Environmental temperatures, physiology and behavior limit the range expansion of invasive Burmese pythons in southeastern USA

Abstract

A well-established population of Burmese pythons resides in the Everglades of southern Florida. Prompted in part by a report that identified much of southern USA as suitable habitat for expansion or establishment of the Burmese python, we examined the plausibility of this snake to survive winters at sites north of the Everglades. We integrated daily low and high temperatures recorded from October to February from 2005-2011 at Homestead, Orlando and Gainesville, Florida; and Aiken, South Carolina, with minimum temperatures projected for python digestion (16 °C), activity (5 °C) and survival (0 °C). Mean low and high temperatures decreased northward from Homestead to Aiken and the number of days of freezing temperatures increased northward. Digestion was impaired or inhibited for 2 months in the Everglades and up to at least 5 months in Aiken, and activity was increasingly limited northward during these months. Reports of overwinter survivorship document that a single bout of low and freezing temperatures results in python death. The capacity for Burmese pythons to successfully overwinter in more temperate regions of the USA is seemingly prohibited because they lack the behaviors to seek refuge from, and the physiology to tolerate, cold temperatures. As tropical Southeast Asia is the source of the Everglades Burmese pythons, we predict it is unlikely that they will be able to successfully expand to or colonize more temperate areas of Florida and adjoining states due to their lack of behavioral and physiological traits to seek refuge from cold temperatures.

Weighing empirical and hypothetical evidence for assessing potential invasive species range limits: a review of the case of Burmese pythons in the USA

“…Considering that reptile species richness across the world can be mostly explained by temperature (Qian 2010), thermal tolerance information would be valuable for assessing range expansion potential. The critical thermal minimum, the low temperature where activity is impaired sufficiently to make survival unlikely, is especially important in this context (Avery 1982;Cowles and Bogert 1944;Huey 1982;Jacobson et al 2012). Combined with physiological thermal tolerances, survival of a reptile depends on its behavioral flexibility in the face of thermal challenges.…”

“…For the Burmese python to expand its range outside South Florida, the species must be able to tolerate or behaviorally protect itself from increasingly cold temperatures further north along the Florida peninsula and beyond into the rest of the USA mainland. Addressing the cooler temperatures may well pose a serious behavioral test for the species, because thermoregulation is relatively passive in the tropical and subtropical regions of Southeast Asia where the Burmese python is native, with ambient temperatures generally stable and supportive of normal activity throughout the year (Avery 1982;Huey 1982;Jacobson et al 2012;Shine and Madsen 1996).…”

“…Thus, it is essential to accurately define the appropriate native climate data to use in range predictions. Different models have been used to infer possible limits of Burmese python distribution in the continental USA and have produced widely divergent range predictions (Jacobson et al 2012;Pyron et al 2008;Rodda et al 2009;Van Wilgen et al 2009). …”

“…In contrast to strictly hypothetical climate modeling approaches, Jacobson et al (2012) analyzed the question of cold tolerance and potential range expansion by integrating multiple hypothetical and empirical factors: Assimilating all factors of physiological and behavioral function with historical norms and extremes in current and possible colonization localities, they concluded that it is unlikely that Burmese pythons would be able to expand to or colonize areas beyond their current range in South Florida.…”

“…Various range predictions for the Burmese python in the USA have been far from concordant (Jacobson et al 2012;Pyron et al 2008;Rodda et al 2009;Van Wilgen et al 2009). Here, we use the case of the Burmese python in Florida to examine issues concerning disparities among modeling results and between model predictions and available empirical information as well as concepts for making cogent choices in model development and for deciding among alternative range predictions.…”