Sharks play a vital role in stabilizing their ecosystems. This role may become even more important as climate change continues to intensify.
Written by: AMY MCKEEVER
The grey reef shark is a common shark in the Indo-Pacific ocean that feeds mainly on short-beaked snoutfish. Photo by LAURENT BALLESTA, NAT GEO IMAGE COLLECTION
Sharks are one of the fiercest predators in the ocean and one of the most vulnerable animals. Three-quarters of high-seas shark and ray species are threatened with extinction, largely because of overfishing.
For years, scientists have warned that a sharp reduction in sharks could have disastrous consequences. Because sharks are the top predators that help control the food chain, they are logically key species — species that have a huge impact on ecosystems. Without key species, ecosystems change or even disappear.
For sharks, however, for a long time this view has remained largely theoretical. Sharks and their marine habitats are difficult to study, and special equipment is required just by entering the water to observe. More than 500 species of known sharks vary in size, prey on different animals, and live in very different environments, making it nearly impossible to pinpoint the impact of each shark.
These complex questions have plagued past researchers with research on how sharks affect their ecosystems from the top down. For example, a 2007 study appears to provide clear evidence that the decline in the North Atlantic great white shark has led to the overbreeding of cattle-nosed rays, which in turn has led to the mass extinction of scallops, clams and oysters. However, scientists question whether the decline in bivalves is affected by other factors, such as maritime traffic.
"We wanted to do simple research, but it wasn't possible," said Michael Heithaus, a marine ecologist at Florida International University and founder of the Shark Bay Ecosystem Research Program. For more than 20 years, he has been studying the 23,000-square-kilometre protected area in western Australia.
Research in Shark Bay shows that sharks help protect the ecosystems in which they live, but not necessarily as predators, but as managers. Sharks can maintain the stability and resilience of the ecological environment, which in turn mitigates climate change and reduces the impact of extreme weather events such as heat waves and hurricanes.
Demystify marine ecosystems
Shark Bay is the perfect place to study shark interactions with the environment. Tiger sharks come to Shark Bay seasonally, so researchers have the opportunity to observe how other animals behave as they appear and disappear. Shark Bay is a UNESCO World Heritage Site with healthy predator and prey populations, as well as dense temperate seagrass meadows that slow down water flows, keep the waters clear and provide habitat and food for many species. Seagrass can also trap carbon dioxide, a potent greenhouse gas, and stabilize deposits storing carbon dioxide on the seafloor. Once carbon dioxide is stored in sediment, it can persist for tens of thousands of years.
"I don't know of any other place on Earth like this," said Rob Nowicki, a research scientist at Mott Marine Laboratory who has collaborated with Heithaus in Shark Bay. "It's the perfect environment to test sharks' interactions with the environment at scale."
Since National Geographic explorer Heihaus launched the project in 1997, researchers have been collecting data on all elements of the Shark Bay ecosystem, laying the groundwork for sorting out its intricate relationships.
An important discovery published in 2012 suggested that tiger sharks can scare away herbivores in Shark Bay, such as dugongs (close relatives of manatees) and turtles, into tropical seagrass meadows, which absorb less carbon than temperate seagrasses. In other words, the loss of tropical seagrass is not as harmful to the environment as losing temperate seagrass.
But in areas where sharks are reduced and sea turtles are protected, such as the Caribbean and Indonesia, sea turtles can over-eat seagrass, undermining human efforts to mitigate global climate change, said Trisha Atwood, director of the Laboratory of Aquatic Ecology and Global Change at Utah State University.
"Over the past 20 years, we've realized that ecosystems like seagrass are actually one of the best carbon reservoirs on the planet, and they absorb carbon faster than any terrestrial forest," Atwood said.
In 2015, Atwood built on a 2012 study and found that tiger sharks in Shark Bay also prevented herbivores from disturbing carbon-filled sediments beneath temperate seagrasses.
"We're not saying turtles shouldn't be protected," she said. "We think sharks should be protected so they can help control who they eat."
Recover from extreme weather
Shark Bay also provides researchers with a deeper understanding of how sharks can make ecosystems more resilient to the effects of climate change.
In 2011, an extreme marine heat wave hit Shark Bay, destroying about 90 percent of temperate seagrass, making Shark Bay even more vulnerable. The researchers knew that seagrass would take a long time to recover, so they saw it as a research opportunity.
"We wanted to know what would happen in Shark Bay, a strange world where tiger sharks were fished out," Nowicki said. "Can the dugong come back to do the work that the heat wave hasn't done yet?"
Nowicki and colleagues divided the bay's seagrass beds into small pieces, simulating how they would change with and without sharks. They found that seagrass beds, unprotected by tiger sharks, were eventually on the verge of collapse. At the same time, the area patrolled by sharks is more stable, as sharks provide more recovery time for seagrass.
Research suggests that tiger sharks are truly key species in Shark Bay and possibly elsewhere, Nowicki said.
"This clearly shows that having a healthy shark population is very important for the stability of the ecosystem," said Study co-author Heihaus. "As ecosystems take on more and more blows, it's probably better to cultivate a whole population of predators than to pray that everything will work out."
Combined impact
Shark Bay has given us an insight into the important role sharks play as predators, but Heithaus says the question now is whether this model applies across the globe. "You can't just study sharks, you should study every aspect of the ecosystem."
For example, Heithaus said, there is also evidence that bull shark pups, which can move in freshwater, deposit important nutrients upstream in the Everglades, but it remains to be seen how much of this nutrient can play a role.
Nowicki likens shark diversity in an ecosystem to a pillar that underpins a bridge, and losing one species is the equivalent of cutting off another pillar that supports an entire structure.
"The problem is that we don't know when it will lead to a crash," he said. For predators like sharks, though, "we need to pay attention because we lose more by losing them." ”
(Translator: Stray Dog)