Marine predators such as sharks should be protected to help mitigate the impacts of climate change according to research from Australian and international scientists, just published in the Journal Nature Climate Change.
Removing apex predators can change the structure of food webs, causing serious flow-on effects – altering ecosystem function to the extent that oceanic carbon storage, especially in coastal ecosystems, is lost, thus intensifying climate change impacts, the study says.
The authors say that recognising the importance of preserving marine predators as part of broader climate change mitigation policies is a matter of urgency.
Dr Peter Macreadie, a marine ecologist at the University of Technology Sydney and Deakin University, and author on the paper, says that although such “trophic cascading effects” or “trophic meltdowns” can occur in any ecosystem the impact on carbon stored in the coastal zone is especially concerning.
“Coastal vegetation such as seagrasses, saltmarsh and mangroves, is incredibly efficient at capturing and storing carbon, 40 times more efficient than tropical rainforests in fact,” he says.
“This ‘blue carbon’ can be stored in sediments for millennia. Despite occupying less than 1 per cent of the sea floor it’s estimated that coastal blue carbon ecosystems sequester more than half the ocean’s carbon.”
The authors say that when sharks and other large fish are removed from blue carbon ecosystems populations of herbivorous fish and other animals, that can’t self-regulate, reach out of control populations reducing the capacity for carbon capture and storage. Recreational over-harvesting of predatory fish and crabs in the salt marshes of Cape Cod Massachusetts, for example, triggered marsh die-off and major erosion with an estimated loss of hundreds of years of carbon stocks from the system.
The study highlights the balancing act for coastal policy management in terms of conserving individual species, both predators and herbivores, whilst trying to maximise carbon storage. In Shark Bay Western Australia, tiger sharks create a landscape of fear where sea turtles and dugongs preferentially forage in seagrass microhabitats where the risk of encountering a shark is low. These habitats have lower carbon stocks than those associated with a higher predation risk.
Trade-offs are inevitable, however, Dr Macreadie points out that if just 1 per cent of vegetated coastal habitat was exposed to trophic cascades this equates to the CO2 produced by close to 100 million cars.
“There is no doubt that removing predators has consequences. We have already lost about 90 per cent of the ocean’s top predators yet practices such as shark culling and finning continue,” Dr Macreadie says.
“This is a global issue. Sharks have an amazing impact on our oceans and we should stop removing them and other predators from marine environments. This combined with stronger policies against overfishing and habitat destruction are needed to keep the carbon locked up by coastal vegetation in the ocean, not released to the atmosphere.”