Tiger Sharks Keep Seagrass Tidy


This video is called tiger shark and more shark images by Mike Neumann.

From Discovery Channel:

July 11, 2007 — 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, working under the auspices of the Shark Bay Ecosystem Research Project, calculated tiger shark predation rates on dugongs. They focused their efforts on tiger sharks at least 10 feet long, as these adults would be 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.

12 thoughts on “Tiger Sharks Keep Seagrass Tidy

  1. As seagrasses expand, state considers new rules to protect them

    The Associated Press
    WILMINGTON, N.C.

    An unexpected expansion of underwater seagrass along North Carolina’s coast has come as state agencies work on a revised definition of the habitat, a discussion that could protect more areas from human disturbance.

    Officials with the North Carolina Division of Marine Fisheries have said the state must modify the definition of seagrass to more accurately describe the habitat _ and possibly help identify and protect areas that could support the seagrass.

    But much of the shallow areas ideal for seagrass growth lies in territory ideal for piers or docks. So the discussion among state officials has some regulators and developers worried about what a new definition could mean for coastal development.

    “Unfortunately, it’s an awkward situation,” said Jim Leutze, the former University of North Carolina at Wilmington chancellor who serves on both the Coastal Resources and Marine Fisheries commissions. “But both sides are trying to do the best they can within their responsibilities.”

    The debate comes as clusters of the so-called submerged aquatic vegetation have sprouted in places where it hadn’t been for years. Researchers are trying to figure out whether the grasses are recolonizing old habitat or expanding their range _ and why they’re doing so.

    New beds are popping up around Topsail Island and appear ready to grow in New Hanover’s highly developed tidal creeks and other coastal waters.

    The seagrass, found in North Carolina’s coastal waters from the Cape Fear River north into Virginia, is critical habitat for a range of sea life from fish to flora. Submerged aquatic vegetation usually grows in water less than 6 feet deep. Water clarity, strength of the current and sedimentation are also critical in determining where the grasses may grow.

    Smaller fish seek protection in the beds, other small animals attach themselves to the blades of grass or eat the plants themselves, and big fish and other animals come into the vegetation looking for food.

    “You end up with a big food web that’s very productive and supports a high-diversity of animal life,” Deaton said.

    State regulators have for decades treated the areas like a rare commodity, limiting dredging and dock building around them.

    Fisheries officials have said a new, broad-based definition is important because some of the vegetation is seasonal. Others can appear one year and not the next. And regulators want to make sure that dredging or development won’t cut into the areas where seagrass could grow.

    But that could make it difficult for regulators to determine the locations of seagrass beds that aren’t always present. And other regulators have worried about an avalanche of permit appeals from developers.

    Leutze said officials have to be practical about what a new definition would mean for waterfront property owners and development.

    Officials have formed a small committee to find a compromise for differing interests. They hope to have a unified definition by early next year.

    Mike Durako, a marine biologist at UNC Wilmington, said while dredging through sea grass effectively destroys the seagrass habitat, people can still build smaller docks to limit how much water they shadow.

    “Having seagrass doesn’t necessarily prevent people from developing their shoreline,” Durako said. “But accommodations have to be made.”

    ___

    Information from: The Star-News

  2. Seagrass losses reveal global coastal crisis

    Tue Jun 30, 2009 2:51am EDT

    By Michael Perry

    SYDNEY (Reuters) – Mounting loss of seagrass in the world’s oceans, vital for the survival of endangered marine life, commercial fisheries and the fight against climate change, reveals a major crisis in coastal ecosystems, a report says.

    A global study of seagrass, which can absorb large amounts of planet-warming carbon dioxide, found that 29 percent of the world’s known seagrass had disappeared since 1879 and the losses were accelerating.

    Seagrasses are flowering plants found in shallow waters. They were vanishing at the rate of about 110 sq km (42 sq miles) a year since 1980, said the study to be published in the U.S. journal Proceedings of the National Academy of Sciences.

    The study by Australian and American scientists found seagrass meadows were “among the most threatened ecosystems on earth” due to population growth, development, climate change and ecological degradation.

    It said there were only about 177,000 sq km left globally.

    “Seagrass meadows are negatively affected by impacts accruing from the billion or more people who live within 50 km (30 miles) of them,” said the report received by Reuters on Tuesday.

    The study said the loss of seagrass was comparable to losses in coral reefs, tropical rainforests and mangroves.

    “Seagrasses are sentinels of change” and the loss of seagrass was an indicator of a deteriorating global marine ecosystem. “Mounting seagrass loss reveals a major global environmental crisis in coastal ecosystems,” it said.

    ECONOMIC LOSSES

    It is estimated that 70 percent of all marine life in the ocean is directly dependent upon seagrass, according to U.S.-based Seagrass Recovery (www.seagrassrecovery.com).

    Seagrasses are the only flowering plants that can live entirely in water. They are most closely related to lilies and are very different to seaweeds, which are algae.

    Seagrass meadows provide important ecosystem services, said the study, citing an estimated US$1.9 trillion a year in nutrient cycling, enhancement of coral reef fish productivity, habitats for thousands of fish, bird and invertebrate species and a major food source for endangered dugong and turtles.

    Seagrass beds are believed to rival rice paddies in their photosynthetic productivity or the ability to extract greenhouse gas CO2 and convert it into oxygen and stored carbon matter.

    One acre of seagrass can lock away nearly 8 metric tonnes of carbon per year, which equals the CO2 emissions from a car traveling more than 3,500 miles, says Seagrass Recovery.

    The study said more than 51,000 sq km (19,700 sq miles) of grass had been lost in the past 127 years, with largest losses (35 percent) occurring after 1980.

    “Seagrass losses decrease primary production, carbon sequestration and nutrient cycling in the coastal zone. If the current rate of seagrass loss is sustained or continues to accelerate, the ecological losses will also increase, causing even greater ill-afforded economic losses,” said the study.

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