It is known that changes to the structure of food webs - particularly due to loss of top predators – can alter ecosystem function.  In particular the loss of predators at the top of the food chain, releases animals lower in the chain from top-down population control, leading to increased population levels and overreliance on particular food resources, causing a ‘trophic meltdown’.

However, the research article, co-authored by Professor Graeme Hays formerly of the University’s College of Science and published in Nature Climate Change has also shown that the loss of ocean predators can also have serious, cascading effects on oceanic carbon storage and consequently climate change.  When predators disappear from blue carbon ecosystems, the resultant increases in herbivores can destroy the capacity of blue carbon habitats to capture carbon.

Professor Hays said: “If you knew that there was zero percent chance of being eaten by a shark would you swim more often?  I think it’s fair to say that for most people the answer to this question is a big ‘yes’! The same goes for animals and, for example, in the case of turtles, the fear of being eaten by sharks restricts the movement and behaviour of entire populations. But when the fear of being eaten dissipates, we see that turtles eat more, breed more, and go wherever they please.

“While this might sound like turtle paradise, the research shows that with the loss of some 90% of the ocean’s top predators, the occurrences of trophic meltdowns are now widespread. In our study we report trickle down effects on the capacity of the oceans to trap and store carbon, because as the prey become predator, their food of choice is ‘blue carbon’ ecosystems.”

The study has found that 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.

‌Facts about blue carbon ecosystems:

• Blue carbon ecosystems capture and store carbon 40-times faster than tropical rainforests such as the Amazon, and store the carbon for millennial time scales which makes them one of the most effective carbon sinks on the planet.
• Despite occupying less that 1% of the seafloor, it is estimated that blue carbon ecosystems capture more than half the ocean’s carbon.
• The carbon stored by blue carbon ecosystems is bound within plant biomass and within the ground.
• In seagrass meadows in Bermuda and Indonesia, for example, relaxed predation on herbivores has resulted in spectacular losses of vegetation, with           removal of 90–100% of above-ground vegetation.
• Such losses of vegetation can destabilize carbon that has been buried and accumulated over geological time scales. For example, a trophic cascade-induced die-off of saltmarsh in Cape Cod of 1.5 km² liberated approximately 248,000 tonnes of below-ground carbon.
• If only 1% of the global area of blue carbon ecosystems were affected by trophic cascades as in the latter example, this could result in approximately 460 million tonnes of CO₂ being released annually, which is equivalent to the annual CO₂ emissions of around 97 million cars.

‌Professor Hays said: “The realization of the role of predators in carbon capture adds to the growing concern over their plight. For example, it is estimated, that more than 100 million sharks may be killed in fisheries each year, often simply to remove their fins. It is vital, then, that there are stronger conservation efforts and stricter fishing regulations which will help restore marine predator populations and maintain the important indirect role that predators play in climate change mitigation. Also policy and management need to reflect this important realisation as a matter of urgency. It’s about restoring balance so that we have, for example, healthy and natural numbers of both sea turtles and sharks.”

Green Turtles (Chelonia mydas) photos courtesy of BS and RD Kirkby.