Wednesday, January 12, 2011

Thoughts on Invasive Snakes

The USA has more than 160 invasive species of vertebrates (Witmer et al., 2007). Rarely do you see people complaining about feral horses, starlings, or bullfrogs. But, the level of emotion involved in discussing invasive giant snakes during 2010 rivals rhetoric directed at the reintroduction of wolves to Yellowstone National Park. Giant snakes and wolves are both apex predators capable of killing humans and their domesticated species as well as a reminder that humans are not on the top of the food chain.

The USGS’ report on invasive giant constricting snakes by Reed and Rodda was released in October of 2009. Rotella et al. (2010) studied the public reaction to the release on the Internet, using news stories based on the report for one month. They analyzed the news items in terms of key points and interpretational errors. Key points of the original publication were identified by consensus and then tallied through critical reading of the news items. Errors in the news items were categorized as: factual errors, exaggerations, conclusions beyond the data, and political.

Rotella and colleagues results revealed 26 unique news stories produced by 11 syndicated news agencies, and 15 blog posts, newsletters, or as items released by net forums for special interest groups. The articles had a mean word count of 621 (the original report was 302 pages). The team identified the key points as: threat to humans, invasion range, effects of climate change, risk assessment, pet release, removal/capture, reproduction/growth, rights of pet owners, biology in the native habitat, and extinction risk for competing species. The key points mentioned most frequently in the news items were: threat to humans, snake removal/capture, reproduction/growth, and pet release. Nineteen of the 26 news items (73%) had errors. The most frequent errors were exaggerations (13 items), unsubstantiated conclusions (11 items), and factual errors (10 items). Two of the news items included parodies based on the original publication.

Other criticism of the report comes from David and Tracy Barker (2010a, b), who run VPI, a python breeding facility in Texas. They argue for less regulation on the captive snake breeding industry and criticize the report on a variety of levels which range from style and grammar to methodology and conclusions drawn from the study.

Python bivittatus, JCM
Some of the problem with the USGS report and its critics rests in the lack of knowledge and confusion concerning the systematics of the Python molurus complex. It is now relatively clear that the Indian Python, P. molurus is a peninsular Indian species, while the Burmese Python (P. bivittatus) is a Myanmar- Indochinese species. They undoubtedly shared a common ancestor, and are thus morphologically similar, but are in all likelihood distinct species (despite the fact they can reproduce with each other). See Jacobs et al. 2009. The failure to recognize this in the USGS report is understandable give the morphological similarity of the species and the fact that large snakes are poorly studied in their native countries. Much of the literature on these species is a tangled mess that results from writers reworking what others have previously said about the natural history of large snakes. Murphy and Henderson (1997) attempted to unravel part of this by using direct quotes from earlier literature, but even this was only partially successful. Biological field work for western researchers is virtually impossible in countries like Myanmar (Burma). Therefore, advancing knowledge about these snakes is left to observation of captive specimens and the few field studies that do get done within the snake's natural distribution.

Predicting the future is always difficult. The climate modeling work done by Pyron et al, (2008) suggests that pythons won’t be able to expand their distribution much beyond what it is today during the course of this century. And, the study done by Dorcas et al. (2010) would seem to support this – although nobody is talking about sample size. The special interests have latched on to this study in an attempt to find support for their view point.

The well documented ecological disaster brought about by the Brown Treesnake in Guam is certainly a wake-up call to prevent invasive snakes from eating their way through the native fauna of southern Florida or any other place they might survive and thrive.

As for giant snakes surviving and spreading –natural selection will be finding those individuals that can withstand cold temperatures (assuming a small percentage can - it may be only a fraction of 1%). Those individuals will be leaving more offspring in the next generation, so as time goes on it seem probable that the Florida populations of Python bivittatus and Python sebae, as well as the Boa constrictor will be adapting to cold snaps. Any reduction in the Florida giant snake populations due to die off from cold weather in southern Florida is at best temporary and offers only a short reprieve to the native vertebrate fauna.

From my perspective the government should be working to minimize invasive species and protect the native fauna - an important natural resource. Perhaps the snake breeding industry should be working on developing designer snake morphs that self-destruct when they escape or are released from captivity. Or, perhaps more realistically sterile designer snake morphs should be produced. So, escapees and released pets at least cannot reproduce.

As for the danger of snakes to humans, in the USA, it pales in comparison to deaths from auto accidents or firearms. In the United States there are more than 250 million privately owned firearms and that the number increases by about 4 million per year. The CDC estimates 75,000 annual human deaths from firearms. Therefore, outrage or even loud concern about snakes that kill less than a dozen people per year seems a bit over the top. Not that the loss of human life should not be a concern, only that statically being killed and or eaten by a giant snake in the USA is a non-issue.


Literature
Barker, D. G. and T. M. Barker. 2010a. A critique of the analysis used to predict the climate space of the Burmese Python in the United Snakes by Rodda et al. (2008, 2009) and Reed and Rodda (2009). Bulletin of the Chicago Herpetological Society 45(6):97-106.

Barker, D. G. and T. M. Barker. 2010b. The Tympanum. Bulletin of the Chicago Herpetological Society 45(9):144-149. Bulletin of the Chicago Herpetological Society 45(12):97-106.

Barker, D. G. and T. M. Barker. 2010c. A review of: Dorcas, M. E., J. D. Willson, and J. WE. Gibbons. 2010. Can Invasive Burmese Pythons Inhabit Temperate Regions of the Southeastern United States? Biological Invasions. Online at doi10.107/s10530-010-9869-6. Bulletin of the Chicago Herpetological Society 45(12):187-189..

Beschta, R. L., and Ripple, W. J. 2009. Large predators and trophic cascades in terrestrial ecosystems of the western United States. Biological Conservation 142, 2009: 2401-2414.

Jacobs, H. J., M. Auliya and W. B√∂hme. 2009. Zur Taxonomie des Dunklen Tigerpythons, Python molurus bivittatus KUHL, 1820, speziell der Population von Sulawesi. Sauria, Berlin, 2009, 31 (3): 5–16.
  

Murphy, J. C. and R. W. Henderson. 1997. Tales of Giant Snakes. Krieger Publishing, Malabar, Florida.

Pyron, R. A., F. T. Burbrink, and T. J. Guiher. 2008. Claims of potential expansion throughout the U.S. by invasive python species are contradicted by ecological niche models. PLoS ONE 3:e2931 [doi:10.1371/journal.pone.0002931]

Reed, R. N. and G. H. Rodda. 2009. Giant constrictors: biological and management profiles and an establishment risk assessment for nine large species of pythons, anacondas, and the boa constrictor: U.S. Geological Survey Open-File Report 2009-1202, Washington, D.C., USA.

Rotella, A. R., R. A. Connelly, M. D. Marsh, C. C. Wessel, J. W. Murphy, L. L. Canton, and J. O. Luken. 2010. Snake Invasion: Evaluation of an Online News Frenzy. Bulletin of the Ecological Society of America 91:438–441. [doi:10.1890/0012-9623-91.4.438]

Witmer, G. W. et al. 2007. Management of invasive vertebrates in the United States, An Overview. USDA National Wildlife Research Center Symposia. 12 pages. http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1055&context=nwrcinvasive

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