Good news can be hard to come by on the Great Barrier Reef, which is threatened by warming ocean temperatures, acidification and deadly predation by crown-of-thorns starfish.
A recent research paper, published in the journal PLOS Biology, identified a small bright spot. A team of researchers hunted for reef structures that could help heal others after severe impacts. The idea is that these “source reefs” could send healthier coral polyps to recolonize areas that were hit by bleaching or predation.
To find these source reefs, the researchers modeled the ecosystem and looked for individual reefs that would meet three criteria: They were connected by ocean currents to the other reefs around them, relatively sheltered from disturbances and less likely to be colonized by crown-of-thorns larvae.
Karlo Hock, a postdoctoral biologist at the University of Queensland in Australia and lead author of the paper, was surprised that the model identified 112 individual reefs, or about 3 percent of the Great Barrier Reef, that met these standards. That’s not a lot, but he wasn’t sure he’d find any going into the project. They were mostly grouped in the southern part of the reef, because that’s where strong ocean currents were dispersing coral larvae.
Oceans Deeply spoke with Hock about how these source reefs may be used in future ecosystem management.
Oceans Deeply: When you say a “source reef,” what does that really mean?
Karlo Hock: A source reef is a reef that can supply lots of larvae to other reefs. Once the corals spawn, their larvae go up in the water column, and then ocean currents carry them to other reefs. This means that if those other reefs have been damaged by some disturbance, let’s say bleaching or an outbreak of coral-eating crown-of-thorn starfish, these larvae can help those reefs recover. A good source reef is going to supply lots of larvae to lots of reefs and do this consistently over time.
Oceans Deeply: You mention in the paper that this idea of a source ecosystem has rarely been looked at in research. Why?
Karlo Hock: It’s fairly new. Usually people tend to focus on what happens on a particular reef, so you look at whether that reef has lots of fishing, has lots of pollution, whether it had bleaching in the past. But that is just one reef. The Great Barrier Reef is a system of nearly 4,000 individual reefs, so we took a step back and wanted to say, “OK, which of these reefs is going to help other reefs recover?”
It means that you have to look at the system as a whole rather than individual components, so it is actually a complex system with emergent properties. That is very interesting to me, but it can be challenging.
Oceans Deeply: You look at three criteria for source reefs. What about other impacts that can damage corals like pollution and cyclones?
Major currents from the Pacific Ocean push cold water into the southern reefs, making them less likely to be hit by coral bleaching. This likely also helps spread coral larvae and drive out crown-of-thorns starfish.
Hock: Even though we haven’t incorporated them directly into the analysis, the reefs that we have identified should have lower risk of cyclones and low water quality. [The identified reefs] tend to be located offshore. They tend to be away from the river mouths, so they tend to have better [water] quality and they are also located in regions that have lower risk of being hit by cyclones.
Oceans Deeply: How are you hoping this data will be used?
Hock: What we’re hoping is that this is going to provide some additional focus for management. The biggest threat to the Great Barrier Reef, and to coral reefs in general, are the global increases of CO2, which cause elevated sea-surface temperature and ocean acidification. [Those issues] require consistent action among different governments to work together. In the meantime, we can also do local management to ensure that the system can function in the best way possible. We can, for example, choose particular reefs to go try to cull the crown-of-thorn starfish to reduce the threat to coral. This provides a bit more focus, if we want to focus on which reefs are important for recovery.
The management agencies are definitely interested in this sort of advice. It means that you can dynamically adapt local management practices to fit with a particular disturbance to support recovery. You can do this for exact, tactical management, right now.
Oceans Deeply: In the paper, you say that that you were not able to look at the statistical significance of your model. Can you address that?
Hock: A statistical test, in this case, would tell you whether this is an unexpectedly high number of these reefs that satisfy all three criteria. These reefs are there, but whether this is an exceptionally high number of them, it’s hard to say.
Even if we wanted to do something like this, there’s lots of spatial autocorrelation, which means that reefs nearby tend to have similar values for different criteria that we have used. These are very difficult to integrate into statistical tests. We tried to solve it in the previous iterations of the manuscript, but we couldn’t come to an agreement with previous referees, so this is the approach that we ended up taking.
Oceans Deeply: Many of these source reefs are in the southern part of the Great Barrier Reef. What does that mean for the reefs in the north?
Coral showing signs of bleaching, with an entire coral colony turned white from recent stress. (Peter J. Mumby, courtesy of Karlo Hock)
Hock: It’s definitely a concern that there are so few of these source reefs in the north, which means that any system would tend to be more fragile in the north than in the south. The latest two bleaching events have generally affected the north of the Great Barrier Reef – this is why we see so few of these source [reefs] in the north, which tells you that these things could change.
The fact that we have just identified these reefs doesn’t mean that the reef is safe. It doesn’t mean that it is going to be safe – that this sort of systemic resilience cannot be lost or changed.
Oceans Deeply: Is there anything else that you would like to say?
Hock: People tend to ask what they can do for reefs. One of the things is that you can take an interest in reducing greenhouse emissions, support government policies that promote the environment and participate in citizen science projects. Here, we have a Citizens of the Great Barrier Reef group that does citizen science. You can reduce your plastic waste because plastic often ends up on reefs and damages organisms.
People often feel that there isn’t anything that they can do, or that reefs are so far away that they don’t have to worry about these things, but yes there is something that we can all do to help the system be as healthy as possible.