[Coral-List] causes of Acropora loss
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
> 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
> among organisms, being some of them affected and dying much faster
> others, in the presence of the same pathogen; moreover, disease
> 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
> 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.
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.
Barton, A. D. and K. S. Casey (2005). "Climatological context for
large-scale coral bleaching." Coral Reefs 24: 536-554.
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