Future planet |oceans
Shark populations around the world are declining. Increasing their numbers could have a cascading effect, helping to lower carbon emissions and make the oceans more resilient to climate change.
On the westernmost tip of Australia, in the aptly named Shark Bay, at least 28 shark species swim in clear water and rippling shoals of seagrass -the biggestin the world. Tiger sharks in particular often frolic in the rocky bays of Shark Bay. These mammoth predators stalk the seagrass with their 15-foot (4.5 m) bodies, occasionally preying on majestic grazing manatees. Although the presence of tiger sharks poses a threat to their prey, these predators are essential to the health of the marine ecosystem in which both species live.
In fact, despite their bad reputation among humans, sharks can also be a powerful ally in mitigating climate change.
All thanks to the thin strands of seaweed that sway with the waves in the shallow waters of Shark Bay. This seaweed is food for manatees and dugongs, all of which roughly graze40 kg(88 pounds) of seaweed per day - as well as for manatees andGreen sea turtles.
Tiger shark numbers are estimated to have declined by at least 71% off Australia's north-east coast in Queensland
Dugongs, which can weigh up to 500 kg, are a rich source of food for tiger sharks. By controlling the manatee population in Shark Bay, tiger sharks help sea grasses thrive. Blooming meadow with seagrasssaves twiceas much CO2 per square mile as forests typically emit on land.
In 2011, Shark Bay was hit by a heat wave that caused the water level to rise by as much as 5°C for two months (Source: Getty Images)
But globallyTiger shark numbers are declining, including in some populations in Australia. Tiger sharks live off the north-eastern coast of Australia, Queenslandestimated to have fallen by at least 71%mainly due to overfishing and bycatch. The decline in tiger shark numbers means that herbivores will graze more seagrass, which means that marine vegetation will absorb less carbon dioxide. In the Caribbean and Indonesia, where shark populations are declining, overgrazing of herbivores such as sea turtles is already a serious threat to seagrass habitat and has led to90 do 100%loss of seaweed.
Loss of seagrass means not only less carbon uptake, but also less capacity of the habitat to absorb itreconstruction after extreme weather events caused by climate change,like heat waves.
Decreasing number of sharks
There is clear evidence that shark populations are declining worldwide, with humans being the main culprit. A recent reassessment by the IUCN Red List found that 37.5% of all shark and rays species are now threatened with extinction. Catherine Macdonald, a marine conservation biologist and associate professor at the University of Miami, points out that ocean sharks and rays have declined by 71% since 1970.
The greatest threat to them is overfishing, but loss of coastal habitat, loss of prey and deteriorating water quality also contribute.
One of Western Australia's worst heatwaves in 2011with a sea temperature increase of 5°Cfor two months. The heatwave was devastating to the seagrass species found in the bay.Amphibolis Antarctydawhich creates rich, dense meadows that store sediment and provide food for pastures. Over 90 percentAmphibolis Antarctydawas lost, the largest known loss in the entire gulf.
This loss of seagrass has been a perverse boon for the Gulf manatees, who love the smaller and harder-to-find species of tropical seagrass that is usually shielded from access by tall, dense seagrass speciesAmphibolis Antarctyda. When tropical seagrass is more available, manatees enthusiastically feed on it in what is known as "digging".digging up rhizomes of your favorite seaweedand makes it difficult to denseAmphibolis Antarctydabeds to be reformed.
In Shark Bay, the tiger sharks managed to restore the balance somewhat by reducing manatee numbers, so not all of the bay's kelp went extinct. But the question arose: what if there were no sharks in the bay?Amphibolis AntarctydaWill the dominated ecosystem survive?
To find out, scientists led by Rob Nowicki of Florida International University spent time in eastern Australia, where shark numbers were lower and manatees grazed mostly undisturbed. There, divers would come down and pluck seaweed, simulating grazing manatees when there were no predators to stop them - enthusiastic, destructive foraging in excavations. In fact, they observed a rapid loss of seagrass cover, especially withAmphibolis Antarctyda, and the ecosystem began to change to a more tropical one, dominated by tropical seagrass.
"We found that dugong grazing, if not controlled, can quickly destroy large areas of seagrass when digging," says Nowicki. These changes can be long-lasting. "When the seagrass regenerates, the seagrass community looks different, with other species dominating than before."
These results highlighted the role sharks played in Shark Bay. "Without the tiger sharks to keep the dugongs at bay, the bay would probably turn into mostly tropical seagrass," says Nowicki.
Manatees and dugongs can be destructive predators, destroying seagrass species that help keep ecosystems together (Source: Getty Images)
If the shark population continues to decline at the current rateAround the World (see Decreasing shark numbers)Nowicki's team concluded that the resilience of carbon-rich marine ecosystems to extreme climatic events such as heat waves is likely to be compromised.
However, Becca Selden, assistant professor of natural sciences at Wellesley College, says the impact on Shark Bay could be more severe than most because of its unique ecosystem. "The strong effect may have been enhanced by a relatively simple food web in the seagrass ecosystem, where predators limit grazing by the megaherbivore," explains Seldon. In other words, other coastal habitats may not fare as badly as Shark Bay under similar pressures.
In addition to reducing manatee numbers and making seagrass ecosystems more resilient, tiger sharks play another key role in maintaining habitat health. They act as an effective fertilizer when buying or dying in meadows.
"Long-lived vertebrates can act as carbon sinks as carbon used up at the sea surface finds its way into the deep sea with faeces and/or dead carcasses falling to the seabed," says Selden.
This phenomenon is called carbon sequestration, is most common inJust, but there are studies showing that the same benefits existHedge.(Read more about how whale deaths and droppings can help lower ocean carbon emissions.)
Research led by Jessica Williams of Imperial College Londonfound that gray reef sharks, which are common in shallow reef ecosystems, transport nutrients such as nitrogen to their habitats through their droppings. They estimated that a population of more than 8,000 gray sharks in Palmyra Atoll provided about 94.5 kg of nitrogen per day.
Since tiger sharks in Shark Bay spend most of their time foraging and moving around seagrass beds, it is likely that they provide similar fertilizing benefits to these plants. "Large pelagic sharks may be the biggest contributors to this effect, including blue sharks, mako sharks and hammerhead sharks," says Selden.
As shark numbers decrease worldwide, the need to understand how they support their ecosystems becomes even more urgent (Source: Getty Images)
When it comes to the growing shark population, conservationists have a formidable opponent: the fishing industry.
According to Nowicki and Selden, there is a move towards more sustainable fishing, but a large percentage of the industry has not changed its methods, which is a problemMain reasonwhy many marine apex predators still doWaste. Different, stringent animal protection laws in different countries also play a role.
"Since many predatory fish are also widely distributed, they may span jurisdictions of many countries, some of which may not protect them or use sustainable fishing practices," says Nowicki.
Reducing illegal and unsustainable fishing has been a difficult task, even as consumers strugglemore environmentally consciousIDecision on Sustainable Fisheriesoh unsustainable.
“Sustainable, coordinated and ecosystem-based fisheries management is an important tool for protecting these predators and their ecological role. Ordinary citizens can do this by educating themselves, advancing in science, demanding that fisheries become or remain sustainable, and by making sustainable purchases of seafood,” says Nowicki.
If you're unsure which seafood is truly sustainable, the Marine Stewardship Council (MSC) rates fishing at an international level. So if the retailer is certified for sustainability, the blue MSC seal will appear on the packaging.
Besides supporting sustainable fisheries, according to Nowicki, the only way to truly protect marine life is to reduce global greenhouse gas emissions. "Ultimately, if we are to protect our ecosystems for centuries, we must manage climate change while protecting species."
Even if shark populations increase again, their contribution to reducing greenhouse gas emissions and mitigating their effects will only be a small part of the climate change mitigation efforts. However, shark abundance has an undeniable impact on many marine ecosystems that depend in one way or another on healthy, abundant seagrass. By leveling the ecological playing field, sharks strengthen these ecosystems against the threat of climate change, enabling them to store carbon for another day.
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