[Coral-List] The truth about top down vs bottom up effects on coral reefs

Steven Miller smiller at gate.net
Tue Oct 17 17:02:12 EDT 2006

Personally, I think the top down vs bottom up debate is useful mostly 
because it makes it easier to write papers and grant proposals. However, 
I think that if you adhere too strongly to either view then you will 
miss out on the more interesting stuff at best, or you'll be wrong at worst.

For those with a dog in the fight regarding factors affecting coral reef 
condition (top down vs bottom up) I think it's important to note that 
much of the decline story in Florida centers around the acroporids.  
Florida reefs began their downward slide in the late 1970s and early 
1980s.  And as in much of the Caribbean the cause was whiteband 
disease.  The demise of Diadema in the early 1980s was significant too.  
And overfishing in  many places predated the demise of Acropora and 
Diadema.  Then later in the decade significant coral bleaching began and 
global warming became a consensus view not too long after.   We are 
quickly entering into the realm of a multi-factorial problem and I 
haven't even mentioned algae or nutrients.  Did nutrients kill the 
acroporids?  No, whiteband killed the acroporids, near and far from 
sources of nutrient pollution.  Did algae kill the acroporids?  No, 
algae did not overgrow the live acroporids but instead followed their 
demise as new dead coral habitat became available.  How much algae 
developed and where it prospered needs to be evaluated based on local 
conditions related to nutrients and grazing.  And coral bleaching killed 
acroporids too.

What about other coral species in Florida?  I find it useful to think of 
the acroporids as the "canopy" species in the shallow to 10 meter depth 
range, where it was once abundant and common in many places.  What 
happens to a community (think forest) when you remove the canopy?  Good 
question for another time.  And what about deeper coral habitats in the 
Keys, beyond 10 meters to say 40 meters?  Better, the same, worse? Few 
studies exist for these deeper - but widespread - habitats.  And what 
about mid-Channel patch reefs, some of which remain in relatively good 
condition in the Keys but are located closer to proposed sources of 
nutrient pollution than the offshore reefs?  And don't forget that reefs 
in Florida are near their northern limit of geographic distribution, 
they are hit by hurricanes, some are periodically bathed by the 
classically inimitable waters of Florida Bay, others are influenced by 
the Gulf of Mexico, cold and nutrient rich upwelling occurs frequently, 
and more. 

Can nutrients kill reefs?  Obviously, and examples were cited previously 
in other list-server posts.  Can algae overgrow corals?  Obviously, but 
the relative importance of this phenomenon is seldom quantified.   Reef 
recovery related to nutrients, algae, and Diadema is another story and 
certainly one that's important at the intersection of science and 

Steven Miller
UNC Wilmington

Here is the newspaper headline that would likely appear based on my 
above comments...

Nutrients kill reefs.  Algae overgrow corals.  Dogs involved too.

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