Marine predators take bite out of climate change

When it comes to mitigating climate change, marine predators could be a key factor.

Coastal habitats full of vegetation, including seagrass beds, salt marshes and mangroves, are some of the best absorbers of atmospheric carbon dioxide, according to FIU marine scientists Mike Heithaus and James Fourqurean. Coastal habitats bury carbon 40 times faster than tropical forests. These same habitats are believed to store as much as 25 billion tons of carbon, making them the most carbon-rich ecosystems on the planet. Yet, when the predator population is low, these areas fall victim to overgrazing and sediment disruption. The findings were published this week by Nature Climate Change.

Many of the world’s oceans are experiencing significant declines in predator populations due, in large part, to overfishing. Without adequate numbers of predators, grazers, such as turtles, are left to roam and devour vegetation freely, also disturbing sediments and soils. This is of particular concern considering that sediments are excellent at storing carbon.

“People typically think of predators and people as being in conflict,” said Heithaus, who also serves as the dean of FIU’s College of Arts & Sciences. “But the presence of marine predators is good for us in many ways. They help to preserve ecosystems that support human uses like fisheries. And the evidence continues to stack up that their contributions to the long-term stability of these ecosystems can help mitigate climate change through preserving carbon stores.”

In the past 50 years, land-use changes, climate change and other factors have resulted in the loss of as much as 50 percent of the world’s vegetated coastal habitats. The effects can be seen throughout the world. In Cape Cod, Mass., hundreds of years worth of carbon stocks have been lost due to the overfishing of predatory fish and crabs. The grazers devoured much of the carbon-storing vegetation, leading to marsh die-off and major erosion. The high rate of predator loss means Cape Cod marshes have lost about 17,000 tons of carbon dioxide storage capacity per year, equivalent to the annual emissions of about 3,000 cars.

“Scientists and policy makers around the world are just beginning to understand the importance of carbon stored in coastal vegetated habitats to climate policy,” said Fourqurean, an international advocate for the Blue Carbon initiative — storage of carbon in seagrasses and other coastal habitats. “This new study highlights how fisheries practices have a surprising and strong impact of the stability of the coastal carbon stores. In effect, fishing activities can influence the rate of global climate change.”

The FIU team, along with scientists from Utah, Australia and the United Kingdom,   conduct much of their research in Shark Bay, Australia, where pristine conditions have left the area’s food web largely intact. Here, the team found areas where predators are most active also account for some of the most active carbon storage — as much as 60 percent greater than in areas where the threat of predators is low. Similar findings have been recorded in locations throughout the world.

While the study focuses on vegetated coastal habitats, the authors argue the results are likely indicative of effects on a variety of other marine habitats including kelp forests, coral reefs and open oceans. Strong conservation efforts and stricter fishing regulations of top predators could be the key in protecting a major resource in carbon storage for the entire planet. Heithaus points out that such endeavors could be a win-win — helping to break the cycle of extinction while also defending the planet against climate change.