A Snake Invasion? Debating the Risks

In Sunday’s paper, I write about the controversy over the federal government’s move to ban imports of nine species of snake and their transport across state lines. Its decision was based largely on a risk assessment by two government scientists of the impact those snakes could have on native ecosystems.  As I explain in the article, snake breeders and other snake enthusiasts have widely criticized one aspect of the report in particular: the suggestion that pythons could be suited to a range of climates across a band of the American South.

The two scientists behind the report, Gordon Rodda and Bob Reed, are eager to answer their critics. In fact, they will have a peer-reviewed rebuttal in print by the end of the month.

First, they point out that climate was only one part of the risk assessment. Had they focused solely on the impact that invasive snakes like the Burmese python have already had on places like the Everglades, for example, that would have been enough to produce an assessment of significant risk.

They cited brown tree snakes in particular as a harbinger of havoc. They were accidently transported to Guam about 50 years ago, and for decades they appeared harmless. Now they are uncontrolled pests that have decimated populations of birds and other small vertebrates native to the island’s forests.

But the scientists also defend their climate models. They emphasize that the models are not meant to factor in every variable that would affect the ability of the species to thrive, like the availability of prey and human development of the land. In other words, a climate model is not a prediction that the animal will spread to those areas, but an outline of the limits of the areas where they can survive the cold and dryness.

They also argue that scientists who did the alternative climate model incorporated too many variables, far more than standard practice would dictate was necessary, in assessing risk — and that this led them to underestimate the area through which the snake species might spread.

While some pythons have died in winters to the north of the Everglades, Dr. Rodda and Dr. Reddy say, their fate does not necessarily reflect the survival capacity of the species as a whole. Pythons learn adaptive behaviors early in their life cycle, they note, whereas the ones taken from the Everglades had matured without exposure to the cold and therefore may not have known how to protect themselves.

Finally, they point out that many of the wild pythons in the Everglades survived the cold snap of January 2010 just fine. They note that hatchling counts in the summer of 2010 were equivalent to those of the previous year, which indicates that the Burmese python population is still expanding — enough to make a grown alligator shudder.