Estimating context-dependent ecological thresholds and risk of sea urchin barren formation in Mediterranean macroalgal communitiesResistance, Resilience and Phase Shifts

Mario Minguito-Frutos¹, Jordi Boada², Sophie Gourguet³, Martin Marzloff¹

¹Ifremer, France - ²Blanes Centre for Advanced Studies (CEAB-CSIC), Spain - ³ Ifremer, Univ Brest, CNRS, UMR 6308, AMURE, Unité d’Economie Maritime, IUEM, France

Regime shifts are widespread in coastal ecosystems exposed to multiple stressors. Complex interactions between anthropogenic stressors and environmental conditions facilitate these critical transitions between alternative ecosystem states, challenging our ability to understand and anticipate such shifts, as well as to promote effective management interventions. Ecosystem or multi-species simulation models are valuable tools to capture how space and time variability in anthropogenic stressors, biotic interactions, and abiotic conditions define context-specific thresholds of ecosystem collapse. Here, we apply the TRITON framework to explore, through simulation models, the dynamics of barren formation in the Mediterranean Sea. In Mediterranean benthic ecosystems, herbivory pressures exerted by two sea urchin species, Paracentrotus lividus and Arbacia lixula, can deplete Cystoseira sensu lato forests. Historical over-harvesting of predatory sea breams has largely contributed to sea urchin population increase beyond critical densities. The decline in Cystoseira s.l. forests facilitate the establishment of shrub- and turf-forming algae that compete for space with Cystoseira spp. and preclude its natural recovery. Our adaptation of TRITON to Mediterranean marine forests captures population dynamics and interactions between six key functional groups: macroalgal assemblages dominated either by Cystoseira spp., shrub- or turf-forming algae; the herbivore guild composed of both urchin species; and predatory sparid fishes. After confronting simulations to field observations to validate model dynamics, these simulations characterise the relative influence of modelled processes on barren formation and provide probabilistic estimates of context-dependent thresholds in terms of sea urchin densities or predatory sparid. This work represents a first step towards developing a decision-support framework to effectively manage and safeguard Mediterranean marine forests.