Showing posts with label Python molurus. Show all posts
Showing posts with label Python molurus. Show all posts

Saturday, February 26, 2011

The Invasive Python Problem, Climate Modeling, and a Revised Map

The following is based upon Rodda et al. 2011. It is taken from their introduction and has been slightly edited for readability.It should be pointed out that Python molurus and Python bivittatus are now regarded as distinct species.

In 2008 the U.S. Fish and Wildlife Service (USFWS) solicited advice from the general public on the potential merits of restricting the importation of nine exotic species of giant constricting snakes. The intent was to reduce the risk of introducing the invasive species into the USA. The species list included the Indian Python (Python molurus) and what at the time was considered to be its subspecies. The best known of which is the Burmese Python, Python molurus bivittatus. At about the same time, Rodda et al. published results of an analysis of the areas of the U.S. that are climatically matched to the native range of the Indian Python. The publication of the map and the USFWS Notice of Inquiry were connected in the sense that USFWS had joined the U.S. National Park Service in funding the U.S. Geological Survey (USGS) study. Some sections of the public perceived the work as interagency collaboration in support of regulation of trade in giant constrictors, despite the fact that USGS had no policy position on invasive species regulation, and was under no pressure, either imposed by the funding sources or self-imposed, to support regulation, or bias the size or extent of the U.S. area that climatically matched the python’s native range. The climate match study was to inform the discussion. Pyron et al. (2008) countered with an alternate map showing areas of the U.S. that climatically matched the python’s native range; their map was embraced by opponents of regulation because it showed a much smaller area of climatic agreement – the area that could be inhabited by the pythons. And,  Pyron et al. concluded that ‘‘The Burmese python is strongly limited to the small area of suitable environmental conditions in the United States it currently inhabits…’’ They also averred, ‘‘The proposed expansion of the python into the continental United States would require an expansion of the actual tropical marshland habitat comprising most of the Everglades, not simply the presence of similar temperature and precipitation conditions.’’ If either of these claims were true, no further areas of the U.S. would be at risk of colonization, and regulation of U.S. trade in this species would be largely moot. Although Pyron et al. did not expressly tie their climate match to policy, they did lay claim to the policy high ground by asserting that, ‘‘The alarmist claims made by USGS could potentially hamper scientific discourse and inquiry into the problem, especially with regard to policy-making.’’ The notice of inquiry and subsequent proposed rulemaking generated a substantial public response, with a large number of comments received (55,600), and most of the criticism focused on the climate matching result for one of the nine species under consideration, the Indian Python. The intensity of the public’s reaction can be used to document climate matching can be a key element in establishing environmental policy. Also, differences among approaches to climate modeling are critical for evaluating the scientific basis for the policy. One element of this controversy is the herpetological facts that were the basis for the models. In these, Rodda et al. and Pyron et al. did not noticeably differ and the herpetological facts will not be discussed further. Another element of the controversy is the modeling approach, for which the two teams took divergent approaches: Rodda et al. adopted a climate suitability algorithm based on first principles, and Pyron et al. used a statistical tool to discover a climate suitability algorithm. Ideally, one would have some method for validating the projections, but there is no obvious way to validate the likelihood of a hypothetical event. Furthermore, the validity of these specific models might rest on factors unique to the Indian Python, and therefore be of limited interest.

Species distribution models are often used to characterize a species’ native range climate, so as to identify sites elsewhere in the world that may be climatically similar and therefore at risk of invasion by the species. Rodda et al. have evaluated a number of species to assess MaxEnt’s  (Maximum Entropy model) utility for vertebrate climate matching.

They found MaxEnt models to be very sensitive to modeling choices and selection of input localities and background regions. As used, MaxEnt invoked minimal protections against data dredging, multicollinearity of explanatory axes, and overfitting. As used, MaxEnt endeavored to identify a single ideal climate, whereas different climatic considerations may determine range boundaries in different parts of the native range. MaxEnt was extremely sensitive to both the choice of background locations for the python, and to selection of presence points: inclusion of just four erroneous localities was responsible for Pyron et al.’s conclusion that no additional portions of the U.S. mainland were at risk of python invasion. When used with default settings, MaxEnt overfit the realized climate space, identifying models with about 60 parameters, about five times the number of parameters justifiable when optimized on the basis of Akaike’s Information Criterion.

Rodda et al. (2011) concluded that when used with default settings, MaxEnt may not be an appropriate vehicle for identifying all sites at risk of colonization. Model instability and dearth of protections against overfitting, multi-collinearity, and data dredging may combine with a failure to distinguish fundamental from realized climate envelopes to produce models of limited utility. A priori identification of biologically realistic model structure, combined with computational protections against these statistical problems, may produce more robust models of invasion risk. The entire article can be found by the linked reference below.

Rodda, G. H., C. S. Jarnevich R. N. Reed. (2009, online 2008) What parts of the US mainland are climatically suitable for invasive alien pythons spreading from Everglades National Park? Biological Invasions 11: 241–252. 10.1007/s10530-008-9228-z.

Rodda G. H., C. S. Jarnevich, R. N. Reed. 2011. Challenges in Identifying Sites Climatically Matched to the Native Ranges of Animal Invaders. PLoS ONE 6(2): e14670. doi:10.1371/journal.pone.0014670

Pyron R.A., F. T. Burbrink, 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. 10.1371/journal.pone.0002931.