Towards ecosystem-based management of Tasmanian temperate rocky reefs : community dynamics models indicate alternative community states and management strategies
Creators: name: Marzloff, MP
Publisher: name: University Of Tasmania
Year: 2023

description: Worldwide, ecosystems have demonstrated the potential for dramatic shifts to an alternative persistent state under gradual long-term environmental changes or following sudden short-term perturbations. Such shifts are documented for numerous marine examples from coral reef to pelagic communities and may become more common as ecological dynamics adjust to climate-driven changes. These shifts are often sudden, challenging to predict and can have disastrous and unpredictable consequences on both ecosystem functioning and the human activities that rely on the associated natural resources. They often result in irreversible dramatic changes in community structure and productivity and represent a growing concern for managers of natural systems. In ecosystems where the presence of an alternative persistent state is well documented, the drivers of these shifts (e.g. anthropogenic stressors or changes in environmental conditions) can be analysed retrospectively so as to address key management questions, as has occurred in several applications on coral reefs. However, phase shifts are often swift and observed a posteriori, i.e. after the ecosystem has shifted to the alternative state. Thus, thresholds in ecosystem dynamics are difficult to identify empirically despite that this is crucial for sound management of marine resources. Additionally, controlled experimental assessment of the effects of alternative management scenarios on community state is hardly ever achievable in marine ecosystems. When they occur, phase shifts are unique to each ecosystem, hence case-specific simulation models present a valuable tool to explore ecological dynamics with alternative persistent community states, test the effects of management scenarios and inform decision-making. On the east coast of Tasmania, shallow rocky reef communities on the exposed coast mainly occur in two alternative persistent states: (1) the seaweed bed state characterised by a dense productive canopy of macroalgae; or, (2) the sea urchin 'barren' state characterised by a poorly productive rocky habitat largely bare of seaweeds as a result of destructive grazing by the long-spined sea urchin (Centrostephanus rodgersii ). The establishment of these widespread sea urchin barrens result from a combination of both: (1) the climate driven range extension of the long-spined sea urchin C. rodgersii from Australia's mainland to Tasmania; and (2) depletion of key reef predators by fishing. Large southern rock lobster (Jasus edwardsii ) individuals constitute the main predator of the long-spined sea urchin in Tasmania. Relative to the seaweed bed state, C. rodgersii barrens represent dramatic losses of habitat, species diversity and productivity, including commercial species such as blacklip abalone (Haliotis rubra) and southern rock lobster, the two most valuable fisheries in Tasmania. Thus, the spread of sea urchin barrens presents a major and pressing threat for the lobster and abalone fishing industries. This thesis presents a suite of models specifically developed to better understand the dynamics of Tasmanian rocky reef communities and inform management interventions to mitigate destructive grazing of seaweed beds by the invasive long-spined sea urchin. Chapter 2 investigates the causal relationships between positive feedback and the occurrence of alternative states in community dynamics. Modelling of community feedback informed by available qualitative knowledge about ecosystem structure constitutes a valuable framework to detect the potential for alternative states in ecological dynamics as illustrated with some examples from Tasmanian rocky reef communities. Qualitative modelling assists to understand the essential features of temperate reef dynamics around Tasmania, and provides a useful first step towards quantitative modelling of rocky reef dynamics. The approach provides an ideal framework to (i) collate all available information about rocky reef ecology, (ii) test model structure uncertainty, and (iii) identify key drivers of alternative states in ecosystem dynamics. The quantitative model presented in the subsequent chapters captures the dynamics of the three key groups or species (i.e. the rock lobster, sea urchin, and seaweed assemblage) directly involved in the positive feedback that drives the shift between alternative states on Tasmanian rocky reef. Chapter 3 describes the development, parameterisation and calibration of a mean field model of the local dynamics (reef area of 100 m2 - 10 ha) of a reef community. The model's ability to capture the potential for phase shifts, from dense seaweed bed to sea urchin barrens habitat and back, is validated against largescale patterns observed on rocky reefs where C. rodgersii occurs. In the simulations, the time for extensive sea urchin barrens to form is of the order of two decades, while restoration of seaweed cover from the sea urchin barrens habitat takes about three decades if relying on management interventions that cannot effectively reduce urchin density to zero. Thus, restoration of seaweed beds seems unrealistic to implement within the current timeframe of management plans. Comprehensive model-independent sensitivity analysis of model behaviour to parameter estimates also suggests that, in addition to lobster fishing mortality, recruitment rates of sea urchins and rock lobsters, which are strongly influenced by large scale oceanographic features and highly variable in eastern Tasmania, are key factors in determining the potential for sea urchin barren formation in the model. In Chapter 4, sets of Monte-Carlo simulations with this model are used to address three sets of questions related to management for mitigation of sea urchin destructive grazing of Tasmanian seaweed beds. Model behaviour suggests that thresholds in shifting from seaweed bed to sea urchin barren and restoration of seaweed cover reveal the existence of a hysteresis in model dynamics. The hysteresis implies that the establishment of sea urchin barrens cannot be reversed easily. These thresholds provide valuable ecological reference points to prevent the establishment of sea urchin barrens. The model indicates that culling of sea urchins appears as the most effective management strategy to minimise the ecological impact of C. rodgersii on Tasmanian reef communities. Indirect interventions relying solely on the rebuilding of rock lobster population (through reduction in fishing or implementation of a maximum legal catch size) perform poorly but, when combined with direct control of the sea urchin population, they can provide optimal outcomes both in terms of minimising barren formation and fishery performance. Finally, the model shows that to allow lobsters to play their critical ecological 'service' role in preventing sea urchin barrens formation, a reduction in lobster fishing mortality from current levels is required. A maximum sustainable yield as estimated from the single species stock assessment model does not account for the ecosystem service delivered by larger lobsters, and the models emphasise the need for an ecosystem-based fishery management approach. This suite of models contributes to the general understanding of mechanisms and drivers that can facilitate shift between alternative states in ecological dynamics. The quantitative simulation model provides specific information to managers about the drivers of shifts between the seaweed bed and the sea urchin barren state in the dynamics of Tasmanian rocky reefs. In particular, the presence of a hysteresis in reef community dynamics means that effort to prevent barrens formation constitutes a more viable and cost effective management strategy than the restoration of seaweed beds once extensive barrens habitat has developed. The commercially-fished rock lobster is an essential reef predator delivering key ecosystem services to Tasmanian rocky reefs and model simulations highlight the necessity for fisheries management to move away from a single species focus and account for the ecological role of targeted commercial species. The tools implemented here to inform an ecosystem-based management of Tasmanian rocky reefs are generic and 'transportable' to other ecosystems with alternative states. While C. rodgersii barrens currently constitute a pressing concern for managers of reef communities and fisheries in Tasmania, the longspined sea urchin is only one example of a species that is dramatically restructuring Tasmanian reef communities. There are many other 'natural' invaders, whose ecosystem roles and impacts are unknown, currently extending their distribution from Australia's mainland to the warming Tasmanian waters. In the coming decades, climate-driven changes are likely to bring more surprises to Tasmanian rocky reefs, and just as many challenges for the associated fisheries and their managers.