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  Discovery Channel 11 Jul 07
Tiger Sharks Keep Seagrass Tidy
Jennifer Viegas, Discovery News

Australian tiger sharks keep a tidy lawn for their marine neighbors by controlling where local herbivores can nibble, according to a study published in the current issue of Animal Behavior.

The discovery adds to the growing list of ways in which sharks benefit ecosystems worldwide.

In seagrass communities in particular, countless other creatures depend on the presence of sharks.

"Seagrasses form the foundation for many near-shore marine ecosystems," lead author Aaron Wirsing told Discovery News. This is the case in Western Australia's Shark Bay, where seagrass is "nourishing and sheltering a host of invertebrates and fishes that, in turn, support top predators like sharks."

Wirsing, a researcher in the Department of Biological Sciences at Simon Fraser University, and his colleagues studied how the presence of tiger sharks specifically affected the feedings of dugongs — large aquatic mammals that somewhat resemble their manatee relatives.

Dugongs spend much of their day chewing on seagrass. Through catch, tag and release methods, the scientists calculated tiger shark predation rates on dugongs.

Working under the auspices of the Shark Bay Ecosystem Research Project with funding from the National Geographic's Expeditions Council, the researchers focused their efforts on tiger sharks at least 10 feet long. Only adults that size are large enough to take on a chunky dugong.

The gentle herbivores prefer to eat segrass in the middle of patches. Growth is lush there and packs more of a nutritional punch due to the presence of extra organic carbon.

Escaping from hungry sharks is difficult from these interior areas, however.

Wirsing and his team found that when large tiger sharks were around, dugongs instead chose to feed around seagrass meadow edges. The grass is not as tasty or nutritious at the edges, but the location allows escape to deeper water if predators are near.

By indirectly controlling where dugongs feed, tiger sharks keep the seagrass mowed down at all areas.

"Dugong grazing can certainly hold seagrass growth in check," Wirsing explained. If left unchecked, however, the herbivores would simply eat all of the seagrass. "That's where tiger sharks come in," Wirsing explained.

Both tiger shark and dugong populations are at dangerous lows in many places, because of human influence.

People often fear tiger sharks, since they have attacked people in the past, but George Burgess, director of the University of Florida's International Shark Attack File, attributes the attacks to tourist recklessness.

He said tourists may often "bring their aquatic recreation to places known to be sharky without asking natives about good and bad places."

World Conservation Society biologist Tim Davenport said the dugong situation is just as bad.

He explained that "dugongs are now critically endangered" in certain regions, such as in Tanzanian waters, primarily due to a combination of fishing net entanglement and habitat destruction.
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