[Coral-List] Great Barrier Reef is rapidly losing coral
douglasfennertassi at gmail.com
Tue Nov 6 20:31:56 EST 2012
* the idea that predators control crown of thorns has relatively little
evidence to support it.
* the hypothesis that nutrient runoffs cause crown of thorns outbreaks has
a great deal of evidence to support it.
* although ecosystems are complex, not all hypotheses have equal evidence
supporting them, and we do know something about the cause of crown of
The evidence for the role of human removal of predators in producing
crown-of-thorns outbreaks has always been quite slim. Originally Triton
snails were known to eat them, and humans to collect Triton shells.
However, Triton shells are pretty rare even where there are no people, and
they are slow to eat a single starfish. No way could a natural population
of Tritons have any effect on an outbreak population of crown-of-thorns. A
handful of Triton shells can't eat millions of starfish, can't be done.
Harlequin shrimp also eat them, takes a pair of these little shrimp a long
time to eat just one, and the shrimp are generally rare. The one place
that predators do have an effect on them is in the eastern Pacific, where
Peter Glynn documented that once the surface of the starfish was broken,
there is a polychaete worm there that burrows in and eats them from the
inside out. There is a Dulvy paper that documents an effect in Fiji, where
villages still exercise the traditional ability of village to exclude
outside fishers, fishing is correlated with crown of thorns. Dulvy's team
correlated the fishing pressure (in the form of village population divided
by the area of the village reef as a proxy of fishing pressure) with the
density of crown of thorns population on the village reef. The heavier the
fishing the higher the population of crown of thorns, a significant
correlation. But as Chuck Birkeland pointed out, higher village population
is also likely to be correlated with more nutrient runoff, so the result
can be explained by nutrient limitation as easily as by fishing pressure.
Some people have speculated that humphead wrasse, which eat a variety of
toxic things, eat crown of thorns and normally control them, if humphead
wrasse are removed, then there are crown of thorns outbreaks. As Hugh
Sweatmann has said, it is very hard to demonstrate any fish eating crown of
thorns (anyone who has been stuck by one of the spines knows why). So far
as I know, the report that humphead wrasse eat crown of thorns derives from
a paper by John Randall in which he examined stomach contents, and found
one crown of thorns in the stomach of one humphead wrasse. The best
evidence is a more recent paper by Hugh Sweatmann in which he reports that
there are fewer crown of thorns outbreaks on reefs that are in MPA no take
areas, than outside the no take areas, on the Great Barrier Reef. How the
no-take area has that effect is not known, but a guess would be that the
fish population is changed in some way that makes it happen. Of course,
correlation is not causation, still it looks like a connection.
The evidence is far stronger for the hypothesis that nutrient runoff
causes crown of thorns outbreaks. Chuck Birkeland came up with the
hypothesis that if there are more nutrients in the water when the starfish
spawn, there will be more plankton for the larvae to eat and more larvae
should survive. After the larvae settle, the tiny starfish hide in holes
and eat coralline algae. As they grow, eventually they start eating
corals. They first start coming out of the holes at night when they are
about 10 cm diameter, and then eventually when they are larger and if there
are many of them, they stay out during the day. Outbreaks typically appear
out of no where, suddenly there are all these adult crown of thorns eating
all the corals. We know they had to start out small, so where did all
these adults come from suddenly? The answer is they were there all along
for 3 years hiding in holes and eventually getting large enough to come out
at night and later come out during the day. In addition, Birkeland
reported that outbreaks are almost always on high islands, almost never on
atolls, and cultures on high islands have names for them and stories about
them, while on atolls they do not. Even atolls that are heavily populated
don't have outbreaks. Further, he was able to correlate big rainstorms
that followed dry periods (and thus nutrients accumulated and then were
washed out) with outbreaks 3 years later. Brodie et al published a paper
that outlined each step in the logical sequence of the hypothesis, and the
evidence for or against each step, and found that every step had good
evidence supporting it, except one that didn't have evidence one way or the
other. Fabricius et al have published a supporting paper subsequently.
If I remember there are now a few reports of relatively small
outbreaks on low islands. I haven't kept up with everything on the topic,
but the runoff hypothesis is much better supported than the predation
hypothesis. As we know, runoff into the Great Barrier Reef is significant.
There is a small section reviewing this topic in:
Fenner, D. 2012. Challenges for managing fisheries on diverse coral reefs.
Diversity4(1): 105-160. (open access)
Glynn, P.W. 1984. An amphinomid worm predator of the crown-of-thorns sea
star and general predation on asteroids in eastern and western Pacific
coral reefs. Bulletin of Marine Science 35: 54-71.
Dulvy, N.K.; Feckleton, R.P.; Polunin, N.V.C. Coral reef cascades and the
indirect effects of
predator removal by exploitation. Ecol. Lett. 2004, 7, 410–416.
Sweatman, H. No-take reserves protect coral reefs from predatory starfish.
Curr. Biol. 2008, 18,
Birkeland, C. 1982. Terrestrial runoff as a cause of outbreaks of *Acanthaster
planci *(Echinodermata: Asteroidea). Marine Biology 69: 175-185.
Birkeland, C. 1989. The Faustian traits of the crown-of-thorns starfish.
American Scientist 77: 154-163.
Brodie, J.; Fabricius, K.; De'ath, G.; Okaji, K. Are increased nutrient
inputs responsible for more
outbreaks of crown-of-thorns starfish? An appraisal of the evidence. Mar.
Pollut. Bull. 2005, 51,
Fabricius, K. E., Okaji, K., De'ath, G. 2010. Three lines of evidence to
link outbreaks of of the crown-of-thorns seastar *Acanthastrea planci* to
the release of larval food limitation. Coral Reefs 29: 593-605.
On Sun, Nov 4, 2012 at 9:01 AM, Eugene Shinn <eshinn at marine.usf.edu> wrote:
> Thanks David for a thoughtful posting re GBR COTS outbreaks I don't
> recall the COTS spines as much as I remember the sand sized grains
> that are the individual components that make up the starfish tests.
> The were the major component of the surficial reef sand at that time
> (1972). I agree pleased that such events have happened in the past
> and that we will probably never isolate a single cause. Such
> outbreaks are likely as complicated as Snowshoe rabbits or Lemming
> population explosions or the various factors that lead to the
> occasional mosquito or lovebug outbreaks here in south Florida. I
> suspect readers can envision other such biological outbreaks. Gene
> No Rocks, No Water, No Ecosystem (EAS)
> ------------------------------------ -----------------------------------
> E. A. Shinn, Courtesy Professor
> University of South Florida
> College of Marine Science Room 221A
> 140 Seventh Avenue South
> St. Petersburg, FL 33701
> <eshinn at marine.usf.edu>
> Tel 727 553-1158----------------------------------
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