Resilience of Long-lived Mediterranean Gorgonians in a Changing World: Insights from Life History Theory and Quantitative Ecology

Abstract

Temperate benthic communities face cumulative impacts from multiple stressors acting both at local and global scales. Understanding how local management and ocean warming affect the dynamics and resilience of dominant habitat-forming species is central to marine conservation. In this thesis, we combined long-term demographic surveys and large-scale distribution datasets with innovative population and spatial modeling approaches, and meta-analyses to unravel the causes and consequences of extreme life-histories. The final goal was to understand resilience patterns and mechanisms, and to assess the effectiveness of widely used conservation tools such as fishing regulations, marine protected areas (MPAs), and active restoration. A comparative analysis across marine sessile species revealed strong linkages between depth occurrence, longevity and demographic stability. These results demonstrated a fundamental role of environmental gradients in shaping the evolution of life-history strategies in the ocean and suggest that deep-sea benthic communities tend to be dominated by long-lived species that are very vulnerable to external sources of mortality. In addition, we demonstrate that life-history strategies play a role in important conservation issues such as recovery processes. For instance, recovery times after fishing or restoration actions for long-lived species can take several decades. Thus, accounting for the temporal dimension and life-history tradeoffs in conservation efforts is essential to avoid potential long-lasting impacts and enhance the recovery of damaged ecosystems. Focusing on the Mediterranean red coral Corallium rubrum, an overharvested precious coral emblematic of coralligenous assemblages, we revealed an extremely slow life-history strategy characterized by low reproduction success, high natural survival, and extended longevity. These traits drive a general pattern of slow population dynamics and suggest low resilience to human-driven stressors. We then show that local and global stressors such as overfishing and warming have strong impacts on different demographic processes of red coral populations. Harvesting causes dramatic decreases in total biomass and strong shifts in size-class distribution towards populations dominated by small colonies. More broadly, MPAs strongly enhance structural complexity of red coral populations but, contrary to prior expectations, have little effect on their long-term viability and associated extinction risk. Population recovery after harvesting is a very slow process that can take decades. Interestingly, recovery is not driven by sexual reproduction but by the capacity of harvested colonies to survive and regrow new branches. This recovery mechanism may explain the persistence of this historically overexploited long-lived coral. Unfortunately, novel impacts associated to climate change threat shallow red coral populations. Recurrent warming-driven mass mortality events had detrimental effects on affected populations, causing long-term declines and potential local extinction. While MPAs have been proposed to enhance the resilience of marine ecosystems to climate change, our simulations suggested that MPAs only have a weak buffering effect to climatic impacts. On the other hand, spatial analyses revealed that future climate change may cause extensive impacts on shallow populations of the red coral and the red gorgonian Paramuricea clavata (another emblematic species of coralligenous assemblages with an important structural role) across the Mediterranean Sea. Importantly, we found that the extent and severity of warming impacts is strongly dependent on depth and the global emissions scenarios. Altogether, our results demonstrate a high vulnerability and low demographic resilience of shallow red coral populations to climate change. The results presented in this thesis suggest that, only by simultaneously acting at both local and global scales, we can we ensure the persistence and enhance the structural role of the long-lived Mediterranean red coral. Finally, this thesis also shows the potential of combining long-term large-scale field data, quantitative tools, and principles of life-history theory to provide new perspectives to advance marine conservation in a changing ocean.