I have been following with interest the previous threads on the Kingsnake Forums regarding the USGS map controversy. The logic they have used to make these potential range maps is extremely oversimplified and also quite in error. The process by which they used average temperature data to construct these maps is, on the surface, a good logical first step. Although other criteria such as available food supply at all life stages, shelter, water, etc. are important factors, climate is still definitely a major determining factor on range distribution. However, they somehow lost sight of the big picture after they moved forward from that point. They obviously made several major errors in their process of determining the assumed maximum extent of the distribution of this species in the U.S.
The first error was using the natural extreme northern range of Python molurus molurus, the Indian python, in their calculations regarding potential range extensions for the Burmese python, Python molurus bivittatus. They are correct in that P.m.molurus has as it’s northern most distribution, the northern Indus valley in northern Pakistan. The farthest north that this species has been found in Pakistan is about 33.5 degrees north latitude. On the other hand, P.m.bivittatus has only been recorded as far north as approximately 26.5 degrees north latitude both in southeast China and in northern Myanmar, formerly Burma.
The average annual coolest month daily temps for the northern most range of P.m.molurus in Pakistan are interesting. The average coolest month temp range is 60 to 65 F as a daily high and about 40 F as a nighttime low. However, the pythons indigenous to this area have evolved to survive in these temperatures, are a separate species, and are in no way related to any burmese pythons in the U.S. When looking at the same data for P.m.bivittatus, the average annual coolest month daily temps for their northern most range are slightly higher. The average coolest month temp range for this species is 65 to 70 F as daily highs and about 45 to 50 F as extreme nighttime lows. Once again however, the python populations native to these northern latitude areas have evolved over hundreds of thousands of years in these cooler climates. I would say without hesitation that probably 99% of the burmese pythons in the U.S. today are from original stock taken from southeast asian latitudes where the average daily coolest month nighttime lows never drop below 60 or 65 F. They are not cool climate representatives of the species, and are not genetically adapted to cooler, yet survivable climates like their northern conspecifics.
The second issue is the obvious mapping error with regards to the northern extent to which the burmese pythons might be expected to move north out of Florida. This is where it was obvious that they took the easy way out and made little effort to try and extrapolate the geographical climate data from the natural range distribution of this species. No disrespect intended, and I know he is not fully responsible for how the study was completed, but I would completely disagree with the comment in Mr. Rodda’s letter that “a rational process was used to derive the climate envelope that was then mapped onto the US”. Furthermore, contrary to what the say on the USGS website, these new “climate match” range maps definitely do not “show where climate in the U.S. is similar to places in which Burmese pythons live naturally”. That is purely bad science. I do not know who constructed this map, but I do know exactly how they came up with it. It appears that all they did was take the most current weather data on the average annual monthly daily low temp for city’s across the U.S. that corresponded to about 32 F, and then overlay that line on a U.S. map. I downloaded the weather data for numerous cities’s across the U.S. and mapped it myself. My map was very close to theirs except in the Oklahoma area where my line went across the southern portion of that state instead of the northern area as in their map. Apparently the scientists working on this project are under the erroneous assumption that P.m.bivattatus can survive anywhere the average daily low winter temps don’t drop below freezing. That shows a complete lack of understanding regarding the autecology of this species.
When I mapped for survivability of P.m.molurus based on climate data alone from their northern most range in Pakistan, I had a maximum northern range line in the U.S. from Savannah, GA to Mobile, AL to Baton Rouge, LA to San Antonio, TX. However, this was based solely on average temperatures and did not take into account suitable available habitat throughout the range, and did not take into account unusually cool years where temps would still fall below lethal limits. When putting together this same map from data for the burmese python, the survivability line is much farther south into Florida. I personally do not believe from this data that the burmese python could maintain a long-term viable, self-sustaining population, anywhere north of an east-west line through about Sarasota. I also intend to contact Mr. Rodda and express my concerns about how the data was compiled and interpreted in their study.
Kelly
