Predator/prey interaction
The pattern of crown-of-thorns starfish outbreaks
and hard coral on the Great Barrier Reef
By
Kate Osborne and Ian Miller
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Crown-of-thorns starfish (COTS) is a highly
specialised coral predator found on coral reefs
throughout the Indo-Pacific. The Reef Monitoring team
has collected data on COTS numbers and hard coral
cover on the Great Barrier Reef annually since 1984.
Click
here for more information
about methods
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For more detailed information on the biology and
life history of Crown-of-thorns see
 CRC
Reef COTS pages
Historically two series of
outbreaks (1966-74, 79-91) were responsible for wide spread
coral mortality on the central Great Barrier Reef (GBR). The
GBR is currently experiencing a third series (from 1994- ). In
this article we summarise some published information on the
relationship between COTS and the coral that they prey upon.
Adult starfish distribution
On the Great Barrier Reef there
are two characteristic patterns of COT distribution. Between
latitude 14 and 20 degrees starfish numbers wax and wane in a
wave from north to south. The animation Crown-of-thorns
starfish (COTS) shows
that the wave of outbreaks was in the middle of a southward
movement when monitoring began in the early 1980s. A new wave
starts in the north around 1994 and is currently centred off
Townsville (Latitude 19ºS). The second pattern is the
separate and continuous series of outbreaks on the southern
Swain reefs around latitude 22ºS. Some of these reefs
currently have high numbers of starfish.
The
southward sequence of outbreaks is probably driven by the East
Australian Current (EAC). Starfish larvae are pelagic so are
transported by the movement of water.
The waves of outbreaks seem to originate around
latitude14ºS, a region where current reversals periodically
occur. Between 15 and 20ºS, the EAC moves consistently from
north to south, presumably transporting larvae from reef to
reef to cause the southerly movement of outbreaks. The large
tidal currents around 20ºS may disrupt the southward
dispersal of larvae.
The effect of COTS
predations of coral on a large spatial scale
Starfish
are described as ‘out breaking’ when a population on a
reef reaches the level where coral is being consumed faster
than it can grow. Surveys
since 1984 have found that the average coral cover on reefs
with a COT outbreak declines at a rate of about 6% per year to
reach a low of about 9%. For comparison, coral cover on reefs
with no history of COT outbreaks ranged from 16 to 40%. Coral
cover has not increased on 25% of reefs that have suffered
COTS outbreaks. Increases
in hard coral after COTS outbreaks on the remaining 75% of
reefs were significantly greater than on reefs with no
outbreak history. Estimates of median recovery times for these
reefs (representing an increase of 30% coral cover above
post-outbreak levels) ranged from 10 to 25 years.(1).
One example of the
relationship between predation and community composition on
the GBR
Of
the 48 reefs between latitude 14 and 19 degrees that have a
history of outbreaks, 27 (56%) have experienced an outbreak in
the current series. Rib Reef and John Brewer reef are two such
reefs. (Table1). AIMS
has surveyed sites on the NE face of these reef since annually
1992, so we can examine the composition of the coral community.
(See the links below to open a separate window with coral
cover graphs). In these two examples where the time between
outbreaks is short, predation was focussed on one variety of
hard coral (Tabulate Acropora spp). Other coral types
were both less abundant and experience proportionally less
change over time. Studies
of the feeding preferences of COTS have shown that Acropora
spp. are highly favoured, especially tabulate species. The
least favoured coral type was the genus Porites (2).
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Table 1a:
Historical cover of coral and COTS density at John
Brewer Reef.
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Table 1b:
Historical cover of coral and COTS density at Rib
Reef. |
You can view a panoramic
footage of survey sites at Rib reef before, during and one year after a COTS
outbreak. You will need Quicktime 5 to view this movie.
 Crown-of-thorns
Starfish QuickTime movie (6.8Mb)
Click
here to view video footage of Rib reef before
and after COTS predation (steve clarke)
References
- Lourey
MJ, Ryan DAJ and Miller IR (2000) Rates of decline and
recovery of coral cover on reefs impacted by, recovering
from and unaffected by crown-of-thorns starfish Acanthaster
planci: a regional perspective of the Great Barrier
Reef. Mar.Ecol.Prog.Ser 196:179-186.
- De’ath
G and Moran PJ (1998) Factors affecting the behaviour of
crown-of-thorns starfish (Acanthaster planci L.) on
the Great Barrier Reef: 2: Feeding Preferences. J.Exp.Mar.Biol.Ecol
220:107-126.
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