Shark attacks get all the headlines. Less appreciated until recently is just how much trouble many species of sharks are in. So are their close relatives, the rays. A quarter of species are at risk of extinction, 100 million or more sharks are removed from the oceans every year, and many of the largest species have declined 90 percent or more and likely no longer fill their past ecological role.
This is something we should care about. Although much remains to be learned about the importance of sharks to marine ecosystems, what we do know suggests that losing sharks could be bad for oceans and people.
For example, studies in the vast seagrass meadows of Western Australia have revealed tiger sharks, which still patrol the waters in large numbers, are a keystone. Tiger sharks hunt the shallow seagrass beds, causing some of their favorite prey – the dugong (a sea cow) and green turtles – to largely avoid these areas or to graze the grasses in a way that puts them at less risk and does less damage to the seagrass beds.
This allows huge pastures to develop with tons of habitat for fish, crabs, and shrimp that grow up to be caught by fishers and support a thriving ecosystem. The fully developed seagrass beds also trap carbon dioxide (in the form of seagrass and dead seagrass in the sediment), meaning that sharks and climate change are linked.
Recent studies suggest that if tiger sharks were lost and the seagrass beds were stressed, the big grazers could cause the entire ecosystem to shift to one with closely cropped seagrass that provides little food or shelter for small animals and poor stores of carbon. Tiger sharks have a strong influence on the quality of the ecosystem even without eating many turtles and sea cows. The mere presence of large sharks acts to shelter the seagrass by dissuading hungry grazers from feeding, much like the return of wolves has resulted in increased ecosystem diversity by causing deer and elk to shift where and how they forage.
Read more on why we should be thanking sharks for making our oceans healthier.