by My Georgia Tech colleague Mark Hay recently published a study showing that the removal of sea cucumbers from reef communities resulted in an increase in organic waste materials and a 15-fold increase in coral mortality. Protecting sea cucumbers, which are over-exploited as a food source, could help keep coral reefs healthy.
The role of ocean connectivity
Coral reefs are not isolated entities. When fish and corals spawn, they release millions of larvae that flow on currents and are exchanged across reefs through mixing and transport processes. These exchanges form the connectivity of coral reefs.
Some reefs provide larvae for many others. Other reefs are more isolated, so scientists may need to artificially introduce larvae to help the reefs recover from bleaching and other threats.
Research shows that well-connected reefs recover more quickly from stresses such as bleaching. Obtaining large supplies of coral and fish larvae, carried by ocean currents from afar, helps to restore bleached areas and maintain diverse reef communities. Preserving this connectivity is critical to preserving reefs.
But measuring connectivity is time-consuming and expensive. Researchers collect coral samples from potentially connected reefs and analyze the coral’s DNA to reconstruct its genetic history. This gives a picture of correlation between different populations.
We also use computer models to simulate ocean currents that transport virtual larvae from their release points to their nesting sites. But streams are not fixed like highways: Their strength and direction change over time, depending on the season and the year. Studying reef connectivity requires multi-year model simulations covering small areas in detail, which is computationally intensive.
Insights from machine learning
Now, machine learning offers a new way to analyze reef connections. This subfield of artificial intelligence enables computers to learn tasks or associations without being specifically directed. Instead, they use algorithms to tackle different tasks.
My research group has developed a tool that takes information about ocean currents, derived from satellite data; it generates a network of ecoregions, or areas where the ecosystems are generally similar; and calculates its connectivity for the last 30 to 40 years. We then identify better-connected reefs by using PageRank centrality – the algorithm developed by Google to measure the popularity of web pages.
Once we have defined an ecoregion network, it is possible to determine how “popular” – that is, connected – known reefs are. And we can verify that our algorithms are working well by seeing if “popular” reef communities have more diverse and healthy coral populations.
We have found that in Southeast Asia’s Coral Triangle – the world’s most biodiverse hot spot – periodic transitions between El Niño and La Niña climate patterns promote biodiversity, and have probably done so for 5 million to 7 last million years. Connectivity during El Niño events is very different from connectivity during La Niñas, because ocean currents are different. This dynamic helps reefs by ensuring that they receive larvae from many different places in different years.
Understanding which coral and environmental characteristics help reefs to be resilient, and coupling this knowledge with connectivity networks opens up new ways to help corals live a little longer. Together, these approaches indicate how and where to prioritize monitoring and reform efforts.
In the long run, the only ways to stop dying of coral reefs and protect the reef habitats that support so much sea life are limiting water pollution and limiting climate change. But more localized actions that make reefs healthier can buy them time, and perhaps make them more resilient to ocean warming.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
It was written by Annalisa Bracco, Georgia Institute of Technology.
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Annalisa Bracco benefited from funding from the National Science Foundation and the National Oceanic and Atmospheric Administration for this work.
