Florida Keys News - Islamorada/KL Free Press
Wednesday, May 30, 2012
Study: 'Commuter' gators may feed the Glades

SOUTH FLORIDA -- The growing number of crocodiles in the Florida Keys has been making lots of headlines recently.

But the surprising role that alligators likely play in linking the Everglades' saltwater and freshwater food chains will be a focus of a documentary to air as part of WPBT2's "Changing Seas" series, which resumes with four new episodes in June.

The episode, titled "Coastal Carnivores," is co-produced by Florida International University biologist Mike Heithaus, who heads the university's School of Environment, Arts and Society. It explores the findings of a study Heithaus has been conducting since 2006 on the role played by top predators in Everglades' transition zones between saltwater and freshwater.

Heithaus and his team have focused their work on both alligators and juvenile bull sharks who reside in the Shark River basin of southwest Everglades National Park.

To monitor behavior, the team captures individual animals and attaches transmitters to them so that they can track their movements in and near the Shark River Slough. The transmissions are augmented by listening systems the scientists located strategically in the slough.

One of Heithaus' most interesting findings, he said in an interview last week, is that contrary to popular belief, individuals from each species adopt differing lifestyles.

"Not every one of the sharks and every one of the alligators behaves the same," he said.

The juvenile bull sharks of the Shark River area stay there until they are 3 to 5 years of age, when they become large enough to hack it against the larger predators that lurk in the ocean. But while some juvenile bull sharks choose to take continuous shelter in the safety of the river, other more adventurous souls head back and forth from the mouth of the river, where it meets the Gulf of Mexico, in order to hunt. Doing so makes the juveniles vulnerable to attack but also gives them access to a wider variety of prey.

Surprisingly, Heithaus found similar behavioral patterns among alligators, despite the fact that the species is mostly intolerant of saltwater.

In a 2011 peer-reviewed paper that Heithaus co-authored last year with FIU grad student Adam Rosenblatt, the duo detailed their findings from alligator monitoring in the Shark River Estuary between November 2007 and December 2009. Over that timeframe, Heithaus and Rosenblatt were able to continuously track 16 individual alligators for periods of at least six months.

Out of those 16 gators, they observed that nine regularly moved between the saltwater of the Gulf of Mexico and the brackish water environment upstream. Some even traveled further up, as much as 25 kilometers, into freshwater territory.

Heithaus calls these transient alligators "commuters," and as with the more mobile juvenile bull sharks, he believes their purpose is to access the more abundant feeding grounds of the ocean.

"Therefore, commuting alligators may link marine food webs with those of the estuary and marshes in the coastal Everglades," he and Rosenblatt wrote in their 2011 study.

Just like any dutiful office worker trying to make a living, these commuters didn't just journey to their working grounds once. Rather, the 16 alligators traveled between the downstream saltwater areas and the brackish waters of the mid-estuary between 13 and 52 times over the life of the study, staying in the gulf waters an average of 6.6 days per trip, but in a few cases more than 60 days.

Unsurprisingly, alligators spent more time on the gulf's edge during the wet season, when rainwater flow from upstream brings salt levels down. During the wet season, the commuter alligators spent nearly half of their time in the saltwater zone, compared to just 16 percent of their time during the dry season.

That finding allowed Heithaus to hypothesize that Everglades restoration, which seeks to increase freshwater flows into the Gulf of Mexico and Florida Bay, will be a boon for commuter gators, making downstream transit easier and the destination more comfortable, not unlike an office worker gifted with an extra lane of highway for his morning drive and his own parking space upon arrival.

Though his study only monitors Shark River alligators, Heithaus said there's no reason to believe that alligators who occupy Florida Bay's watersheds, including its largest historical tributary, Taylor Slough, should behave any differently.

The local coastal regions, with their seagrass and plethora of species, are abundant in nutrients. So, the discovery that many alligators and juvenile bull sharks use the ocean as hunting grounds has led Heithaus to believe that they play a significant role in fertilizing the nutrient poor estuaries of the southern Everglades.

As the gators and sharks move upstream, they "deposit" their nutrients, Heithaus said delicately. Non-science types might use different language to describe what the animals are doing in the Shark River's brackish and freshwater zones. But in either case, such deposits are food for plankton, at the base of the estuary's food chain, as well as for mangroves, sawgrass and small trees.

The "Coastal Carnivores" episode will air at 7:30 p.m. Wednesday, June 27 on WPBT. The "Changing Seas" series will resume Wednesday, June 6, with an episode on tiger shark behavior. The season will also features programs on the mysterious microorganisms that inhabit coral reefs and the last great spawning site of the Nassau grouper.

rsilk@keysnews.com

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