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

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Marzloff, MP (2012) Towards ecosystem-based management of Tasmanian temperate rocky reefs: Community dynamics models indicate alternative community states and management strategies. PhD thesis, University of Tasmania.

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Abstract

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 climatedriven
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.

Item Type: Thesis (PhD)
Keywords: marine ecosystem modelling - Tasmanian rocky reefs - rock lobster - sea urchin - seaweed bed - fishing - climate change - phase shift
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Copyright 2012 the Author

Date Deposited: 17 Aug 2012 04:45
Last Modified: 15 Sep 2017 01:06
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