[Coral-List] Poor terminology in coral reef research 5: Phase shifts
jmcmanus at rsmas.miami.edu
Mon Nov 6 17:21:37 EST 2006
It looks like it would be a good idea to clarify what most of us mean by a
"phase shift" in ecology. The definition provided by Tom Goreau is indeed
used in fields such as Electromagnetics. Our use of the term in ecology
stems from the field of Dynamics. It refers to a persistent change in the
values of two or more variables describing a state of something, from the
standpoint of a phase diagram (representing a phase space), also known as a
state diagram (representing a state space). In ecology, these diagrams have
been used for many decades, particularly in the context of plotting
predator-prey abundance relationships, where the 'something' is a defined
area in which the predators and prey dwell.
To understand how we view the coral-algal phase shift from a dynamics
standpoint, one could make a two-dimensional plot of the amount of coral
(x-axis) vs. fleshy macroalgae (y-axis) in a portion of a reef over time.
Over a few years, a cluster of points from field measurements might form a
cloud in a region of the plot high in coral and low in algae. Then, a
hurricane might come along, and for many years another cloud of point might
be plotted in a region high on the algae axis and low on the coral axis.
Each cloud represents a phase in the phase space defined by coral and algae.
A phase shift occurred in the form of the abrupt (in this case) transition
from one cloud of points to the other. Resilience would be the tendency, if
it existed, for the portion of reef being studied to shift back from the
second cloud to the first (back to coral dominance) in later years.
Other terms used in various subfields of Dynamics are state change, phase
transition, phase change, etc. Phase shift is also correct, and is the term
most widely used in coral reef ecology.
These terms describe phenomena, not causality. The reef community may have
shifted to algal dominance after the storm because of nutrient loading,
changes in the food web, or some combination of the two. The persistence may
be due to natural factors, such as exclusion of coral planulae by the
macroalgae, or it may be driven by human activities leading to nutrient
loading, fishing for herbivores, etc. It is important to attempt to separate
these factors out, because one needs to know whether or not changing one or
more human influences will result in the desired shift back to coral
dominance, and how quickly. There is a lot of inter-reef and intra-reef
variability to deal with, so fully understanding these dynamics is going to
take a good combination of experimentation and comparative reef research. In
the meantime, though, it is very helpful to have some phase space
terminology and the analytical tools that are based on phase spaces
(including ordination, biplot analysis, etc.) to help us make sense of what
is going on.
John W. McManus, PhD
Professor, Marine Biology and Fisheries
Coral Reef Ecology and Management Laboratory (CREM Lab)
Director, National Center for Coral Reef Research (NCORE)
Rosenstiel School of Marine and Atmospheric Science
University of Miami, 33149
Office: 305-421-4814/4820, Fax: 305-421-4910, Website: www.ncoremiami.org
If I cannot build it, I do not understand it. -- Richard Feynman, Nobel
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