Commentary on Reported Mammal Declines Coinciding with the Proliferation of Burmese Pythons in Everglades National Park
Florida Fish and Wildlife Conservation Commission
April 6, 2012
The paper by Dorcas et al. (2012. Severe mammal declines coincide with proliferation of invasive Burmese pythons in Everglades National Park. PNAS Early Edition, Accessed 1/31/2012, http://www.pnas.org/content/109/7/2418.full.pdf+html ) reported “severe apparent declines in mammal populations that coincide[d] temporally and spatially with the proliferation of Burmese pythons in Everglades National Park” (ENP) (p.1). They concluded their findings “suggest that predation by pythons has resulted in dramatic declines in mammals within ENP” (p.1). This review is to assess the extent to which these conclusions are supported by the data they provided.
Methods Documented in the Report
From February 1996 to January 1997, weekly systematic surveys of live and road-killed mammals were conducted in ENP by driving a 130-km route at night on Main Park Road and Research Road. A total of 6,599 km was driven in 51 nights. Again during the period 2003−2011, 56,971 km was surveyed on these roads. Data collected before 2000 were considered to be “pre” python invasion; data collected after 2002 were considered to be after pythons became common in ENP. Comparisons of mammal abundance between the 2 time periods were made by calculating the number of individuals of each species (2 rabbit, 2 fox, and rodent species were combined, respectively) observed per 100 km and determining the percent change in this statistic. In addition, the authors reported raw counts of road-killed mammals collected by park rangers from 1993−1999. Survey effort (number of observers, number of miles driven, roads driven, number of days, and procedures to avoid double counting), however, was not recorded for these surveys. The authors also conducted 4,794 km of surveys on 26 nights in 2009−2011 in 4 “peripheral” locations assumed to be recently colonized by pythons and where python numbers were presumed to be lower: Chekika area, Big Cypress National Preserve, Collier-Seminole State Park, and Florida City to Key Largo. They also included 1 night of survey data (278 km) from the Immokalee area and 9 nights of data (539 km driven by other researchers) from Corbett Wildlife Management Area; these “extralimital” locations were considered to be outside the known range of the python.
Limitations of the Data
- The authors report substantial percentage decreases in encounter rates between the single year of standardized pre-python observations (February 1996 to January 1997) and the 9 years of post-python data. Although the differences in encounter rates are striking, these data do not confirm declines in mammal populations. The single year of pre-python data may represent a period when mammal populations were high along roads, possibly due to high water levels forcing animals to higher ground. Indeed, analysis of water levels at five water stage gauges monitored by the South Florida Water Management District (DBHYDRO Browser, South Florida Water Management District, Accessed 2/22/2012, http://www.sfwmd.gov ) in proximity to Main Park Road (NP-P01, NP-P38, NP-P44, NP-P46, and NP-P72) indicate that average water levels were higher in 1995 than in any year between 2003 and 2009 (2010 and 2011data were not available). Thus, it is possible that the high mammal encounter rates the authors found in 1996 relative to 2003 to 2011were an artifact of the animals having moved to the elevated road bed in response to high water in 1995.
- Although the authors refer to “the apparent incredible density of pythons in ENP” (p.5.), no data are provided to document current densities or the timing of an increase. Data on number of pythons captured per year are provided, which show an exponential increase in captures beginning about 2001. Although the capture effort associated with these capture rates is not available, the apparent implication is that these increases in capture rates represent increases in python numbers. If this is the case, and the decrease in observations between 1996−1997 and 2003 (Table S1) reflect precipitous declines in mammal populations, then the declines occurred prior to the exponential growth of the python population. Whatever is the case, the timing of the python increase and mammal declines is not sufficiently documented to support a hypothesis of cause and effect. Furthermore, the authors do not explain how the python population could continue to increase dramatically after 2003 in the apparent absence of mammalian prey.
- No mention is made of the role that coyotes may have played in the decline in mammal populations. Coyote populations increased in South Florida over roughly the same period, and they are known predators of most of the species studied.
- The authors cite the evidence that mammal encounter rates in the peripheral areas were intermediate between those from the 1996−1997 and 2003−2011 ENP surveys, and that encounter rates in the extralimital areas were similar to historical rates in ENP, as supporting the hypothesis that pythons are responsible for differences in mammal abundance. It is equally plausible, however, that habitat differences between ENP and these other sites were responsible for the differences in mammal encounter rates.
- The authors cite the relatively low numbers of deer encountered in recent surveys at peripheral and extralimital sites (i.e., encountered at 2 of the 6 peripheral and extralimital sites) as evidence that factors other than pythons may have contributed to declines in deer populations in ENP. They ignore, however, the similarly low numbers of rabbits, bobcats, and foxes at the peripheral and extralimital sites (i.e., 0 rabbits and bobcats at 5 of the 6 sites and 0 foxes at 4 of the 6 sites) in suggesting pythons caused declines of these species in ENP.
- No attempt was made to correlate the mammal road-kill data with actual abundance or density of animals off the road, and the authors acknowledged this as a shortcoming of their data. Along much of its length, Main Park Road represents the most significant expanse of high ground in ENP. Its substrate is conducive to turtle nesting, which attracts egg predators. For this and other reasons, the road shoulder is a favored foraging site for raccoons and opossums, and python numbers are probably higher in the vicinity. Elsewhere in the park away from roads, mammal populations may not be impacted as much by a more dispersed python population.
Given the limitations in the data, it is difficult to conclude, as the authors did, that “the preponderance of evidence supports the hypothesis that pythons have severely reduced mammal populations within ENP” (p.3). The circumstantial evidence that declines in certain mammal species occurred in ENP over the last decade is compelling, however. The disparity in detection rates between 1996−1997 and 2001−2011, although potentially due to unique habitat characteristics during the 12-month initial survey period, is substantial. Although not proposed as evidence for a decline by the authors (probably and appropriately because of the lack of estimates of effort for the earlier surveys and apparent lack of provisions for avoiding double counting), the disparity in raw numbers of raccoons, opossums, and rabbits detected between the 1993−1999 and 2003−2011 periods is cause for concern. In addition, anecdotal reports support declines in rabbits and raccoons in ENP. Further investigation would be necessary to confirm low population numbers for mammals in ENP and determine whether they were due to hydrological changes, coyotes, disease, pythons, or a combination of factors.
As the authors note, this study, does show the need for baseline monitoring efforts of even common species in ENP to allow accurate assessment of temporal trends in wildlife populations resulting from invasive species, climate change, disease, hydrological management, or other factors.