perturbations. Higher variability likely stabilized the biofilm biomass. The exposition to a variable-thermal environment provided stability for biofilm to future pulse perturbation. In Chapter 5, to assess the relative contribution of endogenous and exogenous processes in the emergence of spatial patterns, an intertidal assemblage of algae and invertebrates was exposed for 2 years to various combinations of intensity and spatial patterns of disturbance. Localized disturbances impinging at the margins of previously disturbed patches and homogenous disturbances without any spatial pattern generated heterogeneous gap- and patch-size distributions, characterized by a truncated or a pure power-law scaling. Spatially varying disturbances produced a spatial gradient in the distribution of algal patches and, to a lesser extent, also a power-scaling in both patch- and gap-size distributions. These results suggest that exogenous and endogenous processes are not mutually exclusive forces that can lead to the formation of similar spatial patterns in species assemblages.