About half the Burmese pythons found in the park in the past few weeks were dead.

FORT LAUDERDALE, Fla. — Vultures circled over Everglades National Park’s Anhinga Trail, where thousands of dead nonnative fish floated in the marshes.

About half the Burmese pythons found in the park in the past few weeks were dead.

Dead iguanas have dropped from trees onto patios across South Florida. And in western Miami-Dade County, three African rock pythons — powerful constrictors that can kill people — have turned up dead.

Although South Florida’s warm, moist climate has nurtured a vast range of nonnative plants and animals, a cold snap last month reminded these unwanted guests they’re not in Burma or Ecuador any more.

Temperatures that dropped into the 30s killed Burmese pythons, iguanas and other marquee names in the state’s invasive species zoo.

Although reports so far say the cold has not eliminated any of them, it has sharply reduced their numbers, which some say may indicate South Florida is not as welcoming to invaders as originally thought.

“Anecdotally, we might have lost maybe half of the pythons out there to the cold,” said Scott Hardin, the Florida Fish and Wildlife Conservation Commission’s exotic species coordinator. “Iguanas definitely. From a collection of observations from people, more than 50 percent fatality on green iguanas. Green iguanas really got hit hard. Lots of freshwater fish died; no way to estimate that.”

The cold snap has played into a highly politicized debate over how to prevent nonnative species from colonizing the United States.

Reptile dealers and hobbyists strongly oppose a proposal by the U.S. Fish and Wildlife Service to ban the import of and interstate trade in Burmese pythons and several other large snakes. They say South Florida’s cold snap shows these species don’t threaten to spread north, as some claim, and a federal crackdown is unnecessary.

“Pythons are tropical animals,” said Andrew Wyatt, president of the United States Association of Reptile Keepers. “When temperatures fall below a certain level, they are unable to survive. It reinforces the idea that the pythons can’t exist more than a short period of time north of Lake Okeechobee. Even the pythons in the Everglades are dying during the cold snap.”

Wyatt said scientists are downplaying the effect of cold weather on the pythons because that would undermine their ability to win grants to study a problem that has received international publicity.