Building resilience against climate-driven shifts in a temperate reef system: staying away from context-dependent ecological thresholds

As climate-driven environmental changes and anthropogenic perturbations increasingly affect ecological systems, the number of abrupt phase shifts in ecosystem dynamics is rising, with far reaching ecological, economic and social effects. These shifts are notoriously difficult to study, anticipate and manage. Although indicators of impending phase shifts in ecosystems have been described theoretically, they have only been observed empirically either after the fact or under controlled experiments. Here we demonstrate the usefulness of case-specific simulation models to estimate tipping points in the dynamics of real ecological systems, characterise how these thresholds may vary depending on local conditions and derive safe management targets associated with low risk of undesirable phase shifts. Under the combined effects of ocean changes and fishing, inshore rocky reefs in eastern Tasmania can transition from dense seaweed beds to sea urchin ‘barrens’ habitat, realising severe local loss of habitat, productivity and valuable fisheries. Using Monte-Carlo simulations with a validated model that realistically captures reef dynamics, we characterise the hysteresis in community dynamics and the variability in ecological thresholds along the gradient of environmental conditions. Simulation suggests that prevention of ongoing sea urchin destructive grazing of macroalgal beds is achievable but the yet-to-be-observed restoration of seaweed beds from extensive sea urchin barrens is highly unlikely. To guide management against undesirable phase shifts, we define target points associated with low risk of widespread barrens formation and show that, along with sea urchin culling, recognising the role of lobsters in mitigating sea urchin destructive grazing through predation is key to maintain reef productivity.