MIAMI – When a major hurricane, extreme heat or other climate threat devastates an ocean's lifeblood, its recovery could be aided by the presence of sharks.
Sharks as oceanic medics?
That's the idea behind a study on sharks and their role in the ocean. The study, led by researchers at Florida International University, was published last week in the Journal of Animal Ecology.
Decimate sharks and you've made oceans less resilient to extreme climate events, said the FIU scientists. Researchers at the University of Washington and Deakin University in Victoria, Australia, also contributed to the study.
The study was conducted from May 2013 to August 2014 and funded through a combination of a National Science Foundation Rapid Response grant, the PADI Foundation, fellowship awards and public donations.
The research found that predators, like some sharks, including tiger sharks in this study, are critical for maintaining stability and biodiversity in the world's oceans. The study found sharks are important in helping ecosystems recover when devastation hits from hurricanes or marine heat waves.
Sharks eat grazing animals that feed on aquatic plants like sea grass — which helps maintain water clarity, stores carbon dioxide and houses fish and other organisms that can keep seas healthy, the researchers say.
To reach its conclusion, the team conducted a 16-month study in Shark Bay, Australia, an area populated by tiger sharks and dugongs, a cousin to the Florida manatee. Dugongs are strictly marine animals, whereas manatees can thrive in freshwater areas.
"Grazing animals, including turtles and dugongs, eat sea grass. Sharks eat the grazers. Grazers fear the sharks. So, when sharks are around, the grazers often avoid the area. While the grazers are away, the aquatic plants have time to grow and recover. When an extreme climate event strikes, the ecosystem must deal with a whole new set of variables that requires time to recover," the study said.
Shark Bay was selected because a historic heat wave had decimated much of the bay's sea grass canopy in 2011. Recovery was slow but was aided by the seasonal presence of sharks, researchers found.
The grazers feed on shallow sea grass meadows but swim to deeper waters until sharks leave in winter. But the 2011 heat wave destroyed much of the shallow sea grass canopy, and the dugongs munched much of what was left. Researchers wanted to know what would happen if the sharks didn't return during the summer and the dugongs had free rein to eat up the grass.
In April and May 2013, the team placed 30 experimental plots 2 to 3 meters deep in two sea grass banks in Shark Bay. By using previous calculations of how many dugongs were around and how much they ate, researchers were able to mimic the way dugongs feed on the sea grass in the summer and what it would be like if there were no sharks around, FIU reported.
"Their experiment left the area with no recovery time — meaning if the dugongs grazed year-round, they'd end up inadvertently destroying the critically important sea grass canopy. The research shows that when top predators are gone, not only does the structure of the ecosystem break down, but it's also all-but-impossible for that ecosystem to stage a comeback," FIU said in a news release.
"One of the reasons we did this study is because we think it's important to be thinking about how everything is linked and sometimes those linkages are surprising," said lead author Rob Nowicki, a research affiliate at Sarasota's Mote Marine Laboratory. "But they show climate resilience is not something that happens on its own. It happens in conjunction with species conservation."
