Coral species struggle when they’re alone


Restocking efforts should focus on diversity, a new study suggests. Nick Carne reports.


The experimental plots of coral in Fiji.

Table for one (species)? The experimental plots of coral in Fiji.

Georgia Tech / Cody Clements

The effective extinction of many coral species may be weakening reef systems and siphoning life out of the corals that remain, US researchers warn.

Studies in the shallows off Fiji's Pacific shores suggest that there is safety in numbers, as corals with a mixture of species fared much better than monocultures.

Seaweed has conquered this plot of single-species corals.

Seaweed has conquered this plot of single-species corals, which are dead or sickly and surrendering their table to the competing microalgae.

Georgia Tech / Cody Clements

In fact, after just 16 months some of the monocultures were in a pretty bad way, getting weak and fragile and even dying off completely.

"One of the species had entire plots that got wiped out, and they were overgrown with algae," says Cody Clements, a marine researcher from the Georgia Institute of Technology.

"Rows of corals had tissue that was brown: that was dead tissue. Other tissue had turned white and was in the process of dying."

In contrast, the polycultures “all looked great” and the researchers believe these insights could aid ecologists in restocking crumbling reefs with corals.

Past replenishing efforts have often deployed patches of single species that have had trouble taking hold.

The results of the study, by Clements and principal investigator Mark Hay, are published in a paper in the journal Nature Ecology and Evolution.

"This was a starter experiment to see if we would get an initial result, and we did," says Hay.

"So much reef death over the years has reduced coral species variety and made reefs more homogenous, but science still doesn't understand enough about how coral biodiversity helps reefs survive. We want to know more."

To set up the study, Clements used a kayak to transport (one at a time) 48 concrete tables he had built on land. Once they were in place underwater he dived down with 864 jaggy corals in planters fashioned from the tops of plastic bottles that had to be screwed into place.

There were 12 tables each of three different species, with the other 12 holding a balanced mixture of the three. Data was collected at four and 16 months.

While all the polycultures had prospered at the 16-month mark, only one monoculture species, Acropora millepora, showed nice growth, and even that may not be as positive as it sounds.

Hay says that species is more susceptible to disease, bleaching, predators and storms. It may have sprinted ahead in growth in the experiment, but long-term it would probably need the help of other species to cope with its own fragility.

"Corals and humans both may do well on their own in good times," he says. "But when disaster strikes, friends may become essential."

Explore #coral reef
  1. https://www.nature.com/articles/s41559-018-0752-7
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