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Climate forcing and sudden change in marine ecosystems


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Litzow, MA 2014 , 'Climate forcing and sudden change in marine ecosystems', PhD thesis, University of Tasmania.

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Rationale—Continental shelf ecosystems occasionally undergo "regime shifts"
– abrupt reorganization events that can have deleterious social and economic effects
on fishing communities that rely on affected species. Regime shifts are often
interpreted as transitions to alternative ecosystem states after external perturbations
such as fishing pressure cross a critical threshold; they have also been related to shifts
in modes of internal climate variability, such as the Pacific Decadal Oscillation and
North Atlantic Oscillation. However, hypotheses explaining regime shifts are
extremely difficult to test, given the multivariate nature of both stressors and
community response, the frequent paucity of data at adequate spatial and temporal
scales, possible non-linear and non-stationary relationships, and the use of
observational data that preclude strong inference. Furthermore, regime shifts have
often been invoked to explain ecological change without the consideration of
competing models, such as the accumulation of more gradual change over time. As a
result, both the nature of community-level biological change in continental shelf
systems (regime shifts vs. more gradual change), and the identity of factors producing
observed change, remain poorly resolved. Given increasing anthropogenic
disturbance to continental shelf ecosystems globally, there is a need both for better
understanding of the dynamics underlying sudden ecological change, and for tools
providing early detection of ecosystem change to allow for proactive management
measures that might minimize associated socio-economic disruption. Finally, the
regime shift concept is largely based on work in North Atlantic and North Pacific
ecosystems, and there is a need for studies in other areas to determine if the regime
shift model is widely applicable.
Approach—In many continental shelf systems, long-term biological
observations that are necessary for testing hypotheses concerning decadal-scale
ecological change are extremely limited. The first four chapters of this thesis use the
northeast Pacific as a model system, as this region is home to one of the best extant
datasets of long term, large-spatial scale biological observations globally. I compiled
a set of 38 climatic time series (regional climate parameters and large-scale indices)
and 78 biological time series (mostly production or abundance estimates for commercially-important fish and invertebrate populations), from the 1960s to the
present, covering the continental shelf between 30°N and 65°N. This dataset allowed
me to evaluate internal climate variability, commercial fishing and incremental
climate change as factors explaining decadal-scale biological variability (Chapter 1);
test competing models of gradual change and regime shifts for explaining decadalscale
ecological variability (Chapter 2); develop an approach for evaluating possible
ecosystem shifts at the ends of time series (Chapter 2); test for non-stationary
biological responses to climate perturbations (Chapter 3); and test statistical tools for
early detection of ecosystem transitions (Chapter 4). In Chapter 5 I use the
methodology developed in the previous chapters to evaluate a data-poor situation in
southeast Australian continental shelf ecosystems, using nine climatic and 12
biological time series (seabird reproductive parameters and recruitment estimates for
commercially-important fish stocks) for the period 1967-present. This chapter tested
competing hypotheses invoking secular change and regime shifts to explain regional
climate-biology covariation.
Results—Analysis of northeast Pacific data showed that: the ecological change
that is frequently related to regime shifts in the literature is in many instances better
described as the result of incremental change over time (Chapters 1 and 2); commonly
held assumptions concerning the dominant role of internal climate variability as an
external driver on the system are poorly supported by data (Chapters 1 and 2);
assessment of incipient ecosystem shifts may be supported both by model-derived
statistical indicators and by formal evaluations of the length of observation needed to
evaluate ecosystem state (Chapters 2 and 4); and assumptions of stationarity are
inadequate for understanding community responses to climate variability (Chapter 3).
In southeast Australia (Chapter 5), I found that secular change best described decadalscale
climatic and biological variability, with little evidence for a role of regime shifttype
variability in the system.
Conclusions—My findings from the northeast Pacific suggest several revisions
to current understanding of decadal-scale variability in continental shelf ecosystems.
Even in ecosystems that are generally free of overfishing and heavily loaded by
leading modes of global climate variability, anthropogenic drivers (fishing, secular
climate change) are at least as important as climate variability as agents of ecological
change. Furthermore, gradual change over time is at least as important to consider as
more obvious regime shifts when analyzing decadal-scale ecological variability. The tests of early indicators for sudden change included in this thesis are the first application of these proposed methods to actual fisheries management data, and demonstrate their potential usefulness in this context. Finally, while my findings
confirmed the importance of secular change in southeast Australian ecosystems,
analysis of northeast Pacific data shows that the small sample of biological time series
in Chapter 5 is likely to produce an underestimate of the complexity of temporal
variability in the system.

Item Type: Thesis - PhD
Authors/Creators:Litzow, MA
Keywords: climate change, regime shifts, marine ecosystems, north Pacific, south-east Australia
Copyright Holders: The Author
Copyright Information:

Copyright 2014 the author

Additional Information:

Chapter 1 appears to be the equivalent of the peer reviewed version of the following article: Lowitz, M.A., Mueter, F.J., Hobday, A.J., 2014, Reassessing regime shifts in the North Pacific: incremental climate
change and commercial fishing are necessary for explaining decadal-scale biological variability, Global change biology, 20(10, 38-50., which has been published in final form at 10.1111/gcb.12373 This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

Chapter 2 appears to be the equivalent of the peer reviewed version of the following article: Litzow, M.A., Mueter, F.J., 2014, Assessing the ecological importance of climate regime shifts: an approach from the north Pacific Ocean, Progress in oceanography, 120, 110-119.

Chapter 4 appears to be the equivalent of the peer reviewed version of the following article: Litzow, M.A., Mueter, F.J., Urban, J.D., 2013, Rising catch variability preceded historical fisheries collapses in Alaska, Ecological applications, 23 (6), 1475- 1487, copyright by the Ecological Society of America

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