Cold weather limits potential range of Burmese python invasion

An American alligator and a Burmese python locked in a struggle to prevail in Everglades National Park. Credit, Lori Oberhofer, National Park Service.The well-publicized invasion of Burmese pythons in the United States is unlikely to spread farther north than south Florida according to a new study by scientists from the National Wildlife Research Center.

The infamous invasion of the giant snake to the Everglades and neighboring areas likely occurred from illegally released pets and perhaps accidental escapes during Hurricane Andrew.

The growing population of Burmese pythons (Python molurus bivittatus) in the region has sparked serious concerns about potential ecological and public safety issues and has played a role in spurring new legislation to control the reptile trade.

However, controversy has emerged about whether or not the snake could actually establish farther north than the portion of southern Florida it currently inhabits. Habitat models developed by scientists tell conflicting stories.

One model based on mean temperatures predicts potential expansion of the python range to as far north as Delaware while another model based on temperature extremes shows possible snake habitat confined to south Florida and the southern tip of Texas.

This past year, an unseasonably cold winter provided an ideal experimental setting for Michael Avery and fellow scientists to gather sorely needed empirical data on this question.

A year earlier, nine Burmese pythons had been caught in the wild in Everglades National Park and transferred to pens in Gainseville located in the northern part of the state. The snakes had access to the outside as well as heated refugia inside their pens.

Temperatures during this past winter were particularly cold frequently falling below freezing. During the course of the study 3 snakes died, 2 snakes developed infections and were euthanized, and 2 snakes developed potentially fatal respiratory illnesses and were removed from the study.

Only 2 male snakes survived unscathed. Interestingly, the researchers observed that some snakes would sit outside during periods of very cold weather despite the heated refuge in the pen.

The authors hypothesize that a lack of genetic variation among the south Florida python population has perhaps led to a reduced behavioral and ecological flexibility to adapt to climatic changes. Given the overall study findings the authors write,

"Our empirical observations cast doubt that Burmese pythons can become established and persist beyond the southern portion of the Florida peninsula."

--by Rob Goldstein

Avery, M., Engeman, R., Keacher, K., Humphrey, J., Bruce, W., Mathies, T., & Mauldin, R. (2010). Cold weather and the potential range of invasive Burmese pythons Biological Invasions DOI: 10.1007/s10530-010-9761-4