Exploring ecological shifts using qualitative modelling: Alternative states on Tasmanian rocky-reefs

Alternative stable states characterise many natural ecosystems. Subtidal rocky-reefs on the east coast of Tasmania persist in a range of different configurations, including so-called sea urchin ‘barrens’ and dense seaweed beds with a closed canopy. In creating and maintaining barrens habitat on temperate reefs, sea urchins induce major losses of production, biodiversity and physical structure. The invasive long-spined sea urchin (Centrostephanus rodgersii) is able to form barrens across its newly extended range in Tasmania. Formation of urchin barrens on the east coast has been a rising concern in recent decades, in particular because the two most valuable fisheries in the state, for blacklip abalone (Haliotis rubra) and southern rock lobster (Jasus edwardsii), are not viable on barrens. Thus, identifying triggers of barrens formation is critical in the management of these reefs. Here we explore the dynamics of Tasmanian rocky-reef communities, including testing the effects of fishing, using qualitative modelling informed by a deep empirical knowledge of interactions among species. Loop analysis offers a holistic approach to address the structure and dynamics of the system. It sacrifices precision of particular dynamics to maximize reality and generality in providing a causal understanding of complex systems. The network topology forms emergent feedback patterns that cause meta-stable properties in this rocky-reef system, and the models capture formation of urchin barrens and the mechanisms of the phase shifts. Fishing is identified as a perturbation that can reduce resilience of the system in its original highly productive seaweed-dominated state. We show that qualitative loop models can be highly valuable in identifying the kinds of system dynamics that managers need to consider in ecosystem based management, but emphasise that they are not designed to act as definitive management models for specific instantiations.