REEFS OF HOPE STRATEGY
Facilitating the Adaptation of Coral Reefs to Climate Change
Austin Bowden-Kerby, PhD
Corals for Conservation
17 February 2018
Corals have been around for about 500 million years. During this time, corals have survived at least three mass extinction events. Modern reefs, with most of the coral species we see today, have been around for some twenty million years. Coral reefs have dominated the tropical oceans for this period, including four ice ages and vast swings in climate, but corals and coral reefs have survived these climate changes and the rapid changes in sea level that took place, with sea level going up and down, more than a hundred meters each time.
But today corals are in deep trouble. The scientific community views coral reefs as being the most sensitive of all ecosystems to climate change and other man-made alterations of the environment. This is due to corals having a great sensitivity to temperature and chemical alteration of the oceans. Corals are a combination of animal and plant, with tiny algae living in their tissues, providing the coral with food through photosynthesis. These microalgae are very sensitive to hot water, and most corals lose their algae and become white when the water temperature goes above 32°C. This is called ''coral bleaching''. When corals bleach severely, they most often starve to death or succumb to disease. Mass bleaching has hit many of the planet's reefs already, with the GBR bleaching severely in 2016 and 2017, resulting in the death of over 90 percent of the corals on some reefs.
Rather than focusing on all of the dead corals after mass bleaching, it is important to realize that some corals survive - they do not bleach at these high temperatures. Some corals are adapted to live and be healthy in the extremely hot waters of reef flat tide pools and shallow lagoons, and at temperatures over 36°C, and no one is predicting that temperatures of the open ocean to go up to 36°C, so to think that corals will go extinct as a species group in the coming few decades seems rather absurd from that standpoint, although their abundance may indeed go down drastically to the point where the major geological structures we call 'reefs’ may cease to grow.
One consideration needs to be made- over the past 20 million years, when coral reefs were able to survive great changes in climate, the reefs were in a pristine condition, not polluted nor over-fished. Over-fished coral reefs contaminated with mud, fertilizers, and plastics may not be able to adapt to a changing climate. Indeed the more impacted Caribbean reefs have suffered a drastic decline in coral cover over the past 40 years, and the two most important corals that produce the structure of the reefs are now considered by IUCN to be endangered species, extinct or virtually so from most reefs in the Caribbean.
Some conservationists have drawn the conclusion that there are so many problems so there is really no hope for the coral reefs of the planet. However, as coral reefs are the most vulnerable ecosystem to climate change, then this is the system where we must now make our stand. If the coral reefs go, then the sea grass, mangrove, and beach ecosystems go - as will entire atoll nations.
Much progress has been made in reversing coral reef decline and bringing back the Acropora corals from the brink of extinction in the Caribbean, and this strategy is now being implemented in sites in Fiji and Christmas Island, Kiribati, where temperatures remained over the bleaching threshold for corals for 14 months in 2015-16. The strategy is outlined below.
The "Reefs of Hope" strategy is a climate change adaptation strategy that works to actively help coral reefs adapt to ocean warming by identifying and cultivating hot water adapted, bleaching-resistant corals, and to establish these 'super corals' on well managed coral reefs where other stresses are not a problem.
The strategy begins by searching the wider reef area for surviving Acropora corals, as this species group is the most vulnerable to local extinction based on our findings in both the Pacific and Caribbean, and as the branching forms make it among the easiest of the corals to cultivate in field nurseries. Once as many genotypes as possible are collected and enough biomass of each species is produced, after about one year, the outplanting work begins.
The strategy does not aim to replant corals to extensive reef areas, rather it focuses on important ecological biological factors which inhibit the natural recovery of coral populations to the reefs after mass coral bleaching.
The lines of action are as follows:
1. An underlying problem preventing the recovery of coral populations once a species has become rare, is that the few remaining corals are so far apart from one another that they do not spawn effectively, as self-fertilization is inhibited.
To overcome this bottleneck in coral species recovery, corals of each species are out-planted from the nursery to 5x5 meter areas to create genetically diverse patches of bleaching resistant corals of each Acropora species. These corals grow into adults within 2-3 years and begin to spawn, and as the patch is genetically diverse, high rates of fertilization should occur, restoring effective sexual reproduction and the formation of large numbers of coral larvae, which spread throughout the wider system.
2. A second underlying problem inhibiting coral reef recovery is that without corals, the settlement of coral larvae appears to be delayed. It has only recently been recognized that adult corals provide a strong settlement signal for incoming coral larvae: http://science.sciencemag.org/content/345/6199/892
As every coral restoration researcher knows, Acropora corals are particularly smelly, and this smell attracts coral larvae to settle, which then jump-starts the recovery of the reef. Creating dense patches of corals will thus present a strong settlement signal to incoming coral larvae so that they settle nearby. We can help damaged reefs fix themselves naturally through larval recruitment.
3. A third aspect of facilitated restoration is that coral larvae come in 'naked', without the tiny internal algae that they need for photosynthesis, so they have to acquire their symbiotic algae from the environment after they settle onto the dead reef rock. The algae they acquire leak out of nearby corals, so if the corals already there have bleaching resistant algae, the algae leaking out will inoculate the new corals, and so the new corals, regardless of species, will become bleaching resistant as well. This process accelerates adaptation and helps spread bleaching resistance into other coral genera.
4. Lastly, we have found that after mass bleaching, the bleaching resistant corals which have survived have a very hard time persisting due to greatly increased predation rates by coral predators, especially crown of thorns starfish (COTS), and drupelid snails. Therefore it is vital that collections of bleaching resistant corals be made as soon after bleaching as possible, to create the gene bank nurseries needed for the restoration and facilitated adaptation of the reef.
A coral predator control programme may be needed as part of the strategy. If a year has passed since the mass bleaching very few of the initial survivors will be left. However, as Stegastes damselfish keep COTS predators out of their algal territories, bleaching resistant survivors can sometimes be found there.
In summary, the goals of the "Reefs of Hope" strategy is to keep the bleaching resistant surviving corals alive, increase their reproduction and the formation of coral larvae, call coral larvae of diverse corals back to settle on the denuded reefs, to encourage the new recruits to become bleaching resistant by acquiring resistant algae. In short, the goal is to give Mother Nature a helping hand, not to replace natural processes.
Addressing over-fishing, destructive fishing, agricultural runoff, deforestation, and other sorts of poverty-driven, ignorance-driven, or greed-driven coral reef decline are other strategies as well, and putting these various strategies together in the restoration and adaptation sites will become the winning formula needed to solve the present crisis, or to at least give the reefs the fighting chance that they deserve.
Last updated 20 March 2021