Ocean ecosystems rely on sharks to keep the populations of other species in check. Toby Daly-Engel, an assistant professor of Ocean Engineering and Marine Sciences at the Florida Institute of Technology and the Director of the Florida Tech Shark Conservation Lab, is sounding the alarm.
“Big predators like sharks are sentinel species. They’re the canary in the coal mine for climate change, “ she says. “In 2021, a study found that 70% of sharks are already lost in terms of abundance and species. That includes sharks going extinct before they are officially described and named. Of the known species, over a third are close to extinction—that’s a new finding as of 2021.”
In a 2021 study conducted by the British Ecological Society, researchers used data from a 16-month field experiment to simulate impacts should the tiger shark, a key apex predator in Western Australia’s Shark Bay, become completely extinct. The findings were shocking: should tiger sharks die out, the population of sea cows, which tiger sharks primarily feed on, would explode. This would lead to the overgrazing of seagrass, a plant that has already been irreparably damaged by climate change.
The combination of searing temperatures and an explosion in the sea cow population would lead to a seagrass shortage. Other seagrass-reliant animals would start dying out, and those reliant on seagrass meadows for shelter would be displaced. Eventually, the consequences would also reach sea cows. Ultimately, biodiversity would be irreparably damaged.
Big predators like sharks are sentinel species. They’re the canary in the coal mine for climate change.
A new January 2022 study by Global Change Biology proves that ocean warming affects the distributional range, migratory timing, and spatial protections of the tiger shark. Climate change is moving the habitats of shark’s around, mostly towards the poles or to deeper water.
Sharks’ role as apex predators is essential to the ecological balance in the oceans. “When you have loss of a predator, taking the top off the food chain, the prey that those predators were culling will grow out of control and eat themselves into starvation. They can lead eventually to what we call an extinction cascade,” explains Daly-Engel.
However, sharks play other roles in the ecosystem as well. Most of the 500 known species of sharks are smaller “mesopredators.” While these smaller sharks are in the middle, rather than the top, of the food chain, they are nonetheless important, as they play the roles of both predator and prey.
In a February 2021 paper, researchers at the Monterey Bay Aquarium in California reported a significant northward shift in young white sharks’ range due to the need to find a more suitable thermal habitat. Climate change has rendered the waters in the southern California Current—juvenile white sharks’ historic habitat—too warm, forcing them to venture north.
Juvenile sharks are especially vulnerable to warming waters. Even massive sharks that are found everywhere and able to swim thousands of kilometres in a season aren’t using their whole range. They return to the same habitats to reproduce, so are creatures of habit. When climate change affects the shallow reproductive habitats that they use as nurseries, that affects the whole population.
“Many sharks use nurseries, where mother sharks drop babies off at shallow coastal areas where the adults usually don’t go, and the babies join the adults when they’re full-grown. So, the babies stay in shallow coastal areas that make them particularly vulnerable to human activity,” explains Daly-Engel.
A 2021 study published in Nature has recommended protecting 30% of the world’s land and oceans. This is a good start but those concerned with the ocean’s longevity remain deeply concerned about the future.