The Pocillopora Paradox on the Great Barrier Reef
Corals are changing on some Northern GBR reefs
AIMS Reef Monitoring
By visiting the same reefs regularly AIMS reef monitoring can detect changes that have occurred as a result of climate change and other environmental stressors.
What is the Pocillopora paradox?
This story is about changing coral communities on offshore reefs between Cairns and Lizard island. Recently we discovered the coral Pocillopora is thriving on some reefs where other corals are struggling. The paradox is we don't yet know if increases in Pocillopora are too much of a good thing for "The Reef": we need to keep collecting good quality long-term data to understand the effects of changes that are occurring now.
There is more than one explanation for increases in Pocillopora
Historically on the Great Barrier Reef (GBR) Pocillopora dominance has been limited to small parts of reefs and the coral. On some northern GBR reefs coral is increasing but in 2021, Pocillopora had become more dominant than the more typical Acropora corals. Examples from other regions show that in high disturbance environments on offshore reefs, Pocillopora can become the dominant coral. Sometimes this is transitory and sometimes a long term change is occurring.
Survivors, selective mortality, more flexible reproduction
Use the drop-down to explore increases in Pocillopora on 3 reefs on the northern GBR. Each reef potentially has a different mechanism to explain the increase in Pocillopora and these are discussed below. Its likely that the different mechanisms will result in diverse outcomes in coral assemblage structure in years to come.
Adult Pocillopora are enduring: a survival story from Agincourt Reef
At Agincourt Reef,Pocillopora have survived whereas other corals have died. In 2021, coral was increasing, but Pocillopora is increasing faster than the previously most abundant Acropora coral. This increase is likely to be from higher survival of adults as the number of juvenile corals has not increased.
Pocillopora's competitors are dying: a selective mortality story from Thetford Reef
Between 2014 and 2020, Thetford Reef was exposed to crown-of-thorns starfish (COTS) and heat stress causing declines in coral. In 2021, coral increased on Thetford Reef with adult Pocillopora being more abundant than Acropora. This shift was not unexpected as Acropora is targetted by COTS resulting in large declines in that coral type. However, juvenile Acropora corals were more abundant than Pocillopora juveniles and it is likely Acropora will become the dominant coral during future recovery, unless the reef experiences more "shocks". Acropora dominance during the recovery phase has happened previously.
New Pocillopora are arriving: a recruitment story from No Name Reef
On No Name Reef coral was increasing in 2021. In 2019, juveniles of Pocillopora were more abundant than the dominant coral Acropora for the first time since surveys started in 1994. Pocillopora has an advantage when coral spawning is disrupted (as happens after coral bleaching events), as it can produce baby corals every month (not just once a year like Acropora). It is possible that Pocillopora dominance will continue to increase at No Name Reef.
What's been happening on the northern offshore reefs of the GBR?
Disturbances drive coral decline. Recovery of coral takes place when the reefs are free of cyclones, heat stress and predation from crown-of-thorns starfish (COTS). Since 2014 cyclones and heat stress have caused coral decline, and recovery periods have been short.
The web map and the graphic for crown-of-thorns below shows known disturbances between 2014 and 2020 for reefs featured in this story. Click on each reef to see the reef name. The colours on each point shows the number of years where Degree Heating Weeks (DHW), a measure of heat stress was greater than four. DHW>4 is expected to cause coral bleaching. The lines show cyclones since 2014.
Estimates of crown-of thorns (COTS) densities from manta tow surveys
Degree heating weeks plotted from NOAA data accessed at ftp://ftp.star.nesdis.noaa.gov/pub/sod/mecb/crw/data/5km/v3.1/nc/v1.0/.
The Pocillopora Paradox
On GBR reefs the effects on reef function and likely longevity of the change in the coral community from Acropora dominance to Pocillopora is unknown. However, previous studies by AIMS have shown that a loss of Acropora can reduce the abundance and diversity of associated reef fish assemblages.
Continued monitoring will help to determine if Pocillopora becomes the "fittest" dominant coral on the northern offshore reefs and how that will influence the reef ecosystem.
Credits
Emma Kennedy, Kate Osborne, Mike Emslie, Manuel Gonzalez Rivero
NOAA data ftp://ftp.star.nesdis.noaa.gov/pub/sod/mecb/crw/data/5km/v3.1/nc/v1.0/. Map displays known disturbances to AIMS Reef Monitoring survey reefs between Cairns and Cooktown-Lizard Island from 2014-2020. The data diplayed is the number of years with more than 4 DHW based on NOAA data.
