September 21, 2010

Sea Gulls Snatch Top of Food Web

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The top of the food chain – reserved for the animal kingdom’s most ferocious hunters and most lethal killers – offers residency to an unlikely new predator:  gulls.

Though most people think of these sea birds as simple nuisances, easily thwarted by an occasional shoo-ing, Shoal’s Marine Lab sea bird ecologist Julie Ellis discovered these birds’ potential predatory dominance, calling them “the apex predators” in the marine ecosystem of Maine’s Appledore Island. Ellis researches the impact of the predatory behavior of two gull species, the Great Black-backed Gull (Larus marinus) and the Herring Gull (Larus argentatus), on the rest of the island’s wildlife.

She proposed that the gulls’ absence on the island would cause a disturbance, cascading throughout the island’s food web, altering its other animal populations.

In addition to gulls, Appledore Island is home to multiple species of crabs, fish and snails.  Without the presence of any other natural predators, the gull population remains unchecked.  Gulls survey the skies, feed from the waters, and when on land, assert their presence with a loud call, reminiscent of a lion’s roar.  This predator feeds over six times a day, swooping into the water to catch its food.  Gulls also get a huge portion of their meals from picking around at the shoreline during tidal changes.

Ellis has been researching gull predation on Appledore since 2004, focusing on the animal’s coastline cuisine.  She investigates the possible impact that these gulls’ absence would have on the members of the marine food web present during low tide.  According to Ellis, the gulls’ predation in these intertidal zones suppresses their main prey, the Johna Crab (Cancer borealis), from overpopulating and overfeeding on Periwinkles Snails (Larus littorea).

“Crabs themselves are important predators in the intertidal and subtidal zones,” Ellis stated. “They eat urchins, mussels, snails, and other crabs.”  She asked, “If gulls are removing all of these crabs in the intertidal zones, what impacts are these having on other members of these marine food webs?”

To test the significance of gull predation, Ellis and a team of volunteers set up enclosed gull-free zones at the water’s edge. Ellis predicted that the gull-free zones, guarded from gull predation by wire and water-gun wielding volunteers, would contain more Johna Crabs at low tide.  Consequently, she expected the number of the crabs’ prey, Periwinkle Snails, would dwindle as a result of the increased crab predation in these areas.  Lastly, she expected that a decrease in the Periwinkle population would  allow their main food source, algae, to flourish.

Of the three test sites established over four months, only the largest plot (744 meters-squared) showed the predicted effects.  A glance showed the abundance of crabs and the scarcity of snails throughout this gull-free zone, and subsequent statistical tests indicated that there was both a significant increase in the Johna Crab population and a significant decrease in the Periwinkle population.

The algae amount, contrary to predictions, remained unchanged, but following the effects of this disturbed food web, the Johna Crab’s other food sources, like mussels and Dog Whelk snails,  decreased in number throughout the test zone.

For Ellis, the most surprising result was a decrease in the number of Green Crabs – a middle level predator.  This change likely occurred due to increased competition and possible predation caused by the increased number of Johna Crabs.

The flourishing Johna Crab population and the diminished Periwinkle and Green Crab numbers showed that the gulls’ absence caused a population disturbance felt throughout the food web.

Ellis’ study ranked the nuisance seabirds at the top of the food web, the presumed position of nature’s most reknown predators, like lions and tigers.

Original Author: Nicholas St. Fleur