The mysterious case of the dead shark and the disappearing coral reefS

You've probably heard that killing apex predators is bad for ecosystems. But how does that really work? Now we know. The story begins with a lot of dead sharks.

For years, scientists have been trying to establish how sharks — being top predators — affect animals at the bottom of the food chain, such as corals. But they've had limited success, said Jonathan Ruppert, a marine ecologist at the University of Toronto.

Previous research has suggested that when you increase the population of people on islands surrounded by coral reefs, you see multiple impacts on aquatic habitats, including a reduction in sharks and a decline in coral reefs. However, researchers weren't able to connect the loss of sharks with the loss of the reef.

After hearing about the research and speaking with shark experts, Ruppert learned that scientists also haven't been able to pinpoint a well-defined role for sharks in the ecosystem. "It's all well and good to say they must be important, but we haven't actually quantified why," he told io9.

So Ruppert and his colleagues decided to find out if the overfishing of sharks affects coral reefs (a top-down effect), and how this relates to bottom-up effects, such as when coral reefs experience large die-offs that affect the populations of fish that survive off the reefs.

Revealing the Shark-Coral Relationship

Using data from a decade-long monitoring program, the team compared the populations of various types of fish from two atoll-like reefs off the coast of northwestern Australia: the Rowley Shoals and the Scott Reefs. The Rowley Shoals are part of a marine protected area, where no fishing is allowed. The Scott Reefs, on the other hand, lie within the "Australian-Indonesian Memorandum of Understanding Box 74" — a 50,000 square km (19,300 square mile) area where Indonesian fisherman are allowed to harvest fish using traditional techniques.

Though traditional, the fisherman's methods are very effective. Worse yet — they often harvest sharks just for their fins. "I have videos of men on these boats, which are just glorified rafts," Ruppert said. "There's no refrigeration to store fish protein for any length of time — they're not removing other fish and they're not taking back most of the body of the shark."

The shark differences between the reefs were striking: The abundances of the reef sharks at the Rowley Shoals were approximately three times those seen at the Scott Reefs. Additionally, the team saw that the Scott Reefs had significantly fewer herbivorous or algae-eating fish, including parrotfishes and rabbitfishes, than the Rowley Shoals.

It may seem counterintuitive that the herbivorous fish would decline along with the sharks, but the researchers discovered that the loss of the sharks on the reef resulted in fundamental changes to the food web. Specifically, the death of the sharks allowed "mesopredators," such as groupers and snappers, to flourish. These carnivorous fish ate the algae-eating fish, cutting down their numbers.

Bottom-up processes also impacted the ecosystem — a bleaching event reduced the coral cover of the Scott Reefs from about 60 percent to less than 10 percent, while the Rowley Shoals experienced similar losses from a Category 5 cyclone.

These benthic disturbances worked in concert with the top-down effects. With its lack of herbivorous fish, the Scott Reefs struggled to recover after the cyclone, as the algae overwhelmed young corals that tried to repopulate the area. The Rowley Shoals faired much better with its healthy populations of algae-eating fish.

"Fishing and benthic disturbances have independent effects on the ecosystem, but they also seem to work together to influence what's going on," Ruppert said. "I would say they impact the reef habitat equally."

The main implications of the study, Ruppert explained, is that sharks have a very important role in maintaining a balance within the ecosystem. "As soon as we start removing them from coral reefs, we are actually tipping the scales that are causing cascading effects that could trickle through the community," he said.

The research also highlights the potential value of marine protected areas. Previous work has suggested that many reef shark species stick to small areas on the reef. This means, essentially, that people wouldn't necessarily need to set up large reservation zones to protect sharks and prevent snowball effects on the ecosystem, Ruppert said.

Check out the full study in the open-access journal PLOS ONE.

Image via NOAA Photo Library/Flickr.