Reefs at Risk - Guilds questions

Thaddeus J. Murdoch tmurdoch at
Sat Jun 27 22:38:31 EDT 1998

Dear Dr. Steneck,

In answer to your questions:

>1.  How can patterns of decline be determined from a single survey?

It was not my intent to show guild-specific differences in decline over 
time, although I would love to be able to do so.  Additional surveys of the 
entire reef tract are needed to accomplish this.

>2.  Since percent cover of coral varies for many reasons (including
>morphogenetic reasons) how do you sort out reefs that always have had low
>cover from those that have declined to that level recently?

My intent was to examine whether all coral guilds responded equally to 
whatever disturbances resulted in the reef-to-reef differences in total 
coral cover.  If all of the guilds were equally affected by the slew of 
disturbances that assaulted each reef up to the point in time when we 
sampled, one could reasonably expect to see all guilds demonstrate higher 
cover on reefs with high total coral cover, and lower cover on the reefs 
with low total coral cover, when compared over the entire reef tract as a 

The results, on the other hand, show that only the massive spawners 
demonstrate coral cover which varies linearly with total coral cover per 
reef.  All other guilds do not vary.  In other words, reefs which we would 
qualitatively classify as healthy (i.e. high coral cover) have the SAME 
cover of massive brooders, plating corals and branching brooders as a 
reef we would qualitatively classify as unhealthy (hardgrounds too!).  
This implies not only that all guilds do not respond equally to whatever 
factors determine total coral cover from reef to reef, but that most 
guilds are hardly affected at all, as long as there is rock to live on.

The next question is - Have only the massive spawning corals and the 
branching spawners been declining over Recent time in the Keys, or have 
all guilds?  The above results indicate that all guilds might not have 
been declining.  I don't have the data to answer this question, yet.  
Perhaps others do.

>3.  Are you surprised at the low acroporid abundance at 20 m? etc...

Sorry, I should have been more specific.  What I meant was that _Acropora 
cervicornis_  was absent. Many of the reefs we sampled had large amounts 
of A. cervicornis rubble on them, implying that the reefs had possessed 
A.cervicornis in the past.  I think that the graph I presented would have 
looked different in regards to this guild had we sampled a few decades ago.

>4.  Perhaps I don't understand your figure but wouldn't that pattern
>develop if massive spawners are the dominant corals at depths of 20 m.

They are the dominant corals.  Since the massive spawners (and 
branched spawners) possess the properties that make them the dominant 
corals, and since they appear the most affected by water quality, the 
corals of these guilds are the best choice for forced-recruitment in the 
Florida Keys. Subordinate guilds, with slower growth etc., are less 
likely to increase total coral cover over an entire reef, even though 
they are better able to cope with poor water quality.

>If that's so, wouldn't a similar figure be generated on just about every
>reef in the Caribbean?

I would be thrilled if these results scaled up to predict the guild 
compostion of the deeper spur-and-groove reefs over the entire Caribbean, 
relative to reef-by-reef differences in disturbance history.

On a separate issue, I am not sure we need to resort to 
forced-recruitment in the Keys at the moment. For instance, the sexually 
generated offspring of A.cervicornis appear to be recruiting back to the Keys
on their own.  However, steps to improve the water quality of 
the Caribbean should be taken immediately.


Thad Murdoch
Project Manager
No-Take Zones of the Florida Keys - Benthic Monitoring Project

Dauphin Island Sea Lab,
PO Box 369, Dauphin Island, Alabama, 36528, USA.

Tel: (334) 861-7532  Fax: (334) 861-7540

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