I am one of the many scientists who study corals in order to find ways to help them survive and recover. A recent report by the National Academies of Science, Engineering, and Medicine reveals that researchers are exploring a variety of different strategies. Some of these strategies, like managed breeding, which makes corals more resistant to stress, are being tested at small scales. Some, like moving corals into new areas to colonize, haven’t been tested.

In my research, I examine whether corals thrive and survive if there is a greater variety of coral species. In a recent study, my colleague Mark Hay found that yes is the answer. This finding may help inform strategies to make coral reefs more resilient when oceans are altered.

More is more

Do ecosystems that contain more species have a higher quality of life than those with fewer? It is an important question for ecology. Scientists have generally found that ecosystems that are more diverse Foundation species (those that define and are inseparable from a system ), such as the trees in a forest, tend to be healthier and function better.

This test was never applied to coral reefs until recently. We do know, however, that coral reefs in good health are complex and diverse ecosystems dominated primarily by corals. Reefs that are damaged by stressors such as coral-bleaching events become simpler, less diverse landscapes dominated by seaweeds.

We chose for our study a reef on the southwest coast of Fiji’s main island, Viti Levu, in the South Pacific. Overfishing, pollution, and other human activities have severely degraded many reefs on this coast. Seaweeds and corals are now dominant.

Hundreds of coral species are found in the Pacific. However, at smaller scales, we only saw five or fewer species during our preliminary surveys on the degraded site. We chose this site because it reflects the conditions of many reefs around the world. This was an ideal location to test whether coral biodiversity is important for the “new norm” we expect to find on reefs in the future.

Seaweeds can quickly take over coral reefs that are stressed by excessive fishing. Some seaweeds produce chemicals that repel fish and coral larvae. This may prevent reefs from recovering.

Underwater gardens

Our team constructed 48 concrete plots along the seafloor to serve as experimental coral gardens. Each garden contained one coral species: Pocillopora Damicornis (also known as cauliflower coral); Porites Cylindrica (also known as yellow-finger coral); and Acropora Millepora, which is one of the many species of corals known as staghorn. We also planted mixed-species gardens that contained all three species.

These corals were chosen because they are found on reefs all over the Pacific and represent different coral families with varying responses to harmful disturbances. Each garden had 18 corals, totaling 864 corals.

A porites cylindrical Coral was planted in our experiment gardens. The corals were embedded in an upside-down bottleneck with epoxy. This allowed us to attach them or remove them from the garden plots. Cody Clements, CC BY-ND

We needed to periodically remove each coral from its plot in order to assess their performance. We cut the tops off hundreds of soda bottle necks and placed an individual coral inside each bottle using epoxy putty. We embedded bottle caps in our concrete slabs to make it easy to unscrew the necks of the bottles and examine the corals they held. Then we screwed them back into their base. We weighed corals over 16 months and monitored other indicators of their health, such as tissue death and the colonization of each garden with harmful seaweeds.

Coral gardens were created on a reef that was degraded in Fiji. The gardens with a mixture of coral species did better than those containing just one type. Cody Clements, CC BY-ND

Corals in mixed-species plots performed consistently better than corals in single-species fields. In just four months, the coral growth in mixed-species plots was higher than the best single-species plots. This indicates that species can benefit from each other, even in the early stages of coral communities’ development.

During our 16-month trial, we created coral gardens of both single-species and mixed-species. Mixed-species coral gardens outperformed single-species in many ways at four months. They grew faster than the best single-species gardens, Acropora Millepora. Mixed-species and Acropora gardens grew at the same rate by 16 months. However, single-species gardens still lacked performance. Clements and Hay 2019 CC-BY-ND

Why is more always better?

Next, we need to know what caused the observed effects. In future experiments, we hope to explore a variety of possible leads. Farmers often keep that planting diverse crops reduces the spread of infectious diseases between individuals. Could coral reefs be the same?

Our initial findings are both alarming and hopeful for the future of coral reefs. Continued species loss can have a dramatic impact on coral ecosystems, leading to a further decline of reefs. How many parts of the “ecosystem motor” can you remove before it starts to break down?

The National Academies report contains many strategies that use biodiversity, both genetically and at a species level, to increase coral reef resilience. Cross-breeding between coral populations, altering the genes of corals to give them a new function such as heat tolerance, and moving corals or genes that are stress-tolerant to recent locations are examples.

Researchers can assess coral health by using promising advances in technology, such as mapping reefs from above. This information can be used to guide management and restoration efforts better.