Tiger sharks play a crucial role in helping fragile and economically important seagrass ecosystems bounce back after marine heatwaves by scaring off hungry herbivores such as sea cows and turtles, emphasizing the importance of top predators, new research indicates.
Scientists in Australia investigated what would happen if sharks disappeared from recently damaged seagrass meadows. Without sharks to keep them in check, the emboldened grazers would prevent the lush seagrass canopy from growing back in many areas, the team reported on Feb. 23 in the Journal of Animal Ecology.
"You would never think that sharks and climate change were really linked in that way or that the resilience of ecosystems to extreme climate events would hinge on a top predator," said Michael Heithaus, a marine ecologist and dean of the Florida International University College of Arts, Sciences & Education, who is the last author of the study. "So it really drives home that when we are thinking about conserving ecosystems we have to have healthy and intact predator populations."
Aside from sharks, turtles and sea cows such as dugongs, commercially valuable fish and crustaceans also benefit from the shelter that seagrass meadows provide. Around the world, though, seagrass meadows are threatened by climate change, pollution and other human-related disturbances, including coastal development.
Shark Bay in Western Australia, the setting for the new study, is one of the largest and most pristine seagrass ecosystems in the world, with smaller grasses as well as bush-like species that can grow to exceed 6 feet tall.
"If you can understand how Shark Bay works, you can use that information to try to protect and restore other areas that are in trouble," Heithaus says.
In the summer of 2011, marine currents swept abnormally warm water into the bay for two months. The prolonged heatwave wiped out most of the tall grasses, which are especially sensitive to warm temperatures. Under climate change, this kind of extreme weather event is predicted to become more common.
"[If] you lose that seagrass then you don't have the roots there to hold the sediment together," Heithaus said. "The water clarity went way down and it just really kind of unraveled that whole system temporarily."
He and his colleagues had been visiting Shark Bay regularly for nearly 20 years before the heatwave. They'd observed that when tiger sharks return to the meadows every summer, dugongs and sea turtles retreat from the shallow waters of their "salad bar" to safer areas with less choice vegetation, Heithaus says.
The researchers wanted to find out what would happen if a seagrass meadow was hit by a heatwave and there were no sharks to fend off the herbivores, particularly dugongs, which are closely related to manatees.
In 2013, they set up 30 plots within the recently disturbed seagrass meadows of Shark Bay. Over the course of 16 months, the researchers mimicked marauding dugongs and emulated how they would crop and dig up the grasses during times when sharks would normally be plentiful.
"We did an experiment that was basically underwater hedge-trimmers," Heithaus said.
The researchers exposed these areas to varying levels of grazing every 30 to 60 days, taking into account the estimated populations of the grazers. They found that the amount of Amphibolis antarctica — the vital, tall-growing grass — declined by around 30% to 50% on these plots. On seagrass patches that the researchers left alone, A. antarctica declined by 23% overall.
However, the researchers noted, among the seagrass beds that they didn't disturb or had "grazed" less intensely, the declines mostly occurred in areas that had already been impacted more severely by the heatwave.
"What we found is if the sharks are there and they continue to scare those grazers, it kind of gives the seagrass space to recover, but if you were to lose the sharks then the sea cows and sea turtles could eat to their heart's content wherever they wanted," Heithaus said. "That big, important seagrass would never make it back, and instead what we would have is these short seagrasses that are kind of like a finely mowed lawn that don't provide the habitat value that big seagrass does."
Among their other functions, seagrass meadows pull carbon dioxide from the surrounding waters. Seagrasses and similar habitats such as mangroves and salt marshes can store up to 1,000 metric tons of carbon per hectare, more than most terrestrial ecosystems. This means that seagrass can help mitigate the impacts of greenhouse gas emissions. One species has also been found to trap and remove plastic waste from the shallow ocean floor.
Fortunately, in the remote waters of Shark Bay, the tiger sharks that unwittingly protect these meadows are thriving, Heithaus says. However, overfishing has caused sharks around the world to dwindle. The population of sharks and rays in the high seas has dropped by an estimated 71% over the past half-century. And when Heithaus and his colleagues recently surveyed reef sharks around the globe, they failed to find any sharks on nearly 20% of the reefs they visited.
The findings reinforce how important it is to ensure that top predators are abundant enough to fill their ecological roles, Heithaus says.
"We should be protecting [predators] even as we're trying to figure out the where, when and how they are important to their ecosystems," he said.
Meanwhile, Heithaus and his colleagues will continue to track the slow and ongoing return of seagrasses to Shark Bay. Also important will be establishing whether top predators can help other ecosystems stay resilient in similar ways as the climate changes.
"What we need to figure out now is how general is this result — can we apply it to other systems, or is this a quirk of Shark Bay?" he asked.
The study "Loss of predation risk from apex predators can exacerbate marine tropicalization caused by extreme climatic events," published Feb. 23 in the Journal of Animal Ecology, was authored by Robert J. Nowicki, Mote Marine Laboratory and Florida International University; James W. Fourqurean and Michael R. Heithaus, Florida International University; Jordan A. Thomson, Deakin University; and Aaron J. Wirsing, University of Washington.