Sharks are apex predators in aquatic environments, which means they prey on a wide range of species below them in the food web. Through direct consumption they restrict the abundance of other animals, which in turn reduces the impact of those animals on their food source. Sharks also affect the foraging behaviour and distribution of their prey species through fear. Both of these trophic cascade mechanisms have an impact on the marine carbon cycle.
Protecting sharks, across the world’s coral reefs (301,000 km2) can sequester 108 MtCO2/y. That is the total of fossil fuel emissions by Kuwait in 2022 (109 MtCO2/y)¹
Research carried out on Australia’s Great Barrier has shown that sharks cruising near coral reef patches can alter the foraging behaviour of many herbivorous fish species. The perceived threat of predation causes these fish to feed heavily near the coral patches, where they can quickly take shelter should the need arise, but stops them from venturing further. This leads to the creation of so-called “grazing halos” – patches of seabed that are largely devoid of seagrasses and macroalgae. Thanks to reduced levels of vegetation, these halos were found to have 24 percent less carbon stored in their sediments than areas which experienced little or no grazing.
In Shark Bay, in Western Australia, tiger sharks have been found to control the behaviour and numbers of dugong (sea cows), which graze on roughly 40 kg of seagrass a day. By keeping the dugong population in check, and reducing grazing pressure, the sharks help seagrass meadows thrive, thereby boosting CO2 uptake and storage. Seagrass captures carbon up to 35 times more quickly than tropical rainforests – even though it only covers a tiny fraction of the seafloor, it absorbs an astonishing 10 percent of the ocean’s carbon each year.
These studies, and a growing range of others, highlight the need to conserve predator-prey dynamics to help maintain the critical role of marine vegetation and sediment in carbon sequestration. Sharks, just like sea otters, have a critical role to play in this regard.
Yet shark populations are currently in decline in the majority of marine ecosystems. In the Caribbean and Indonesia, for example, dwindling shark populations have led to overgrazing by herbivores such as sea turtles, which has seen the complete disappearance of seagrass in some areas. A recent assessment by the IUCN found that over a third of all shark and ray species are now threatened with extinction. Overfishing is the biggest threat, but loss of coastal habitat, loss of prey, and declining water quality are also contributing factors.
Studies have shown that changes to predator populations can have a significant impact on CO2 emissions
Sharks influence the abundance and behaviour of their prey. This can have a significant impact on blue carbon levels
How do sharks help store carbon?
- Trophic Cascades: As apex predators, sharks play a crucial role in regulating marine ecosystems. By controlling the population and behavior of their prey, sharks help maintain the balance of marine vegetation, such as seagrasses, which are critical to carbon sequestration.
- Behavior Influence: The fear of sharks affects the feeding patterns of herbivorous fish and other prey. For instance, sharks on the Great Barrier Reef cause herbivores to stay close to coral patches, limiting their grazing on seagrass beds. This helps preserve carbon-rich vegetation.
- Seagrass Carbon Storage: Seagrass meadows, which are crucial carbon sinks, absorb carbon up to 35 times faster than tropical rainforests. They cover a small fraction of the seafloor but store 10% of the ocean’s carbon each year.
Current situation – what we need to do
Sharks are key to protecting blue carbon ecosystems. Declines in shark populations due to overfishing and habitat loss lead to overgrazing by herbivores like sea turtles, which can devastate seagrass meadows. Protecting shark populations is crucial to preserving marine vegetation, which in turn helps mitigate CO2 emissions. Maintaining predator-prey dynamics is essential for the health of these ecosystems and for effective carbon sequestration.
(1)Note: These species data are from Table 1 and the Supplementary Appendix 1 of Schmitz, O.J., Sylvén, M., Atwood, T.B. et al. Trophic rewilding can expand natural climate solutions.
Nat. Clim. Chang. 13, 324–333 (2023). https://doi.org/10.1038/s41558-023-01631-6
Next steps
- Return to the main Climate Heroes page.
- Read all the original Climate Hero case studies in ‘Animating the Carbon Cycle: Supercharging Ecosystem Carbon Sinks to Meet the 1.5°C Climate Target’. Download the pdf here.
- Curious to learn more? Watch the video from the UN Climate Change Side Events at COP28: Bringing elephants into the room – why UNFCCC needs to put animals on the table.