[Coral-List] causes of Acropora loss

John Bruno jbruno at unc.edu
Thu Oct 4 20:53:44 EDT 2007

>> Diego L. Gil-Agudelo, Ph.D. wrote:  Please, correct me if I?m  
>> wrong, but
> diseases hardly affect 100% of the individuals of any population, as
> supposedly happened with Acroporas in the 80?s

Hola Diego, You probably know a lot more that me about the etiology  
of white band disease.  And it is true that disease prevalence is  
often less than 100%.  But it can indeed be very high as it was for  
Acropora spp., Diadema, etc.   Outbreaks of infectious diseases that  
kill 95% or more of individuals are not that uncommon in marine  
systems.  There are many well documented examples of comparable  
urchin die offs all around the world, seagrass die offs (such as the  
epidemic that nearly extirpated seagrass in New England in the  
1930s), oyster diseases, etc. Kevin Lafferty has written a number of  
excellent reviews on this (http://www.werc.usgs.gov/chis/ 
lafferty.asp).   And there are countless terrestrial examples; just  
think of American Chestnut Blight (http://www.apsnet.org/online/ 

> (sorry, I was in high
> school).  Besides, it is not good for pathogens to do so (who will  
> they
> infect next?).

True, but many of these examples have resulted from the introduction  
of novel pathogens; the pathogen and host have no evolutionary  
history.   And of course pathogens, like all life forms, are not  
optimized (otherwise they would have evolved wheels)-they do  
sometimes wipe out their host (or nearly so-Ebola anyone) even though  
this leads to their own demise.  We see predators do this all the  
time too.  Just think of what an Acanthaster outbreak does to a coral  

> If our data is right (and I?m sure it is), the WBD pathogen
> is a relatively common Vibrio species (most likely V. carchariae = V.
> harveyi), at least for A. cervicornis.  We found differences  
> (resistance?)
> among organisms, being some of them affected and dying much faster  
> than
> others, in the presence of the same pathogen; moreover, disease  
> stopped
> after couple days.

That is interesting, but not surprising.  Remember you are working  
with the 1% or so of the population (and their offspring) that  
survived the epidemic.  The proportion of resistant individuals is  
surely much higher now that it was 30 years ago before all the  
susceptibles were weeded out of the population.

>   This lead me to think that probably the Acropora loss  
> experimented during
> the 1980?s was the product of bleaching due to thermal events.  I  
> know this
> decade experienced strong Ni?o events, although I?m not aware of  
> analysis
> showing the coincidence or not of these thermal events with reports on
> Acropora die offs.

That is an interesting idea that was also discussed fairly  
extensively in the recent paper in JEMBE by Lesser et al. (2007).    
The trouble is, as far as we know, the early-mid 1980s (when nearly  
all Caribbean coral loss seems to have occurred) was a relatively  
cool period according to an analysis by Barton and Casey (2005); an  
often overlooked and very important paper.  Additionally, the white  
band symptoms were widely observed and documented-i.e., we saw it  
happen and it was not bleaching, overgrowth by algae, sedimentation,  
etc.  It was disease.

> To my mind, this is the only kind of event we know that
> can produce such massive deaths of these corals in such an ample  
> manner, in
> a large geographic scale and in a short period of time.

See my comments above.   We do know that infectious diseases can  
cause such rapid, devastating, and geographically widespread  
population declines .  Did you see what happened to crows in the  
eastern US in the early 2000s?  They and many other species suffered  
rapid population declines due to West Nile Virus.

> Diego

A GENRAL RANT, NOT DIRECTED TOWARD DIEGO:  Diseases are a common and  
very important ecological phenomena.  Pathogens and parasites are at  
least as important in regulating populations as competitors and  
predators.  I think in some cases (not all) human alterations to the  
environment have increased coral disease severity.   But we as coral  
reef scientists don't need to continue to grasp at other explanations  
for the Acropora die off of the 1980s.   White band disease was  
clearly not bleaching, it was not caused by coastal development, by  
nutrient pollution, by fishing or by algae.  It was/is caused by a  
bacterial pathogen (as nicely shown in an excellent  paper by Diego  
(Gil-Agudelo et al. 2006)) and its impacts were identical next to  
large urban centers and on very isolated, unfished reefs.   We don't  
need any convoluted explanations to understand what happened.

John B

Barton, A. D. and K. S. Casey (2005). "Climatological context for  
large-scale coral bleaching." Coral Reefs 24: 536-554.

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