What If There Were No Sharks?

Coral reef environments are another good example of sharks’ importance for overall biodiversity and ecosystem health, said Toby Daly-Engel, an assistant professor in the marine science department and director of the Shark Conservation Lab at Florida Tech.

“If the sharks disappear, the little fish explode in population, because nothing’s eating them,” Daly-Engel said, “pretty soon, their food — plankton, microorganisms, little shrimps — all of that is gone, so all the little fish ultimately starve.”

When that happens, algae and bacteria move into the reef, covering the coral so that it can't photosynthesize. “The coral will die, leaving just its skeleton behind, which eventually turns into limestone,” Daly-Engel said. “Then, in come the animals like starfishes and sea urchins; we call those grazers. So instead of a bunch of different species — sharks, bony fishes, invertebrates and mollusks — you end up with a reef with four to five species in it, tops. That’s a dead reef.”

Sharks serve another important role in ocean food webs: They are food for marine carnivores. Dead great white sharks that washed up on South African beaches without their livers were thought to have been victims of orca attacks. And video footage recently showed a dogfish shark (Squalus clarkae) feeding frenzy on the bottom of the Atlantic that ended with a grouper swallowing one of the shark whole. Even octopuses are known to feed on sharks, as demonstrated in a video that National Geographic posted to YouTube in 2009.

Migrating sharks, such as the gray reef shark (Carcharhinus amblyrhynchos), also provide nourishment for organisms in multiple locations in the ocean, by leaving behind generous helpings of their nitrogen-rich poo, marine biologist Melissa Cristina Márquez wrote in Forbes earlier this year. Márquez is the founder of The Fins United Initiative, which provides educational resources about sharks and their close relatives skates and rays.