Underwater gardens boost coral diversity to stave off 'biodiversity meltdown'

Coral Gardens in French Polynesia Credit: Georgia Tech
The corals that make up tropical reefs around the world are essential ecosystem services. However, stresses such as overfishing, pollution and warming oceans are threatening corals. Scientists know that corals create structures that provide living space for many species. However, coral diversity was not as important as it is today.

Two researchers from the Georgia Institute of Technology have released a new study that offers hope and grim predictions for coral reefs damaged by climate change. The study was published in Science Advances on October 13, by Cody Clements (Mark Hay) and shows that coral survival and growth can be improved by outplanting a variety of coral species. This is especially relevant in the initial stages of coral recovery after large-scale coral losses. It also supports healthy reefs which in turn support fisheries and tourism.

Scientists also recommend additional research to understand and harness these positive species interactions. This will help to increase reef conservation and accelerate the recovery of degraded coral reefs.

However, the ecological pendulum swings in the opposite direction. Clements and Hay call it a "biodiversity meltingdown" if coral species are reduced.

Clements, a Teasley Postdoctoral Fellow at the School of Biological Sciences, stated that corals are the main species in these ecosystems. They provide habitat and food for many other species. Negative effects on corals can have cascading effects on other species that depend on coral reefs. If coral reefs are dependent on biodiversity for their resilience and performance, then a "biodiversity meltdown" could lead to a further decline in reef ecosystems.

Hay and Clements traveled to Mo'orea in French Polynesia in the tropical Pacific Ocean to plant coral gardens. They compared the diversity of coral species to determine the relative importance and competitiveness of corals growing and interacted over time.

Hay, Regents Professor in the School of Biological Sciences and Teasley Chair, said, "We've done all the manipulations and the corals should be competing against each other but they actually do better together than on their own." Hay is also the co-director for the graduate program in Ocean Science and Engineering at Georgia Tech. We are still trying to determine the mechanism behind this unexpected result. However, our experiments have consistently shown that positive interactions outnumber negative interactions in the reef environments where we conduct these experiments. This means that if you remove species from the system, you are removing some of the positive interactions. If you take out important ones, it could make a significant difference.

Coral Gardens close up. Credit: Georgia Techg

In the shade, under the sea in a coral-growing yard

Globally, coral reefs are in danger. Hay points out that the EPA estimates that the Caribbean has lost between 80 and 90 percent of its coral coverage. Over the past 30 years, half of all corals in the Indo-Pacific have been lost. Nearly half of all corals on the Great Barrier Reef died during the 2015-2016 bleaching event.

He said that corals have been dying off more frequently than ever in the past 20-30 years due to large-scale bleaching and heat events. Although there are some hot spots where coral reefs still thrive, they are small and isolated.

Hay and Clements managed coral diversity in French Polynesia by placing them on platforms that resembled underwater chess boards. This was to determine if species density and richness affected survival and productivity.

Hay pointed out that similar experiments were conducted in the past, where corals were brought into a laboratory to pit species against one another. He points out that all of his experiments are done in the real world. We don't care as much about whether it is possible, but whether it happens.

Clements suggested an experimental arrangement that used Coke bottles to help the scientists organize their garden. Hay stated that the end tables have Coca-Cola bottlecaps embedded into their tops. We can then remove the necks from Coke bottles and glue corals to the upside-down necks. Then we screw them into these plots. This allows us not only to arrange the species we wish, but also to unscrew them every few months and weigh them for accurate growth rates.

Clements stated that corals could benefit from higher biodiversity but only to a certain extent. Clements stated that corals planted in gardens with a high number of species (three to six) performed better than those with one or low numbers. We still don't fully understand how these observations were made.

Clements stated that their research needs more attention. Why is it that corals perform better in mixed-species communities than in single-species ones? This biodiversity effect is decreasing rather than continuing to increase at the highest level of coral variety. Clements stated that we need to have a better understanding of the impact of diversity on these processes in order to predict how corals will be affected by biodiversity loss. Also, it is possible to harness biodiversity's positive effect to protect corals.

Continue reading Global warming causes the death of 14 percent world's corals within a decade

More information: Cody S. Clements. Biodiversity has positive, but saturating effects on imperiled coral reefs. Science Advances (2021). www.science.org/doi/10.1126/sciadv.abi8592 Journal information: Science Advances Cody S. Clements, Biodiversity has a positive but saturating effect on imperiled coral reefs,(2021). DOI: 10.1126/sciadv.abi8592