Prof. Craig Johnson1, Professor Steve Dudgeon2
1IMAS, University Of Tasmania, Hobart, Australia, 2California State University Northridge, Northridge, USA
At a time of unprecedented influence of human activity on marine ecosystems, there is heightened interest in persistent shifts in community structure and dynamics. We review concepts of smooth, continuous and discontinuous phase shifts and the relationships between them. We aim to clarify continued confusion about these concepts in the literature, emphasising that they are alternative outcomes of a single model. This presents three principal challenges for ecologists and managers. The first is how to interpret observed change in marine communities, distinguishing outcomes that represent multiple stable states from those of long-transient successional stages, or environmentally-driven, potentially reversible, shifts in the phase portrait of single attractor systems. Second, is how to quantify and estimate the magnitude of three sources of variability that cause switches between community states, viz. (1) ‘noise’ in species’ abundances, (2) ‘wiggle’ in system parameters, and (3) trends in parameters that affect topography of the basin of attraction. Third, is how variability in the system shapes the evidence used to interpret changes in community state. We discuss combining experimental and observational approaches and present a novel approach using critical length scales to potentially answer these important questions. Finally, real-world examples of discontinuous phase shifts in macroalgal systems emphasise the possibility that a system can exist in more than two stable configurations, and that transient successional stages may persist for long time periods necessitating a management focus on the ecology of long transients in addition to stable endpoints. These features provide additional layers of complexity for managers to consider.
Presentation Slides – Craig Johnson
Biography:
Craig is broadly concerned with the space-time dynamics of marine ecosystems, and with predicting the distribution of biodiversity. In both areas the impacts of climate change and possible management responses dominate recent work. He studies interactions among suites of organisms spanning bacteria, algae, invertebrates and fish. The focus has largely been on temperate reefs, but has also included deep mesophotic reefs and pelagic systems in the Southern Ocean. His research is equally divided between field work, modeling marine system dynamics and spatial variation in biodiversity, and work to develop kelp-based industries. He has >200 peer-reviewed publications including several edited books.
