Degraded Reefs May Be Saved By Young Coral

Coral reefs are made up of many types of living creatures including fish, seaweed, urchins and turtles but all of those creatures live on reefs because of the coral, which provides them with homes and food. Coral organisms themselves rely on tiny algae that live inside them for food, while these tiny algae receive a home. When the ocean water becomes too warm, coral organisms and the algae that live inside them get stressed, and sometimes the algae are forced to leave their coral homes. When this happens, the coral turns white, which is called coral bleaching. If the coral organism is separated from its algae for too long, the coral will die. If this happens to many coral organisms on a reef, the reef itself will degrade. Researchers predict that 70–99% of reefs worldwide will have degraded due to bleaching by 2100 [1]. However, some reefs are in areas of the ocean that have not yet heated up as much as others and are not predicted to heat up as much in the future. These reefs are at low risk of degrading in the future [1, 2]. One group of researchers gave every reef around the world a score based on how likely it is to be degraded by climate change (e.g., by bleaching or by storms) [2]. Reefs with a high score are unlikely to degrade because of climate change, so we will call them low-risk reefs for the rest of this article.

Adult coral are stuck to the ocean floor and cannot move, but they produce young coral that are released into the ocean and are carried by ocean currents until they mature and settle on the ocean floor. If they settle where the conditions are just right for them to grow, they will mature into adult coral. If there is a strong enough ocean current between two reefs, young coral from a low-risk reef may be carried to a less healthy reef, possibly saving it from degradation (Figure 1A). In this way, reefs are connected by ocean currents into reef networks, made up of all the reefs that can send or receive young coral to/from each other. Reefs in the same reef network may be able to rescue each other.

Which Reefs are in Which Reef Networks?

We determined which reefs are connected within the same reef networks by calculating how likely it is that many young coral will be carried by ocean currents from any one reef to any other reef. To calculate this, researchers from around the world used a computer model to simulate the movement of young coral among reefs [3]. They then used these movement data to calculate the number of times young coral moved to and from particular reefs between 2003 and 2011. We only included the connections between reefs that young coral traveled along often (based on data collected by other researchers [4]) to determine which reefs were and were not connected into reef networks. This became our present-day young coral movement computer model. Over the 12,292 reefs of the world, we found 604 reef networks, with six particularly large ones (>290 reefs) and many small ones (Figure 2).

What if Only the Low-Risk Reefs Survive?

To determine which reefs are low-risk, we re-calculated the reef scores (from [2]) to only account for how likely a reef is to be degraded by climate-related factors, such as bleaching and dangerous storms. A reef was designated as low-risk if it scored in the top 29% of our re-calculated reef scores. We then determined which reefs were in which reef networks if only the low-risk reefs survived. We found that many of the reef networks remained fairly large. This tells us that the reefs that degraded in our model must have been in either small reef networks or that the reefs themselves were not essential to keeping the surviving networks together (that is, they must have been like reef Y in Figure 1B). This is good news for coral reefs, as it tells us that, if only the low-risk reefs survive into the future, they may be able to send young coral to each other, increasing the chances that those reefs will survive.

Can Low-Risk Reefs Rescue All the World’s Coral?

If only the low-risk reefs survive into the future, how many of the world’s reefs will they be able to send young coral to? We used our computer model to answer this question by calculating which reefs young coral could land on if they spread from the low-risk reefs. This meant that we calculated all the reefs (the “rescued reefs”) that the low-risk reefs could send young coral to, even those that can only receive young coral via formerly degraded reefs (Figure 3A). Unfortunately, we found that over half of present-day coral reefs could not be rescued by the low-risk reefs (“degraded reefs” in Figure 3B). This is because most of the low-risk coral reefs are in the same big reef networks and are unable to rescue the many reefs outside of those networks.

What Did We Learn and Why is it Important?

In this study, we learned that most reefs are either in very big or very small reef networks and that these networks will mostly survive if only the low-risk reefs survive. Knowing which reefs are connected to which other reefs and whether those reef networks will survive is really important, as it helps us determine how to protect coral reefs worldwide—it tells us which reefs should be managed together, and which should not. Our results also stress the importance of finding and protecting at-risk reefs that are good sources of young coral for degraded reefs (see Figure 3B), since we now know that the degraded reefs cannot be rescued by low-risk reefs. We are currently working hard to figure out where these good source reefs are, so that we can protect them from future harm. Climate change is degrading reefs around the world at frightening rates, so any information that tells us which reefs can help us restore the world’s coral reefs will help us avoid riskier and more expensive methods for saving reefs. Overall, these results will help save worldwide coral reef networks by guiding reef-preservation efforts around the world.

Glossary

Coral Reef: ↑ A coastal ocean ecosystem in the tropics dominated by coral, which provide homes and food for a huge variety of fish, invertebrates, and other organisms.

Coral Bleaching: ↑ When coral become stressed due to increased ocean temperatures or pollution, they lose the algae inside them and turn white. If coral stay bleached for a long time, they die.

Climate Change: ↑ Around the world, climates are becoming more variable and more extreme than before (hotter or colder) owing to human development causing increasing levels of greenhouse gases in the atmosphere.

Reef Network: ↑ The set of reefs that send or receive young coral to/from each other often enough that they may be able to rescue each other.

Computer Model: ↑ A set of instructions that a computer uses to simulate how a real-world ecosystem or collection of ecosystems (like a reef network) may behave under changing conditions.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This research was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Canada Graduate Scholarship to AG and NSERC Discovery Grants and Canada Research Chairs to MK (grant no. 4922) and M-JF (grant no. 5134). We thank S. Wood and the other authors of the paper by Wood et al. [3] for sending us the global connectivity matrix that they generated in their study and the coordinates of the corresponding coral reef centroids, without which this work would not have been possible. We wish to thank the Digital Research Alliance of Canada for providing computational resources and C. Brookson for their coding help. We also wish to thank A. McLeod and D. Mouillot for helpful comments on the project. Finally, we would like to thank the Frontiers Planet Prize team and the Jury of 100 for awarding our source article the “National Champion of Canada” prize and thus providing us with the opportunity to share our research as a Frontiers for Young Minds article.