Corals get a boost from other coral species in underwater gardens

Corals are the foundation of tropical reef systems worldwide, where they help form habitat suitable for countless other associated species. In the past 30 years, however, corals have suffered degradation due to pollution, overfishing and global warming. The Caribbean has lost 80-90 percent of its coral cover, a 2015 bleaching event killed nearly half of the remaining corals along the Great Barrier Reef, and the Indo-Pacific coral reefs are under threat.  

Against this backdrop, two researchers from the Georgia Institute of Technology set out to investigate whether newly established coral species grow better in the presence of other coral species, or whether competition between different species restricts growth rates. Cody Clements and Mark Hay traveled to Mo’orea, French Polynesia, in the tropical Pacific Ocean, where they planted coral gardens differing in coral species diversity to evaluate the relative importance of mutualistic versus competitive interactions between the corals as they grew.  

This research has relevance in terms of understanding how to promote reef recovery during the early stages following large-scale coral loss, such as occurs more and more frequently due to the warming of the seas. “The frequency of these big bleaching and heating events that are killing off corals has increased fairly dramatically over the last 20 to 30 years. There are hot spots here and there where coral reefs are still good, but they’re small and isolated in general,” said Hay, Regents Professor and Teasley Chair in the School of Biological Sciences. 

The researchers conducted field experiments in which they established corals on raised, 40 cm x 40 cm platforms, or gardens, in shallow ocean waters. They manipulated coral diversity and density to see whether these factors affected coral survival and productivity.

The team used an ingenious system involving Coca-Cola bottles, suggested by Clements, a Teasley Postdoctoral Fellow in the School of Biological Sciences. The platform surfaces “have Coca-Cola bottlecaps embedded in the top of them,” explained Professor Hay. “We can then cut off the necks of Coke bottles, glue corals into the upside-down necks of these things, and then screw them in and out of these plots. This allows us to not only arrange what species we want where, but every couple of months we can unscrew and weigh them so we can get accurate growth rates.”   

Professor Hay noted that many previous, similar experiments involved bringing corals into a lab to “pit species against each other.” But he points out, “We do all of our experiments in the real world. We’re not as interested in whether it can happen, but whether it does happen.”

The researchers found that corals benefitted from increased biodiversity, but only up to a point. “Corals planted in gardens with an intermediate number of species – three to six species in most cases – performed better than gardens with low, or one, species, or high, as in nine, species,” said Clements. “However, we still do not fully understand the processes that contributed to these observations.”

This finding implies that damaged coral reefs may be re-established more effectively if several different species of corals are grown in proximity to one another. However, it also implies that, if species are lost from a coral reef there may come a point where too few species co-exist and the positive effects of multi-species assemblages are lost.

“We’ve done the manipulations, and the corals should be competing with each other, but in fact they do better together than they do on their own,” said Professor Hay. “We are still investigating the mechanisms causing this surprising result, but our experiments consistently demonstrate that the positive interactions are overwhelming the negative interactions in the reef settings where we conduct these experiments. That means when you take species out of the system, you’re taking out some of those positive interactions, and if you take out critical ones, it may make a big difference.”

“Corals are the foundation species of these ecosystems – providing habitat and food for numerous other reef species,” said Clements. “Negative effects on corals often have cascading impacts on other species that call coral reefs home. If biodiversity is important for coral performance and resilience, then a ‘biodiversity meltdown’ could exacerbate the decline of reef ecosystems that we’re observing worldwide.”

Clements said their research demands more investigation into questions such as   why corals perform better in mixed species communities than single-species communities, and why the biodiversity effect diminishes at the highest level of coral diversity. “We need a better mechanistic understanding of how diversity influences these processes to predict how biodiversity loss will impact corals, as well as how we may be able to harness biodiversity’s positive influence to protect corals,” said Clements.

The study is published in the journal Science Advances.

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By Alison Bosman, Earth.com Staff Writer