Sharks – the Wolves of the Sea

By this point, just about everyone has seen the incredible video explaining how the re-introduction of wolves to Yellowstone National Park dramatically changed the makeup of the ecosystem. The presence of an apex predator like wolves changed how prey species behaved, which led to the recovery of plant species that had been in decline, which led to a shift in how the Yellowstone River itself flows through the park. After millions of years of evolution, every ecosystem has developed a unique method for keeping plant and animal, predator and prey in balance. In the world’s oceans, one apex predator keeping things in balance is the shark.

Depending on the species, sharks eat everything from fish, to crustaceans, to krill, to other sharks. The great white shark surprises its prey, usually sea lions and seals, from below. The whale shark feeds by swimming along with its mouth open and filtering out a meal of plankton and krill. Still other sharks will bottom feed by trawling the ocean floor eating crustaceans, mollusks, and other low-dwelling creatures. Others will focus their dining efforts on schools of feeding-frenzied fish.

Image

Regardless of what it eats, each shark has its own role to play in keeping its ecosystem in check. Sharks will normally hunt old, weak or sick fish and other prey. An easier meal for the shark also means a healthier population of fish since any disease or genetic disadvantage that the eaten fish might have had will not be passed on to others. Sharks also prevent the overpopulation of any one prey species. By eating the excess residents in their territory, sharks keep prey populations at sustainable levels.

Many scientists have found that the mere presence of an apex predator like the shark will alter the behavior of their prey species and prevent them from consuming too many resources in one area. In Hawaiian waters, for example, scientists have found that when there are no tiger sharks around beds of seagrass beds, sea turtles will spend so much time nibbling on one area of the tastiest grass that the bed may be destroyed. When tiger sharks are present, however, the turtles move around more frequently, spreading their meal out more evenly among the beds to avoid becoming an easy target for dinner. This leaves room for other species to make use of the same habitat, allowing a wider variety of fish and other animals to thrive on the seagrass bed. Meanwhile, the sharks themselves will rarely over-hunt a single species because the shark can feed on such a wide variety of prey.

Image

As a top predator, sharks often help to maintain the populations of the next-biggest predators in the food chain. These predators in turn regulate the populations of their prey species, and so on. You can see how this relationship plays out in the Caribbean Sea, where the number of sharks actually impacts how healthy the coral reefs are.

Coral and macroalgae both compete for space on reefs. When enough algae-eating fish live nearby, they eat up the competition, giving coral more room to grow. That’s the bottom of the food chain. At the top are the reef sharks. The sharks eat grouper, which eat herbivorous fish like gobies and parrotfish, which in turn eat the algae. But without sharks, grouper numbers go up, and they eat more and more gobies and parrotfish. Without as many algae-eating fish to chow down on their competition, coral can be overtaken by algae, leaving the reefs less healthy and less able to support such a wide variety of life. This is exactly what happened to Jamaica’s coral reefs, almost 90% of which are now covered by smothering macroalgae.

Similar relationships exist between sharks and other species all across our oceans. Scientists have found direct connections between the loss of sharks and the loss of healthy coral reefs, declining fish populations, crashing shellfish populations, and more. Remove one link in the food chain and the entire system can be thrown off.

Image

So what’s happening to our sharks? Sadly, human activities are taking their toll. Shark finning, a process in which a shark’s fins are cut off for the food trade, kills as many as 73 million sharks each year. Another 30 to 50 million sharks each year become “bycatch,” caught up in nets and lines that are intended to catch other fish. Most sharks don’t reach their reproductive age until they are 12-15 years old, and even then many species only produce one or two pups at a time. This means that shark populations have an especially hard time recovering when so many adult sharks are lost. Of the 465 identified species of sharks, 74 are currently listed as threatened by the International Union for Conservation of Nature (IUCN). That means that without action, we could lose almost 16% of all shark species. Just think of the impact that the loss of these sharks could have across the world’s oceans!

Humans are causing the decline of these important species. It’s up to you and me to put things right. Here are a few things you can do to help:

Promote shark fin bans in your state
Currently 10 states have banned the shark fin trade, making it illegal to buy, sell, or in some cases even possess shark fins. By cutting down on the demand for shark fins, we can save lives. If your state already has such a ban, thank the people who made it possible! And if it doesn’t, keep an eye out for an alert from us. We’ll be reaching out to our supporters as more and more states introduce legislation to ban the possession of shark fins.

Eat sustainable seafood
Did your seafood cost sharks their lives? Ask questions about the food you purchase, and look for brands or labels that indicate your food was caught sustainably.

Spread the truth about sharks
Thanks in no small part to Hollywood, people tend to pay more attention to sharks’ teeth and movie sequels than they do to the role these animals play in our oceans. Talk to people about how much healthy oceans depend on sharks, and we may see more attention given to protecting these important creatures.