Shark alert: Ocean predators protect humans from climate change risks

Sharks are the unlikely heroes protecting humans from the perils of climate change, Deakin University researchers argue in today’s publication of esteemed science journal Nature Climate Change.

According to the Deakin University scientists, when humans kill sharks they cause instability in the ocean’s natural food chain, which can ultimately lead to the release of carbon from the seafloor into the earth’s atmosphere.

With fewer sharks in the food chain, populations of the predator’s food sources, such as sea turtles, flourish.

One of the sea turtles’ main food sources is seagrass, which store vast reservoirs of carbon within sediments. With more sea turtles consuming more seagrass, the carbon is unlocked and can be released into the earth’s atmosphere, thereby accelerating climate change.

After examining available data from across the world, the team from Deakin’s Centre for Integrative Ecology within the School of Life and Environmental Sciences is now calling for urgent research to further investigate the consequences of shark-culling practises throughout the earth’s oceans.

The Deakin project’s lead researcher, Dr Peter Macreadie, said that sharks and other predators were being over-harvested by humans.

“For a long time we’ve known that changes to the structure of food webs - particularly due to loss of top predators such as sharks – can alter ecosystem function,” Dr Macreadie said.

“In science, the consequence is what is known as a trophic meltdown. With the loss of around 90 per cent of the ocean’s top predators from around the globe, the occurrences of trophic meltdowns are now widespread.

“In our article, we report the trickle down effects on the capacity of the oceans to trap and store carbon.

“There are multiple ecosystems by which this can occur, but the most profound examples occur in the coastal zone, within seagrass, saltmarsh, and mangrove ecosystems – commonly known as ‘blue carbon’ ecosystems.

“In the case of sharks and turtles, sharks eat turtles, which in turn eat seagrasses. But when sharks disappear, the turtles have a tendency to run wild and the seagrass ecosystems cannot sustain the turtle populations.

“The turtles overgraze, and, as a consequence, we’re seeing large reductions in seagrass carbon stocks.”

Dr Macreadie said this had played out in Shark Bay, Western Australia, where fewer sharks as a result of hunting had led to carbon storage rates less than half the amount in areas which were abundant with sharks.

“At the extreme level, we see turtles without predation pressure eating themselves out of house and home and destabilising carbon stocks that have been locked away for millennia,” he said.

Dr Macreadie said losing sharks and other top predators from the ocean interfered with the ocean’s carbon cycle and can accelerate global warming.

“Stronger conservation efforts and stricter fishing regulations are needed to reinstate the important role that predators play in the ocean’s carbon cycle,” he said.

“It’s about restoring balance so that we have, for example, healthy and natural numbers of both sea turtles and sharks.”

The researchers say while there is limited data on the topic, they believe further studies will reiterate their findings. This research is the first to explicitly link the loss of major ocean predators, including sharks and other top sea animals, to the loss of stored blue carbon.

The research included collaborators from the University of Queensland, the University of Western Australia, the University of Technology Sydney, Griffith University, Swansea University, Utah State University and Florida International University.