Towards ecosystem-based management of Tasmanian temperate rocky reefs: Community dynamics models indicate alternative community states and management strategies

摘要

Worldwide, ecosystems have demonstrated the potential for dramatic shifts to anudalternative persistent state under gradual long-term environmental changes or followingudsudden short-term perturbations. Such shifts are documented for numerous marineudexamples from coral reef to pelagic communities and may become more common asudecological dynamics adjust to climate-driven changes. These shifts are often sudden,udchallenging to predict and can have disastrous and unpredictable consequences on bothudecosystem functioning and the human activities that rely on the associated naturaludresources. They often result in irreversible dramatic changes in community structureudand productivity and represent a growing concern for managers of natural systems.udIn ecosystems where the presence of an alternative persistent state is well documented, theuddrivers of these shifts (e.g. anthropogenic stressors or changes in environmental conditions)udcan be analysed retrospectively so as to address key management questions, as has occurredudin several applications on coral reefs. However, phase shifts are often swift and observed audposteriori, i.e. after the ecosystem has shifted to the alternative state. Thus, thresholds inudecosystem dynamics are difficult to identify empirically despite that this is crucial for soundudmanagement of marine resources. Additionally, controlled experimental assessment of theudeffects of alternative management scenarios on community state is hardly ever achievableudin marine ecosystems. When they occur, phase shifts are unique to each ecosystem, henceudcase-specific simulation models present a valuable tool to explore ecological dynamicsudwith alternative persistent community states, test the effects of management scenariosudand inform decision-making.udOn the east coast of Tasmania, shallow rocky reef communities on the exposed coast mainlyudoccur in two alternative persistent states: (1) the seaweed bed state characterised by auddense productive canopy of macroalgae; or, (2) the sea urchin ‘barren’ state characterisedudby a poorly productive rocky habitat largely bare of seaweeds as a result of destructiveudgrazing by the long-spined sea urchin (Centrostephanus rodgersii ). The establishment of these widespread sea urchin barrens result from a combination of both: (1) the climatedrivenudrange extension of the long-spined sea urchin C. rodgersii from Australia’s mainlandudto Tasmania; and (2) depletion of key reef predators by fishing. Large southern rock lobsterud(Jasus edwardsii ) individuals constitute the main predator of the long-spined sea urchinudin Tasmania. Relative to the seaweed bed state, C. rodgersii barrens represent dramaticudlosses of habitat, species diversity and productivity, including commercial species such asudblacklip abalone (Haliotis rubra) and southern rock lobster, the two most valuable fisheriesudin Tasmania. Thus, the spread of sea urchin barrens presents a major and pressing threatudfor the lobster and abalone fishing industries.udThis thesis presents a suite of models specifically developed to better understand theuddynamics of Tasmanian rocky reef communities and inform management interventions toudmitigate destructive grazing of seaweed beds by the invasive long-spined sea urchin.udChapter 2 investigates the causal relationships between positive feedback and theudoccurrence of alternative states in community dynamics. Modelling of community feedbackudinformed by available qualitative knowledge about ecosystem structure constitutes audvaluable framework to detect the potential for alternative states in ecological dynamicsudas illustrated with some examples from Tasmanian rocky reef communities. Qualitativeudmodelling assists to understand the essential features of temperate reef dynamics aroundudTasmania, and provides a useful first step towards quantitative modelling of rocky reefuddynamics. The approach provides an ideal framework to (i) collate all available informationudabout rocky reef ecology, (ii) test model structure uncertainty, and (iii) identify key driversudof alternative states in ecosystem dynamics.udThe quantitative model presented in the subsequent chapters captures the dynamics ofudthe three key groups or species (i.e. the rock lobster, sea urchin, and seaweed assemblage)uddirectly involved in the positive feedback that drives the shift between alternative statesudon Tasmanian rocky reef. Chapter 3 describes the development, parameterisation andudcalibration of a mean field model of the local dynamics (reef area of 100 m2 - 10 ha)udof a reef community. The model’s ability to capture the potential for phase shifts, fromuddense seaweed bed to sea urchin barrens habitat and back, is validated against largescaleudpatterns observed on rocky reefs where C. rodgersii occurs. In the simulations,udthe 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 decadesudif relying on management interventions that cannot effectively reduce urchin density toudzero. Thus, restoration of seaweed beds seems unrealistic to implement within the currentudtimeframe of management plans. Comprehensive model-independent sensitivity analysisudof model behaviour to parameter estimates also suggests that, in addition to lobster fishingudmortality, recruitment rates of sea urchins and rock lobsters, which are strongly influencedudby large scale oceanographic features and highly variable in eastern Tasmania, are keyudfactors in determining the potential for sea urchin barren formation in the model.udIn Chapter 4, sets of Monte-Carlo simulations with this model are used to address threeudsets of questions related to management for mitigation of sea urchin destructive grazingudof Tasmanian seaweed beds. Model behaviour suggests that thresholds in shifting fromudseaweed bed to sea urchin barren and restoration of seaweed cover reveal the existence of audhysteresis in model dynamics. The hysteresis implies that the establishment of sea urchinudbarrens cannot be reversed easily. These thresholds provide valuable ecological referenceudpoints to prevent the establishment of sea urchin barrens. The model indicates thatudculling of sea urchins appears as the most effective management strategy to minimise theudecological impact of C. rodgersii on Tasmanian reef communities. Indirect interventionsudrelying solely on the rebuilding of rock lobster population (through reduction in fishing orudimplementation of a maximum legal catch size) perform poorly but, when combined withuddirect control of the sea urchin population, they can provide optimal outcomes both inudterms of minimising barren formation and fishery performance. Finally, the model showsudthat to allow lobsters to play their critical ecological ‘service’ role in preventing sea urchinudbarrens formation, a reduction in lobster fishing mortality from current levels is required.udA maximum sustainable yield as estimated from the single species stock assessment modeluddoes not account for the ecosystem service delivered by larger lobsters, and the modelsudemphasise the need for an ecosystem-based fishery management approach.udThis suite of models contributes to the general understanding of mechanisms and driversudthat can facilitate shift between alternative states in ecological dynamics. The quantitativeudsimulation model provides specific information to managers about the drivers of shiftsudbetween the seaweed bed and the sea urchin barren state in the dynamics of Tasmanianudrocky reefs. In particular, the presence of a hysteresis in reef community dynamics meansudthat effort to prevent barrens formation constitutes a more viable and cost effective management strategy than the restoration of seaweed beds once extensive barrens habitatudhas developed. The commercially-fished rock lobster is an essential reef predator deliveringudkey ecosystem services to Tasmanian rocky reefs and model simulations highlight theudnecessity for fisheries management to move away from a single species focus and account forudthe ecological role of targeted commercial species. The tools implemented here to inform anudecosystem-based management of Tasmanian rocky reefs are generic and ‘transportable’ toudother ecosystems with alternative states. While C. rodgersii barrens currently constituteuda pressing concern for managers of reef communities and fisheries in Tasmania, the longspinedudsea urchin is only one example of a species that is dramatically restructuringudTasmanian reef communities. There are many other ‘natural’ invaders, whose ecosystemudroles and impacts are unknown, currently extending their distribution from Australia’sudmainland to the warming Tasmanian waters. In the coming decades, climate-drivenudchanges are likely to bring more surprises to Tasmanian rocky reefs, and just as manyudchallenges for the associated fisheries and their managers.