When we think of mangrove forests, seagrass meadows and saltmarshes, we don’t immediately think of shark habitats. But the first global review of links between large marine animals (megafauna) and coastal wetlands is challenging this view – and how we might respond to the biodiversity crisis.
Mangrove forests, seagrass meadows and saltmarshes support rich biodiversity, underpin the livelihoods of more than a billion people worldwide, store carbon, and protect us from extreme weather events.
We know marine megafauna also use these habitats to live, feed and breed. Green turtles and manatees, for instance, are known to eat seagrass, and dolphins hunt in mangroves.
But new associations are also being discovered. The bonnethead shark – a close relative of hammerheads – was recently found to eat and digest seagrass.
The problem is that we’re losing these important places. And until now, we’ve underestimated how important they are for large, charismatic and ecologically important marine animals.
Today our review of the connections between marine megafauna and vegetated coastal wetlands was published in the journal Trends in Ecology and Evolution. As it turns out, far more megafauna species use coastal wetlands than we thought.
Before our review, the number of marine megafauna species known to use these habitats was 110, according to the International Union for Conservation of Nature (IUCN) Red List, which assesses species’ conservation status.
We identified another 64 species from 340 published studies, bringing the total number to 174 species. This means 13% of all marine megafauna use vegetated coastal wetlands.
We predominantly documented these habitat associations by electronic tracking, direct observation or from analysing stomach contents or chemical tracers in animal tissues.
Less commonly, acoustic recordings and animal-borne video studies – strapping a camera on the back of turtle, for instance – were used.
In recent weeks, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) released a damming assessment of humanity’s stewardship of the natural world. Up to 1 million species were reported to be facing extinction within decades.
We need to dramatically change how we relate to and engage with species and their habitats, if we are to fix this problem.
But the question is, how can we make global change real, relevant and feasible at local and regional scales? And, as the international community rises to this challenge, what information is needed to support such efforts?
Our study suggests a critical first step to addressing the global biodiversity crisis is to deepen our understanding of links between species and their habitats. We also need to elevate how the evidence is used to both assess extinction risk and prioritise, plan and deliver conservation actions.
More than half of all coastal wetlands have been lost globally and the rest are at risk from a range of serious threats, including deforestation. There is an urgent need to limit and reverse the loss of coastal wetlands to stop biodiversity loss, protect communities and tackle climate change.
Targeting places where high rates of mangrove loss intersect with threatened megafauna could lead to more efficient and effective conservation outcomes. Southeast Asia, Mexico and northern Brazil are such places.
In Southeast Asia, for example, the world’s largest mangrove forest is losing trees at a rate far exceeding global averages, largely due to aquaculture and agriculture. This is threatening the critically endangered green sawfish, which relies on these mangrove habitats.
The IUCN Red List assesses the extinction risk for almost 100,000 species. It provides comprehensive information on global conservation statuses, combining information on population sizes, trends and threats.
The wealth of data collected during species’ assessments, including habitat associations of threatened species, is one of the Red List’s most valuable features.
But our study shows many known associations are yet to be included. And for more than half of the assessments for marine megafauna, habitat change is yet to be listed as a threat.
This is concerning because assessments that overlook habitat associations or lack sufficient detail, may not allow conservation resources be directed at the most effective recovery measures.
But it’s also important to note habitat associations have varying strengths and degrees of supporting evidence. For example, a population of animals shown to consume substantial amounts of seagrass is clearly a stronger ecological link than an individual simply being observed above seagrass.
In our paper, we propose a simple framework to address these issues, by clarifying habitat associations in conservation assessments. Ideally, these assessments would include the following:
Habitat loss is accelerating a global extinction crisis, but the importance of coastal habitats to marine megafauna has been significantly undervalued in assessments of extinction risk.
We need to strive to protect remaining coastal wetland habitats, not only for their ecological role, but also for their economic, social and cultural values to humans. We can do this by strengthening how we use existing scientific data on habitat associations in species assessments and conservation planning.
Michael Sievers, Research Fellow, Global Wetlands Project, Australia Rivers Institute, Griffith University; Rod Connolly, Professor in Marine Science, Griffith University, and Tom Rayner, Science Communicator, Griffith University
Oscar Serrano, Edith Cowan University; Carlos Duarte, King Abdullah University of Science and Technology; David John Gregory, National Museum of Denmark; Dorte Krause-Jensen, Aarhus University, and Eugenia Apostolaki, Hellenic Centre for Marine Research
For more than 6,000 years, seagrass meadows in Australia’s coastal waters have been acting as security vaults for priceless cultural heritage.
They’ve locked away thousands of shipwrecks in conditions perfect for preserving the fragile, centuries-old timbers of early European and Asian explorers, and could even hold secrets of seafaring by Aboriginal Australians.
Seagrass meadows accumulate marine sediments beneath their leaves, slowly burying and safeguarding wrecks in conditions that museum curators can only dream of. It’s a process that takes centuries, as mats of seagrass and sediments cover the wrecks and all their buried treasure.
Seagrass sedimentary deposits also hold archives of wider environmental change over millennia and are important sinks for atmospheric carbon dioxide, known as Blue Carbon.
But human development, climate change and storms are threatening fragile seagrass meadows around the world, and that risks the loss of the important cultural heritage they protect as well as some of the world’s most productive marine ecosystems.
Our research, carried out by an international team of scientists in Australia, Denmark, Saudi Arabia and Greece, shows that seagrass meadows, hidden beneath our oceans, gradually build up the seafloor over millennia by trapping sediments and particles and depositing those materials as they grow.
The organic and chemical structure of seagrass sedimentary deposits is key to its ability to protect shipwrecks and submerged prehistoric landscapes. These structures are extraordinarily resistant to decay, creating thick sediment deposits that seal oxygen away from archaeological sites, preventing ships’ timbers and other materials from rotting away.
Seagrass meadows are under environmental stress due to climate change, storms and human activity. Recent disturbances and losses have exposed shipwrecks and archaeological artefacts that were previously preserved beneath the sediment. Once the protective cover of seagrass is gone, the ships and other sites begin to break down. If you lose seagrass, you lose cultural heritage.
Seagrass meadow losses in the Mediterranean have exposed Phoenician, Greek and Roman ships and cargo, many of which are thousands of years old. Unless these effects can be stemmed, the frequency of exposures is likely to increase. This has already put European archaeologists and marine scientists in a race against the clock.
Around 7,000 shipwrecks are thought to lie in Australia’s coastal waters. Seagrass disturbance led to the unearthing in 1973 of the James Matthews, a former slave ship that sank in 1841 in Cockburn Sound, Western Australia, and the Sydney Cove, which ran aground off Tasmania’s Preservation Island in 1797, forcing survivors to walk 700km to Sydney.
Artefacts and pieces of the James Matthews’ hull have been recovered and studied at the WA Museum. Meanwhile, the recovery of beer bottles from the Sydney Cove has led, remarkably, to 220-year-old brewing yeast being cultivated and used to create a new beer – fittingly enough called The Wreck.
We and our colleagues are aiming to match shipwreck data with seagrass meadow maps. From there, we hope new acoustic techniques for below-seabed imaging will allow exploration of underwater sites without disturbing the overlying seagrass meadows. Controlled archaeological excavation could then be undertaken to excavate, document and preserve sites and artefacts.
We also believe there’s significant potential to find archaeological heritage of early Indigenous Australians buried and preserved in seagrass meadows. Sea level around Australia rose around 6,000 years ago, potentialy submerging ancient indigenous settlements located in coastal areas, which may now be covered by seagrass.
The danger of not putting these protections in place is evidenced by treasure-hunters off the Florida coast, who have adopted a destructive technique called “mailboxing” to search for gold in Spanish galleons. This involves punching holes into sediment to find and then pillage wrecks, an action that damages seagrass meadows and archaeological remains.
The accumulated sediments in seagrass meadows could also help build a record of environmental conditions, including fingerprints of human culture. These archives can be used to reconstruct prehistoric changes in land use and agriculture, mining and metallurgical activities, impacts of human activities on coastal ecosystems, and changes associated with colonisation events by different cultures. Think of it as a coastal equivalent to polar ice cores. Seagrass records could even help us understand, predict and manage the effects of current environmental changes.
But to do all this, we first need to realise what a truly valuable resource seagrass is. Granted, it doesn’t look spectacular, but it can do some pretty spectacular things – from sucking carbon out of the skies, to underpinning entire ecosystems, and even guarding buried treasure.
Oscar Serrano, Doctor of Global Change, Edith Cowan University; Carlos Duarte, Adjunct professor, King Abdullah University of Science and Technology; David John Gregory, Senior Researcher, National Museum of Denmark; Dorte Krause-Jensen, Senior Researcher, Marine Ecology, Aarhus University, and Eugenia Apostolaki, Researcher, Institute of Oceanography, Hellenic Centre for Marine Research
Just like birds and mammals carrying seeds through a rainforest, green sea turtles and dugong spread the seeds of seagrass plants as they feed. Our team at James Cook University’s TropWATER Centre has uncovered a unique relationship in the seagrass meadows of the Great Barrier Reef.
We followed feeding sea turtle and dugong, collecting samples of their floating faecal matter. Samantha then had the unenviable job of sifting through hundreds of smelly samples to find any seagrass seeds. These seeds range in size from a few centimetres to a few millimetres, and therefore can require the assistance of a microscope to be found. Once any seeds were found, they were stained with a chemical dye (Tetrazolium) to see if they were still viable (capable of growing).
Green sea turtles and dugong are iconic animals on the reef, and seagrass is their food. Dugong can eat as much as 35 kilograms of wet seagrass a day, while sea turtles can eat up to 2.5% of their body weight per day. Without productive seagrass meadows, they would not survive.
This relationship was highlighted in 2010-11 when heavy flooding and the impact of tropical cyclone Yasi led to drastic seagrass declines in north Queensland. In the year following this seagrass decline there was a spike in the number of starving and stranded sea turtles and dugong along the entire Queensland coast.
The seagrass team at James Cook University has been mapping, monitoring and researching the health of the Great Barrier Reef seagrasses for more than 30 years. While coral reefs are more attractive for tourists, the Great Barrier Reef World Heritage Area actually contains a greater area of seagrass than coral, encompassing around 20% of the world’s seagrass species. Seagrass ecosystems also maintain vibrant marine life, with many fish, crustaceans, sea stars, sea cucumbers, urchins and many more marine animals calling these meadows their home.
These underwater flowering plants are a vital component of the reef ecosystem. Seagrasses stabilise the sediment, sequester large amounts of carbon from the atmosphere and filter the water before it reaches the coral reefs. Further, the seagrass meadows in the Great Barrier Reef support one of the largest populations of sea turtles and dugong in the world.
Samantha’s research was worth the effort. There were seeds of at least three seagrass species in the poo of both sea turtles and dugong. And lots of them – as many as two seeds per gram of poo. About one in ten were viable, meaning they could grow into new plants.
Based on estimates of the number of animals in the coastal waters, the time it takes for food to pass through their gut, and movement data collected from animals fitted with satellite tags, there are potentially as many as 500,000 viable seeds on the move each day in the Great Barrier Reef. These seeds can be transported distances of up to 650km in total.
This means turtles and dugong are connecting distant seagrass meadows by transporting seeds. Those seeds improve the genetic diversity of the meadows and may help meadows recover when they are damaged or lost after cyclones. These animals help to protect and nurture their own food supply, and in doing so make the reef ecosystem around them more resilient.
This research shows that these ecosystems have pathways for recovery. Provided we take care with the environment, seagrasses may yet recover without direct human intervention.
This work emphasises how much we still have to learn about how the reef systems interconnect and work together – and how much we need to protect every part of our marvellous and amazing reef environment.
Samantha J Tol, PhD Candidate, James Cook University; Alana Grech, Assistant Director, ARC Centre of Excellence for Coral Reef Studies, James Cook University; Paul York, Senior Research Scientist in Marine Biology, James Cook University, and Rob Coles, Team leader, Seagrass Habitats, TropWATER, James Cook University
The following link is to an article that looks at how much carbon is stored by seagrasses.