FIU: When devastation strikes the oceans, sharks can hold the key to recovery

MIAMI, Feb. 26, 2021 /PRNewswire/ -- A world without sharks is a world less resilient to extreme climate events, scientists say.

Predators, including some sharks, are known to be critical for maintaining stability and biodiversity in the world's oceans. But according to a new study, they are also critical in helping ecosystems recover when devastation hits from hurricanes or marine heatwaves.

In a unique study, a team of scientists tested whether an ecosystem could recover if sharks were no longer there to keep other animals in check. The answer is no, according to Mike Heithaus, a co-author of the study, marine ecologist and dean of Florida International University's College of Arts, Sciences & Education.

The scientists conducted their study in Shark Bay, Australia — a pristine and largely untouched region of the world — where tiger sharks like to spend their summers, making it quite uncomfortable for resident grazers, especially dugongs. The grazers prefer the shallow seagrass meadows. But this is also the most dangerous place to be, so the dugongs head for safer waters until the sharks leave in winter and it's safe for them to return to the shallows.

But in 2011, the dugongs were living in a very different Shark Bay. A historic heatwave decimated much of the bay's seagrass, which is more than just dugong food. The seagrass helps maintain water clarity and serves as a habitat for commercially lucrative fish and other organisms. It also happens to be really great at storing CO2 emissions.

Recovery from the heatwave has been slow but aided by the seasonal presence of sharks. The initial die-off of the seagrass canopy, which can grow as high as 6 feet, paved the way for more heat-tolerant seagrasses. Dugongs prefer these new seagrasses and so grazing in the shallow areas was expected to continue. Unfortunately, the heat-tolerant seagrass doesn't provide the same level of services to the bay as the large, canopy forming seagrasses that were decimated in 2011.

The scientists, wondering what would happen if sharks didn't return during the summer, decided to create an 'eternally safe' Shark Bay. To do this, they used previous calculations of how many dugongs were around and how much they ate to perform the role of the grazer themselves, mimicking the way dugongs feed on the seagrass during the summer. The experiment left the area with no recovery time — meaning if the dugongs grazed year-round, they'd end up inadvertently destroying the critically important canopy species.

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.

To read more, please visit: https://news.fiu.edu/2021/when-devastation-strikes-the-oceans,-sharks-can-hold-the-key-to-recovery 

Media Contact:
Angela Nicoletti
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SOURCE Florida International University

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