Firefighters in Florida's Everglades National Park are encountering large, dangerous reptiles and poisonous trees as they battle a fire that has consumed about 39,000 acres (16,000 hectares) since last week.

Fighting the Mustang Corners blaze in the remote, trackless Everglades has "posed a lot of challenges," said Mike Dueitt, a firefighter from Florence, Mississippi.

"We're seeing everything from boa constrictors and pythons to iguanas and a few alligators."

When they cross paths with a large reptile, firefighters "do the best we can to work around it and move on, and wait until it clears the area before we go in," Dueitt said.

Poisonwood trees, whose effect Dueitt described as "poison ivy on steroids," also pose a hazard.

At the same time crews are struggling to keep the flames away from stands of invasive melaleuca trees, which can grow more than 60 feet (18 meters) tall.

"Melaleuca does create a challenge because of the very flammable, papery bark that it has," said David Hallac, chief biologist for Everglades National Park.

Firefighters fear that melaleuca stands near the park's northeastern boundary could help the fire spread into the area near Fort Lauderdale and Miami, where about six million people live.

Helped by Fire

The melaleuca tree, sometimes called the paperbark tree, is native to Australia.

The tree absorbs enormous quantities of water and was introduced into the Everglades in the early 20th century to help drain the vast region for development.

But the fast-spreading species quickly became an environmental nuisance in what should be a mostly grassy swamp.

Fire can actually benefit the tree, because flames cause it to drop large numbers of seeds. There are often more melaleucas after a fire than before.

The current blaze, which has been burning since May 14, recently reached the edge of a melaleuca stand before workers stopped it with fire-retardant chemicals.

"It's been dicey the last three or four days," Duiett said.

Bridget Litten, a National Park Service public information officer, said the fire was about 50 percent contained by Wednesday.

But the wind direction is expected to change Thursday and blow from the east, and that will keep the 200-plus firefighters there on their toes, she said.

"We're not quite sure what will happen tomorrow," Litten said. "We're hoping everything will hold."

Native Species Safe?

So far experts are optimistic that the fire will have a minimal impact on the roughly 20 species of native endangered wildlife that live in the Everglades.

For example, the flames have not done much damage to the habitat of the endangered Cape Sable seaside sparrow, Everglades biologist Hallac said.

About 3,000 of the birds, which live only in the swamp, are left in the wild.

"Overall, we've been extremely lucky, I guess, because the more important areas where the birds are nesting have been relatively unaffected," Hallac said.

"Some of that was luck, but it's also due to the really hard work of the firefighters, who fought very hard to keep the fire out of those areas."

In addition, rare Florida panthers probably would move away from the fire, and crocodiles live in an area that is not threatened by the blaze, Hallac said.

The park's alligators could lose some habitat, but Hallec said he didn't think there would be population declines because of the fire.

A few dozen Miccosukee Indians also live within the national park boundaries, but they have not had to evacuate, the National Park Service's Litten said.

Park service and firefighting officials think the blaze was caused by human carelessness or arson.