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One year after Hurricane Katrina smashed into Louisiana and the Gulf Coast, washing away hundreds of communities and lives, FOXNews.com's Catherine Donaldson-Evans visits Slidell, La., to find out first hand how one town is determined to rebuild. This is the fifth of her series of exclusive reports.

SLIDELL, La. — After the winds quieted and the rains stopped, even after the high floodwaters receded, nature was not itself in Slidell nor in any of the surrounding areas pummeled by Hurricane Katrina.

"The saltwater came in and killed all the vegetation — all of it," said Paul Trahan, who runs Honey Island Swamp Tours in the Pearl River and has become an expert in the local environment and ecology. "Nothing was alive. It looked like someone dropped an atomic bomb. All you could hear were cats crying. It was a real eerie feeling."

Residents of the area, including Trahan — who has been leading the boat ecotours through the swamp for almost two decades — say everything looked gray for months after the monstrous storm made landfall on Aug. 29, 2005, as a Category 3 hurricane.

There was little or no greenery. Flowers died. Yards were covered in thick layers of slimy mud that later dried and cracked. Trees were down everywhere; many that remained standing were stripped of their leaves and began to decay. Huge portions of the area’s marshes (which Trahan describes as flooded prairies) and swamps (flooded forests) disappeared.

"The marsh was really detrimentally affected," Trahan said.

Local wildlife took a hit, too, with some native species losing half or more of their population. The bodies of raccoons and rodents lay in the streets. Birds fell silent. And at least 1,000 dead fish littered the area, casualties of the massive storm surge that came from Lake Pontchartrain.

"The saltwater intrusion killed all the freshwater fish," Trahan said. "You could see them a week after the storm, floating up and down the river, which left the water rancid for a couple of weeks."

The environmental damage Katrina inflicted on the Slidell area and the wide swath of the Gulf Coast it slammed into was significant. And though nature has a way of reviving itself, those familiar with the local environment say it’s too early to predict the lasting ecological impact of the storm.

"As for the long-term effects, we just don’t know," Trahan said.

One thing is clear, however: It will take a long time — in some cases, decades — for many plant and wildlife species that once thrived here to fully recover, though several have already started to repopulate the area. Some, however, may never return. And much of the lost swamp and marshland is gone for good.

"It’s going to take years to come back," Trahan said. "The permanent impact is going to be on the marsh that was washed away."

Trahan, 60, along with his wife Brenda, 59, runs the swamp tour company founded by a local ecologist named Dr. Paul Wagner, who retired three years ago. The Trahans lived one town over from Slidell, in Chalmette in St. Bernard Parish — which was almost totally destroyed in the hurricane.

Their house was not only flooded with seven feet of water but also covered in a few inches of crude oil from a nearby Murphy Oil Corp.refinery. Refinery tanks burst because they floated up out of the ground during Katrina’s storm surge, spilling about 1 million gallons of oil in St. Bernard Parish. The company has denied claims of negligence and paid settlements to some affected homeowners, including the Trahans, who opted out of the class-action lawsuit Murphy is currently embroiled in.

The Trahans had to move to Picayune, Miss., just over the Louisiana/Mississippi border — which is about 12 miles from Slidell — but they’re trying to come back. For now, though, real estate prices, which are up by between 20 percent and 30 percent from what they were pre-Katrina, are too high.

Aside from the destruction to their home, the Trahans were in danger of losing the business that introduces visitors to the regional environment and the prized local swamp.

Though the site had seven feet of water, the buildings are 27 feet above sea level, so there was no flooding. Shortly after the electricity came back on, the Trahans reopened, on Oct. 1, 2005. But for months, nobody came. The tours that had seen as many as 40 to 60 people a day — many of them visitors to New Orleans from all over the world — only had at most four to six a week post-Katrina.

"It was scary," Trahan said. "I didn’t think we were going to make it. But we hung in there." He had to go back to work as a pharmacist to make ends meet. But now Honey Island is back up to a daily average of 10 to 20 people.

Today, the site’s still waters, green with the reflection of all the plants and trees, run calmly through what feels like an enchanted forest. Blue and white herons perch quietly in clumps of marsh grass. Alligators glide through the bayou.

Gators, Birds and Pine Beetles

The late-summer scene belies what happened here last August. But Trahan has watched the swamp and its inhabitants closely in the year since the hurricane.

The alligators that thrive there have done well, he said. In fact, new gators have joined longtime swamp residents like the 130-pound, 8-foot-long female that was circling a tour boat gliding through the bayou — the natural passageway through the swamp — on a recent excursion, and the 1,000-pound, 10-foot-long male that lives farther in.

Freshwater fish like catfish, perch and bass still haven’t returned since they were killed in the saltwater storm surge Katrina caused, but Trahan expects them back over the next few years. The Pearl and its swamps and bayous are all fresh-water systems.

Furry creatures that live by the river but aren’t strong swimmers or climbers — like raccoons, minks, otter, nutria (large South American water rats) and marsh rabbits — were killed in large numbers, and so far not too many have come back, according to Trahan. The region’s wild hog and white-tailed deer populations also declined.

Bobcats and gray squirrels did better, most likely because they were able to climb high into the treetops to safety.

Many birds that had nested in the swamp flew away in the months after Katrina, but a good number have returned. Trahan has noticed new species moving in.

"The bird population has increased tremendously," he said. "They lost a lot of habitat, so I think they moved up here."

Currently herons, egrets and ibis are among the varieties that have been spotted in the swamp. Others, like swallow-tailed kites, which used to nest in the once-prevalent pine trees, have had to leave because so many of the pines fell during the storm or have died during the year since because of their exposure to saltwater.

Tree frog populations have also thinned, but crawfish are doing well — and so are unsavory critters like spiders, pine beetles and snakes.

"Pine beetles are now taking over," said Suzanne Le Breton, a reporter at the local St. Tammany News, which combined with The Slidell Sentry-News after the hurricane. "You’re seeing a lot of brown trees."

Le Breton’s trailer home in a nearby town in St. Tammany Parish (where Slidell is located) was surrounded — and smashed — by pine trees, so she has had firsthand experience with the beetles. The concern is that they will ravage many of the trees that are left.

"This was a tree-lined city," said Slidell Mayor Ben O. Morris. "The storm — and this is a conservative estimate — left 30,000 to 35,000 trees on the ground, the majority of them pines. The saltwater is killing the [remaining] trees."

Officials — including those at the Federal Emergency Management Agency — have begun "de-greening" the city, taking down dying trees and planting new ones, according to Morris.

Oaks also "took a real beating," according to Trahan, who said their numbers dwindled by about 40 percent.

"The hardwoods in the northern part of the swamp were devastated," he said. "We lost willows, a lot of red maples."

But other kinds in the wetter part of the swamp, like the 1,000-year-old cypress trees and the tupelo gums lining the bayou, weathered Katrina better.

"In the area where we do tours, there were not a lot of down trees," Trahan said. "The majority survived."

He estimates that the pines will return in about five to 10 years, but the oaks and other hardwoods will take a decade to 15 years to repopulate the area.

Some invasive species have moved into the swamp that are a concern, including a clover-like weed called salvenia that dotted the top of the water before Katrina and now covers it thickly in parts.

"This was spread all over the area and before it was limited," Trahan said. "An invasive species does alter the ecology. It’s going to affect the vegetation underneath [by killing it] because sunlight can’t get through and so it can’t grow."

What environmental experts are particularly concerned about in the region are the vastly sinking wetlands below New Orleans (which is 30 miles south of Slidell) — which include barrier islands that had served as natural protection against winds, floodwaters and storms.

They have been sinking at an alarming rate for decades because of global warming, oil drilling and other factors. The levees built to protect people from violent weather after the great flood of 1927 have actually made the city, most of which is an average of eight feet below sea level, more susceptible to flooding because they have interfered with the natural movement of the river and the sediment.

By now, those levees have become notorious because they were punctured in a few places under the force of Katrina's storm surge — and overtopped in other spots by the surge — making the devastation in parts of New Orleans unimaginable and completely wiping out entire neighborhoods there.

"The wetlands are compacting," Trahan said. "South of New Orleans — that will never recover. There are no levees here, so it can recover. Canals and levees destroy wetlands. They change the flow of the water."

He has faith in nature’s ability to regenerate — and believes it will happen at his swamp and elsewhere in the region, albeit slowly. What he doesn’t have as much faith in is people’s handling of the environment.

"Nature is pretty resilient when left alone," Trahan said. "When man interferes, that’s when things get messed up."

Part I: One Year After Katrina, Slidell, La. Shows Gulf Coast’s Determination to Rebuild

Part II: Katrina-Battered Slidell, La., Must Also Rebuild Arts Scene

Part III: Slidell, La. Small Businesses Among Katrina's Many Victims

Part IV: Slidell, La., Sheriff Fights to Keep Down Crime After Helping Town Survive Hurricane Katrina