(1)Department of Biology, Palm Beach Atlantic University, West Palm Beach, FL 33401 ... and outside of the python's southern Florida range, in similar habitats, ...
AN EFFECT OF BURMESE PYTHONS (PYTHON MOLURUS BIVITTATUS) ON MAMMAL POPULATIONS IN SOUTHERN FLORIDA JOSHUA HOLBROOK(1) (1)
AND
THOMAS CHESNES(1)
Department of Biology, Palm Beach Atlantic University, West Palm Beach, FL 33401
Biological Sciences
AN EFFECT OF BURMESE PYTHONS (PYTHON MOLURUS BIVITTATUS) ON MAMMAL POPULATIONS IN SOUTHERN FLORIDA JOSHUA HOLBROOK(1)
AND
THOMAS CHESNES(1)
(1)
Department of Biology, Palm Beach Atlantic University, West Palm Beach, FL 33401
ABSTRACT: The invasive Burmese python has been firmly established in southern Florida for several years, has undergone a population explosion, and is spreading northward. Unanswered questions include the python’s effect on native biota, and what future effect can be expected in areas annexed into the python’s range. To address these questions, mammal counts were conducted inside and outside of the python’s southern Florida range, in similar habitats, to determine their effect on mammal species. Samples revealed far fewer mammals within Everglades National Park, especially small mammals which are a suitable size for consumption for most of a python’s life.
Key Words: Python molurus bivittatus, Everglades, mammals, trophic effects, herpetology SOUTHERN Florida is plagued by one of its best attributes: it is a warm, subtropical climate which seldom freezes and, despite heavy development, still maintains many thousands of acres of adequate habitat for plants and animals. However, these factors that support a great diversity of native species have also served as a foothold for a number of invasive species which cost millions of dollars to attempt (many times unsuccessfully) to control (Ferriter et al., 2008). One such species which has gained much attention in the past few years is the Burmese python, Python molurus bivittatus. One of the problems which has arisen is managing the python population, for which no well-supported scientific population estimate exists. However, well over 1,000 pythons have been removed from southern Florida to date and yet they remain extremely common (Harvey et al., 2008). Current estimates suggest a southern Florida population in the tens of thousands of individuals (FFWCC, 2009). Burmese pythons are native to Southeast Asia, can attain a maximum length of over 20 feet (though none this size have yet been encountered in Florida), and are capable of producing clutches from 8 to 107 eggs, with the average clutch being 60–80 eggs (Harvey et al., 2008). Nests have been recorded in the Everglades since 2006 (Harvey et al., 2008). They are a subspecies of the Indian python (P. molurus), a species which ranges across much of the Asian continent. The subspecies bivittatus ranges from the southwestern Chinese coast east to Bangladesh, with disjunct populations in India, between 30 degrees north latitude and 10 degrees south latitude. In their native range, Burmese pythons primarily inhabit tropical lowlands, mangroves, and other tropical and subtropical habitats up to 1200 meters. The single most limiting 17
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factor for the pythons seems to be their dependence on wetlands and bodies of water (Barker and Barker, 2008). The initial site of introduction of the pythons in Florida seems to have been Everglades National Park (ENP) (Collins et al., 2008), and the majority of python sightings occur inside ENP. However, reported sightings at and north of Tamiami Trail are becoming increasingly common (especially on internet forums where reports of field trips from recreational reptile enthusiasts are posted, such as www.Fieldherpforum.com). ENP is a bastion of mostly undisturbed habitat – approximately 600,000 hectares - and is home to 76 mammal species and over three hundred taxa of birds (Brown et al., 2006). Many of these species are actual or potential prey items for the pythons (Ferriter et al., 2008; Snow et al., 2007). This study attempts to detect the possible effects of the pythons on southern Florida mammal populations, with hopes that future studies will be conducted on the pythons’ effects on native species. MATERIALS AND METHODS—Study Sites—Because the pythons were likely first introduced into Everglades National Park, it was selected as the area that will likely have suffered the most ecological impact. ENP consists primarily of freshwater marsh with tree islands, but also tropical hardwood hammock, pinelands, cypress swamp, mangrove swamp, and coastal prairie (Lodge, 2005). As a comparison, J.W. Corbett Wildlife Management Area (CWMA) (Figure 1) was selected since it is ecologically similar to ENP, yet well outside of the range and impact of the main Everglades python population. CWMA is a 60,000 acre wildlife management area managed by the Florida Fish and Wildlife Conservation Commission (FFWCC) and like ENP consists primarily of freshwater marsh with tree islands, pinelands, cypress swamp, and tropical hardwood hammock. Both areas have a main road with side roads allowing access to many of the parks’ plant communities. Sampling Protocol—Mammals were sampled using roadside counts on the main roads and tributaries of the two field sites. Road sampling has been effectively used as an index of abundance for numerous organisms including birds (Hanowski and Niemi, 1995; Keller and Scallan, 1999), mammals (Kline, 1965; Seagle and Close, 1995), and termites (Pomeroy, 1977). It was an efficient way to view both mammals and Burmese pythons and also allowed the investigators to cover many miles through a variety of habitat types in relatively little time. Mammals cross roads on a regular basis (Beier et al., 2008) often leading to road mortality; however, no data were collected on deceased pythons or mammals. Road sampling involved driving the selected roads at a relatively constant speed (65 kph in ENP, 50 kph in CWMA) beginning within an hour of sunset until 2–5 hours past sunset. This time frame was used because it tends to be an optimal time for wildlife viewing across many species (Blem and Kelleen, 1993; Perelberg et al., 2003). Encountered pythons were to be collected, measured, and per condition of our permit were given to National Park Service officials for further study. Nine surveys were completed in each area between February and July 2009 on evenings when daytime temperature was at or in excess of 23uC. Data analysis was conducted using SPSS statistical software.
RESULTS—Nine individuals representing three mammal species were counted over the nine surveys in Everglades National Park. On five of the ENP surveys, no mammals were encountered. Six pythons were collected in total, all of them in ENP (Table 1). In Corbett WMA, 40 individual mammals representing seven species were counted (Table 2). No pythons were encountered at Corbett WMA.
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FIG. 1. Location of the Corbett Wildlife Management Area (CWMA) and Everglades National Park (ENP). Dashed lines represent the estimate established population ranges of the Burmese python, as of mid 2009, according to www.EvergladesCISMA.org, a database of captured exotic species.
A Mann-Whitney-Wilcoxon test was conducted to evaluate the hypothesis that there were fewer mammals encountered per kilometer in Everglades National Park, on the average, than in Corbett Wildlife Management Area. The results of the test were in the expected direction and significant (z 5 23.62, p , 0.001). DISCUSSION—Any evening drive in Florida is nearly always interspersed with the presence of mammals crossing the road – not only in urban areas, but also in the sparsely inhabited rural areas of Florida. Within the Burmese python’s Florida range though, it seems that the sighting of any mammal (especially small mammals) is a rare occurrence. This is surprising since only a few years ago seeing mammals was common in ENP, and small mammals such as raccoons (Procyon lotor) were numerous (Brown et al., 2006; Lodge, 2005). No P. lotor were encountered in the nine surveys in ENP (Table 2). The lack of P. lotor sightings where they were once common could potentially affect reptile populations in a positive way, as P. lotor is known to prey upon reptile egg clutches on a frequent basis (Marchand et al, 2002; Davis and Whitting, 1977) and reptiles do not constitute a large portion of an Everglades python’s diet (Snow et al., 2007).
Km
257.5 450.6 321.9 289.7 321.9 354.1 321.9 450.6 321.9 Species Total
Date
09-Mar 27-Mar 04-Apr 09-Apr 13-Apr 09-May 28-May 04-Jul 07-Jul
0 0 1 0 0 0 0 0 0 1
Didelphis virginiana 0 0 0 0 0 0 0 0 0 0
Urocyon cineroargenteus 0 0 0 0 0 0 0 0 0 0
Dasypus novemcinctus 0 0 0 0 0 0 0 0 0 0
Procyon lotor 0 0 0 0 0 0 0 0 0 0
3 3 0 0 0 0 0 1 0 7
0 1 0 0 0 0 0 0 0 1
Felis Odocoileus Small domesticus virginianus Unidentified
Everglades National Park
3 4 1 0 0 0 0 1 0 9
Total Ind
0.012 0.009 0.003 0.000 0.000 0.000 0.000 0.002 0.000
Mammals/ Km
0 1 (4.4m) 0 0 0 1 (3.7m) 1 (1.5m) 1 (1.7m) 2 (1.5, 3.2m) 6
Python molurus bivittatus
TABLE 1. Summary of sampled mammals and Burmese pythons by roadside counts in Everglades National Park. An estimate of the relative number of mammals encountered per kilometer is given. Captured python lengths are in parenthesis.
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Km
96.6 96.6 72.4 96.6 96.6 72.4 36.2 96.6 72.4 Species Total
Date
26-Feb 06-Mar 25-Mar 30-Mar 16-Apr 18-Apr 24-Apr 10-May 21-Jun
1 1 0 3 1 3 1 1 1 12
Didelphis virginiana 1 0 0 0 0 0 0 0 0 1
Urocyon cineroargenteus 0 1 0 0 0 0 0 0 0 1
0 0 2 2 4 3 5 3 2 21
0 0 0 0 0 1 1 0 0 2
Dasypus Procyon Felis novemcinctus lotor domesticus 1 0 0 0 0 0 0 0 0 1
Odocoileus virginianus
Corbett Wildlife Management Area
1 0 0 0 0 0 0 1 0 2
Small Unidentified
4 2 2 5 5 7 7 5 3 40
Total Ind
0.041 0.021 0.028 0.052 0.052 0.097 0.193 0.052 0.041
Mammals/ Km
0 0 0 0 0 0 0 0 0 0
Python molurus bivittatus
TABLE 2. Summary of sampled mammals and Burmese pythons by roadside counts in Corbett Wildlife Management Area. An estimate of the relative number of mammals encountered per kilometer is given.
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Other possibilities for a lack of mammal sightings, including predation by native predators, were investigated but the historical presence of an abundance of small mammal species suggest that something other than predation by long coexisting native species is occuring. Additionally, disease was investigated as a possible cause for low mammal counts within ENP, but no record has been found of any epizootic capable of decreasing mammal numbers. However, with increasing python sightings and population estimates, it is extremely possible and likely that mammal population reductions could be due to python predation. The same may hold true for bird and some reptile species within the python’s range, though they were not included in this study. Future studies should incorporate such bird and reptile counts, as pythons have been known to prey upon both in Florida (Ferriter et al., 2008). Another noticeable difference involved whitetail deer (Odocoileus virginianus): numbers in ENP were well above the number encountered in CWMA (Tables 1 and 2). It is likely, however, that the lack of O. virginianus in Corbett WMA is be due to the fact that mammal counts were performed at a time of year when large mammal populations (deer and hogs) are under heavy pressure from hunting. This is not a significant factor in ENP where hunting is not permitted. At all sizes, the pythons seem to be based around water sources and readily utilize them as a highway through terrestrial habitats (Barker and Barker, 2008), and Florida seems perfect for pythons in this respect. Perhaps more troubling to consider is what will happen to the python population should it reach the borders of the Everglades Agricultural Area (EAA) directly south and west of Lake Okeechobee – The EAA contains thousands of acres of disturbed agricultural habitat filled with an enormous concentration of ruderal rodent species, as well as an intricate network of canals and rivers radiating out through the peninsula. The python’s future range in the United States is uncertain – It has been suggested that the P. molurus bivittatus subspecies is more temperature tolerant than traditionally thought, and one-third of the United States is suitable based on temperature and rainfall data – from Florida north to Washington D.C. and west to the San Francisco Bay (Rodda et al., 2008). Attention should not only be given to the northern range potential but also the south – pythons have been captured as far south as Key Largo, where they have been observed to consume the critically endangered Key Largo woodrat (Neotoma floridana smalli) (Harvey et al., 2007). They could potentially be a threat to the endangered key deer (Odocoileus virginianus clavium) if their range expands south to Big Pine Key. CONCLUSION—The fact that large constrictors are established and, indeed, common in Florida still surprises those who encounter them on a regular basis – even the largest and most impressive native Florida snakes such as the eastern indigo snake (Drymarchon corais couperi) and the eastern diamondback rattlesnake (Crotalus adamanteus) are dwarfed by immature pythons. Pythons constitute a threat that needs to be addressed in Florida, and even greater
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efforts need to be made to control the species. Though more study is needed, it is extremely likely based on their projected population size and the amount of food required to sustain such a population, that the pythons are indeed affecting the southern Florida ecosystem in measurable ways. Much research is needed to develop management strategies to contain the pythons, because at this point outright eradication is improbable. Education is also key for managing the pythons, and also preventing future releases and the establishment of additional detrimental species. A number of species of concern as well as threatened and endangered species have been found in the stomachs of pythons, including wood storks (Mycteria Americana), American alligators (Alligator mississippianus), limpkins (Aramus guarauna), and white ibises (Eudocimus albus) (Harvey et al., 2008). Extra effort must be put into protecting such species as well as others such as the Florida panther (Puma concolor coryi) and the American crocodile (Crocodylus acutus) which also may be threatened by the Everglades newfound predator. ACKNOWLEDGMENTS—This project was partially funded by a Palm Beach Atlantic University Quality Initiative research grant. R. Holbrook, J. Thullbery, and D. Filipiak assisted in data collection. J. Swick, G. Goss, and R. Waldner provided valuable input to the manuscript.
LITERATURE CITED BARKER, D. G. AND T. M. BARKER. 2008. The Distribution of the Burmese python, Python molurus bivittatus. Bull. Chicago Herp. Soc. 43(3):33–38. BEIER, P., D. MAJKA, S. NEWELL, AND E. GARDING. 2008. Best management practices for wildlife corridors (updated June 27 2008). http://www.corridordesign.org. Accessed: September 6, 2009. BLEM, C. R. AND K. B. KILLEEN. 1993. Circadian metabolic cycles in eastern cottonmouths and brown water snakes. J. Herpetology. 27(3):341–344. BROWN, M. T., M. J. COHEN, E. BARDI, AND W. W. INGWERSEN. 2006. Species diversity in the Florida Everglades, USA: A systems approach to calculating biodiversity. Aq. Sci. 68(2006):254–277. COLLINS, T. M., B. FREEMAN, AND S. SNOW. 2008. Genetic characterization of populations of the nonindigenous Burmese python in Everglades National Park. South Florida Water Management District Final Report. DAVIS, G. E. AND M. C. WHITING. 1977. Loggerhead Sea Turtle Nesting in Everglades National Park, Florida, USA. Herpetologica. 33(1):18–28. FERRITER, A., B. DOREN, R. WINSTON, D. THAYER, B. MILLER, B. THOMAS, M. BARRETT, T. PERNAS, S. HARDIN, J. LANE, M. KOBZA, D. SCHMITZ, M. BODLE, L. TOTH, L. RODGERS, P. PRATT, S. SNOW, AND C. GOODYEAR. 2008. The Status of nonindigenous species in the South Florida environment. South Florida Environmental Report. South Florida Water management District. FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION (FFWCC). 2009. Burmese Python Removal Program (updated September 2, 2009). http://myfwc.com/newsroom/resources/ news_resources_pythonpermitFAQs.htm. Accessed September 6, 2009. HARVEY, R. G., M. L. BRIEN, M. S. CHERKISS, M. DORCAS, M. ROCHFORD, R. W. SNOW, AND F. J. MAZZOTI. 2008. Burmese pythons in South Florida: scientific support for invasive species management. University of Florida Institute of Food and Agricultural Sciences. HANOWSKI, J. M. AND G. T. NIEMI. 1995. A Comparison of on- and off-Road Bird Counts: Do You Need to go off Road to Count Birds Accurately? J. of Field Orin. 66(4):469–483.
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KELLER, C. M. E. AND J. T. SCALLAN. 1999. Potential Roadside Biases due to Habitat Changes along Breeding Bird Survey Routes. The Condor. 101(1):50–57. KLINE, P. D. 1965. Factors Influencing Roadside Counts of Cottontails. J. Wildlife Man. 29(4):665–671. LODGE, T. E. 2005. The Everglades Handbook: Understanding the Ecosystem, Second Edition. CRC Press, Boca Raton, FL. MARCHAND, M., J. LITVAITIS, T. MAIER, AND R. DEGRAAF. 2002. Use of Artificial Nests to Investigate Predation on Freshwater Turtle Nests. Wildlife Soc. Bull. 30(4):1092–1098. PERELBERG, A., D. SALTZ, S. BAR-DAVID, A. DOLEV, AND Y. YOM-TOV. 2003. Circadian Changes in the Home Ranges of Reintroduced Persian Fallow Deer. J. Wildlife Man. 67(3):485–492. POMEROY, D. E. 1977. The Distribution and Abundance of Large Termite Mounds in Uganda. J. App Ecol. 14(2):465–475. RODDA, G. H., C. S. JARNEVICH, AND R. N. REED. 2008. What parts of the US mainland are climatically suitable for invasive alien pythons spreading from Everglades National Park? Biol. Invasions. 11(2):241–252. SEAGLE, S. W. AND J. D. CLOSE. 1996. Modeling White Tailed Deer, Odocoileus virginianus, Population Control by Contraception. Biol. Cons. 76:87–91. SNOW, R. W., M. L. BRIEN, M. S. CHERKISS, L. WILKENS, AND F. J. MAZZOTTI. 2007. Dietary Habits of the Burmese Python, Python molurus bivittatus, in Everglades National Park, Florida. Herp. Bull. 101:5–7.
Florida Scient. 74(1): 17–24. 2011 Accepted: December 17, 2009 Florida Academy of Sciences. 2011
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