[Coral-List] Great Barrier Reef is rapidly losing coral
douglasfennertassi at gmail.com
Mon Nov 12 22:02:16 EST 2012
*we don't have to choose between top-down predator control of
crown-of-thorns and bottom-up nutrient control, both could happen
*the pattern of some crown-of-thorns outbreaks are hard for predators to
explain. Predators would have to be common enough to control them for
decades, then go away for a couple years or so, then return for decades
*variability in larval survival is well known in other invertebrates and
fish, and can easily explain outbreaks.
We have a tendency to think that we have to decide that either
top-down effects control crown-of-thorns (or algae), or that bottom-up
effects control them. But in fact, as causal factors they are independent,
and both could be acting at once, or one could in one situation and the
other in another situation. Both could be very real causal factorst. In
fact, predator control by a worm is well documented in the East Pacific.
Some crown-of-thorns outbreaks, such as the one that happened in
American Samoa in 1978, are relatively brief events, lasting just a year or
two. For predators to cause the outbreak, the predators would have to have
been common for decades before the outbreak, disappear during the outbreak,
and then return and be there for decades more. But the predator hypothesis
is usually that humans have removed a predator, and doesn't include that
the predator was put back. Even if we assume that the end of an outbreak
is because the starfish have eaten so many corals that the starfish all
starve to death, then how come after the corals recover, there isn't
another immediate outbreak?
That said, there are likely to be predators on crown-of-thorns,
probably more when they are small and young and not so well defended, than
when they are large and well defended. The predators might keep them under
control except when there was a huge recruitment event (which could be
caused by nutrients fueling a plankton bloom providing food for larvae).
So both factors could be important, but outbreaks would be mainly caused by
a recruitment event. I don't remember good documentation on what the
predators of young crown-of-thorns are.
Large recruitment events are known in many marine species, not just
crown-of-thorns. That includes a variety of echinoderms, some other
invertebrates (I remember a tiny sea slug that eats gorgonia (if I remember
correctly) in the west Atlantic, and which is normally rare. But one year
there was a huge number. Following years they were rare again.) High
variability of recruitment in space and time are well documented for coral
reef fish. I remember a few years ago reports on coral-list of huge
settlements of pufferfish in the west Atlantic. We shouldn't be so
surprised by outbreaks of crown-of-thorns, since high recruitment
variability is common in reef organisms.
I got the points about multiple controls and the problem with predator
control hypotheses from Birkeland & Lucas, Chapter 6:
Birkeland C, Lucas J (1990) *Acanthaster planci*: major management
problem on coral reefs. CRC Press, Boca Raton
On Tue, Nov 6, 2012 at 2:31 PM, Douglas Fenner <douglasfennertassi at gmail.com
> * 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
> thorns outbreaks.
> 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.
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Pago Pago, American Samoa 96799 USA
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