With sea levels on the rise, why conserve coral atolls?

With sea levels on the rise, why conserve coral atolls

By: Paul Jacques, Island Restoration Specialist, Island Conservation

In the face of rising sea levels, you might wonder the value of working to protect low-lying coral atolls. These fragile tropical ecosystems typically stand only two metres (six and a half feet) above sea level, and studies that consider sea-level rise alone have predicted that many atolls will be swamped by annual flooding events and become uninhabitable within the next 100 years.  

There is, however, more to the picture.  

Recent studies have shown that healthy atolls are dynamic and may be able to keep pace with rising sea-levels provided that their natural building mechanisms are maintained. To understand this, let’s take a look at the fabric of a coral atoll. 

Atolls – a product of combined forces 

Atolls are ring-shaped strips of low-lying land, often divided into multiple islets, that sit atop a fringing reef enclosing a shallow lagoon. They are found in tropical latitudes, predominantly in the Pacific and Indian Oceans. Atolls are formed by the combined forces of volcanism, coral growth, time and tide.  

Atolls are, in effect, the outline of ancient volcanic islands that have subsided to leave behind a barrier reef of living coral.

The islets that form on the barrier reef are composed of material transported by waves and tidal currents from the living coral reef platforms that surround them.  

Much of this material is the calcified skeletons of dead corals, but fish play their part too – parrotfish munch corals on the surrounding reefs, excreting thousands upon thousands of pounds of white sand each year.  

Wading through waters on Nadikdik Atoll, Marshall Islands.

Flooding – a double-edged sword 

A recent study by a team of researchers from the UK, New Zealand and Canada highlights that, whilst flooding events will increase with rising sea levels, these events may be the means that keep coral atolls above the waterline. This study illustrates that when waves overtop the island during storms, material is thrown up from the reef, increasing the elevation of the island and mimicking the process that created the atoll in the first place. 

Atolls may also change shape and move laterally in response to sea-level rise, with material washed up from the beach causing the atoll to grow higher but narrower. Another recent study conducted in the Maldives shows that reef islets there originated when sea levels were around half a metre higher than present day, probably due to large storms that smashed coral off the reef to be deposited by waves to form the islets. Whilst this sounds promising for the future of coral atolls, researchers point out that this growth can likely only occur on less modified atolls with a low human population – sea defenses constructed to protect settlements from waves also prevent the movement of sediment that can re-build the atoll.  

Both studies also highlight another very important caveat – atolls can only continue to grow so long as the living coral reefs that surround them stay healthy – and that is a big if.

Atoll conservation – restoring the island-ocean connection 

Coral reefs are finely balanced, vulnerable ecosystems that lie between the realms of land and sea, facing threats from both. Human-induced climate change threatens the health of reefs, witnessed by the increasing frequency of drastic bleaching events driven by both acidification and warming of the oceans. Overfishing disrupts the balance of the ecosystem, and removal of keystone species like parrotfish means reefs are not as diverse nor as productive. Huge changes to islet ecosystems caused by the introduction of invasive species have fundamentally degraded the ecological health of both islets and their fringing reefs. 

Nowhere is the interconnectedness of land and ocean more immediate and compelling than on coral atolls. 

In their pristine state, atoll islets team with life, providing habitat for a myriad of creatures including turtles, land crabs and many thousands of seabirds, “connector species” that bring nutrients from the open ocean that then flush back through run-off to nourish the reef. This nutrient input accelerates the growth rate of corals and speeds up recovery from bleaching events.  

Brown Noddy, a key connector species that brings nutrients from the sea to the land, which runs-off to nourish the surrounding marine environment.

Sadly, pristine atolls are now a great rarity; invasive rats and other mammals spread by humans to the naturally mammal-free atolls throughout the Pacific have decimated native biodiversity and broken the cycle of nutrient dispersal. There is hope for coral atolls though, and something we can do to change their uncertain future. Research shows that conservation and restoration of these most vulnerable ecosystems is the best pathway forward for their survival. Thanks to the generous support of our funders, Island Conservation works with local communities and partners to remove invasive species and restore the threatened biodiversity and ecological health of low-lying atolls across the Pacific, in wild and beautiful places such as the Marshall Islands, French Polynesia, and Tuvalu.  

By restoring atoll ecosystems, we see renewed abundance of seabirds, land crabs and turtles, species that join deep ocean, reef and islets in the ongoing cycle of renewal that may keep coral atolls above the water for generations to come. 

This nature-based solution bolsters biodiversity, restores native vegetation, and enhances critical nutrient cycles. Healthier, functioning ecosystems can be more resilient to future climate impacts, benefiting the the people and native species that depend on them. For example, research has found that coral reefs near rat-free island thriving with seabirds grow 4x faster, have 50% more fish biomass, and are more resistant to warming temperatures.  

And there is a vital role for everyone to play, not just those working on islands – minimizing our emissions to help the planet stay below the 1.5-degree Celsius threshold of global warming will protect living corals from the drastic die-offs caused by rapidly warming oceans. 

Want to learn more about the benefits of restoring and rewildling islands? Check out the ambitious and inspiring Island-Ocean Connection Challenge where we are working with partners, funders, communities and governments to restore 40 globally significant islands from ridge-to-reef by 2030.  

Resources. 

Coastal flooding could save atoll islands from rising seas – but only if their reefs remain healthy

Maldives: climate change could actually help coral islands rise again – but they’re still at risk

Tough Teeth and Parrotfish Poop

Harnessing island–ocean connections to maximize marine benefits of island conservation 

Featured Photo: The Maldives, a country of 1,196 low-lying islands arranged into a double chain of 26 coral atolls.

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