[Coral-List] The myth of 100% coral cover

David Fisk davefisk at gmail.com
Sat Mar 24 14:37:18 EDT 2012

 I have two points to raise here. Firstly, I think it is a good point from
Eugene's comments that the often assumed typical ideal conditions for coral
reef development and persistence over time is not as straight forward as we
would like them to be. Eugene mentioned former high coral cover communities
in the Dominican Republic, which sound like typical near shore naturally
turbid water environments where coral reefs can and do thrive today. This
is not exceptional in terms of persistence in such a supposed extreme
environment for corals. nor in terms of high live coral cover. There are
many healthy and resilient near shore reefs on the Australian GBR and
throughout Asia, as well as around continental islands in the Indo-Pacific
which have such coral communities. Many near shore reefs in the GBR are
actually thin carbonate veneers overlying mud deposits - place a heavy
retaining rock wall on top of these fringing reefs and it may just blow out
down the slope (like squeezing a balloon), This has occurred in the
Whitsunday Islands (GBR).

My other point is that the current discussion of one of the cornerstones of
coral reef science - percent coral cover - in this case, the theoretical or
real maximum value possible on contemporary reefs, has not touched on its
limitations in a more general sense. For reasons outlined below, I believe
the discussion on cover ranges is not very meaningful in itself. For a
start, it is possible to have greater than 100% cover, though this cannot
be accurately measured using most in situ methods. I am thinking of
situations where there are overlapping and multiple layers of live plate
form colonies on steep slopes. Such reefs do occur on outer reef slopes in
the Pacific. In any case, trying to define what do healthy and normal reefs
look like, has to include consideration of the other critical variables wrt
measuring coral cover. These are the spatial scale chosen for estimating
cover (ones definition of a site) and the size of the sampling unit used
(quadrat, transect length etc).

Furthermore, though percent cover is a very useful statistic for
descriptive purposes as it is easy to visualise (by integrating colony
sizes and colony numbers of a sample into a simple statistic), I think its
usefulness is limited and frequently over emphasised in reef status and
impact assessment studies. This is because one cannot accurately visualise
the size composition nor the colony growth form composition of the
communities from a simple percent cover estimate. The latter two variables
are key attributes from which coral reef health and resilience should be
assessed. For example, the Pacific examples I mentioned earlier were
regenerating coral communities several years post impact from very
destructive cyclones, and consisted of no more than 2-3 fast growing, high
recruitment, and short lived, opportunistic species. In the Pacific context
this would be considered a relatively poor coral community by most
measures, except percent cover.

It is of concern that most data from current monitoring methodologies (eg,
GCRMN, Reef Check etc) are influencing management policy and advice to
governments wrt preservation using only this simple but potentially
misleading metric. Changes in percent cover can make for sensational
headlines, but is it confusing the issue rather than helping it?

Dave Fisk

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