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Cold-induced mortality of invasive Burmese pythons in south Florida

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A recent record cold spell in southern Florida (2–11 January 2010) provided an opportunity to evaluate responses of an established population of Burmese pythons (Python molurus bivittatus) to a prolonged period of unusually cold weather. We observed behavior, characterized thermal biology, determined fate of radio-telemetered (n=10) and non-telemetered (n=104) Burmese pythons, and analyzed habitat and environmental conditions experienced by pythons during and after a historic cold spell. Telemetered pythons had been implanted with radio-transmitters and temperature-recording data loggers prior to the cold snap. Only one of 10 telemetered pythons survived the cold snap, whereas 59 of 99 (60%) non-telemetered pythons for which we determined fate survived. Body temperatures of eight dead telemetered pythons fluctuated regularly prior to 9 January 2010, then declined substantially during the cold period (9–11 January) and exhibited no further evidence of active thermoregulation indicating they were likely dead. Unusually cold temperatures in January 2010 were clearly associated with mortality of Burmese pythons in the Everglades. Some radio-telemetered pythons appeared to exhibit maladaptive behavior during the cold spell, including attempting to bask instead of retreating to sheltered refugia. We discuss implications of our findings for persistence and spread of introduced Burmese pythons in the United States and for maximizing their rate of removal. Keywords Python molurus -Florida Everglades-Cold temperatures-Invasive species-Mortality-Thermoregulation
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ORIGINAL PAPER
Cold-induced mortality of invasive Burmese pythons
in south Florida
Frank J. Mazzotti
Michael S. Cherkiss
Kristen M. Hart
Ray W. Snow
Michael R. Rochford
Michael E. Dorcas
Robert N. Reed
Received: 17 March 2010 / Accepted: 1 June 2010 / Published online: 15 June 2010
Ó The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract A recent record cold spell in southern
Florida (2–11 January 2010) provided an opportunity
to evaluate responses of an established population of
Burmese pythons (Python molurus bivittatus)toa
prolonged period of unusually cold weather. We
observed behavior, characterized thermal biology,
determined fate of radio-telemetered (n = 10) and
non-telemetered (n = 104) Burmese pythons, and
analyzed habitat and environmental conditions expe-
rienced by pythons during and after a historic cold
spell. Telemetered pythons had been implanted with
radio-transmitters and temperature-recording data
loggers prior to the cold snap. Only one of 10
telemetered pythons survived the cold snap, whereas
59 of 99 (60%) non-telemetered pythons for which we
determined fate survived. Body temperatures of eight
dead telemetered pythons fluctuated regularly prior to
9 January 2010, then declined substantially during the
cold period (9–11 January) and exhibited no further
evidence of active thermoregulation indicating they
were likely dead. Unusually cold temperatures in
January 2010 were clearly associated with mortality of
Burmese pythons in the Everglades. Some radio-
telemetered pythons appeared to exhibit maladaptive
behavior during the cold spell, including attempting to
bask instead of retreating to sheltered refugia. We
discuss implications of our findings for persistence and
spread of introduced Burmese pythons in the United
States and for maximizing their rate of removal.
Keywords Python molurus Florida Everglades
Cold temperatures Invasive species
Mortality Thermoregulation
Introduction
Invasive alien reptiles present an increasing challenge
to conservation of biological diversity (Wilcove et al.
1998; Kraus 2009). The United States dominates the
world trade in live reptiles (Hoover 1998; Franke
and Telecky 2001), and the state of Florida alone
currently hosts more established alien reptiles than
any other state or nation (Meshaka et al. 2004; Kraus
F. J. Mazzotti (&) M. S. Cherkiss M. R. Rochford
Ft. Lauderdale Research and Education Center, University
of Florida, 3205 College Ave., Davie, FL 33314, USA
e-mail: fjma@ufl.edu
K. M. Hart
US Geological Survey, Southeast Ecological Science
Center, 3205 College Ave., Davie, FL 33314, USA
R. W. Snow
National Park Service, Everglades National Park,
40001 State Road 9336, Homestead, FL 33034, USA
M. E. Dorcas
Department of Biology, Davidson College, P.O. Box
7118, Davidson, NC 28035, USA
R. N. Reed
US Geological Survey, Fort Collins Science Center,
2150 Centre Ave., Fort Collins, CO 80526, USA
123
Biol Invasions (2011) 13:143–151
DOI 10.1007/s10530-010-9797-5
2009). Many of the more than 40 exotic reptile
species established in Florida are confined to urban or
otherwise manmade habitats such as backyards and
canals; however, several species (e.g., Burmese
python, Python molurus bivittatus) have successfully
invaded natural habitats. The Burmese python is
native to Southeast Asia and is established in natural
areas in southern Florida such as Everglades National
Park (ENP) (Snow et al. 2007a).
Burmese pythons are habitat and dietary general-
ists (Reed and Rodda 2009). Considerable concern
has been expressed over their impacts on south
Florida ecosystems, particularly in ENP, where
Burmese pythons consume primarily birds and
mammals (Snow et al. 2007b). Ecological and
economic impacts of invasive animals such as
pythons depend on types and magnitude of impacts
(e.g., predation on or competition with native species)
and geographic extent of invasion (Pimentel et al.
2005; Kraus 2009).
Although climate is often proposed as a primary
factor limiting potential geographic extent of invad-
ing species, predicting potential range of an invasive
species is difficult because of a poor understanding of
predictors of invasive ranges (Hayes and Barry 2008),
observations that native range climate may under-
predict invasive range distribution (Fitzpatrick et al.
2007; Duncan et al. 2009), and methodological or
statistical uncertainties (Randin et al. 2006; Beau-
mont et al. 2009; Phillips et al. 2009; Reed and Rodda
2009). As an example of the latter, several attempts
have been made to use native-range climatic vari-
ables to predict potential distribution of Burmese
pythons in the United States, but results have been
inconsistent and even contradictory (Pyron et al.
2008; Rodda et al. 2009; van Wilgen et al. 2009).
The native range of Burmese pythons extends
from tropical zones in Southeast Asia (including
Vietnam, Cambodia, Laos, and Thailand) to warm
temperate zones in China and Nepal (Groombridge
and Luxmoore 1991; Zhao and Adler 1993; Whitaker
and Captain 2004). Winter temperatures may exert
some influence over northern range limits of Burmese
pythons. However, little is known about thermal
physiology and thermoregulatory behavior of pythons
in native habitats (Alexander 2007; Reed and Rodda
2009). A recent record cold spell in southern Florida
(NOAA 2010) provided an opportunity to evaluate
responses of an existing Florida population of
Burmese pythons to a prolonged period of unusually
cold weather.
During 2–11 January 2010, south Florida experi-
enced record cold temperatures (NOAA 2010). During
9–11 January, air temperatures remained at or below
10°C for at least 48 h. A low of 1.6° C on the morning
of 10 January tied an all-time record low for Miami.
Record low maximum air temperatures (9°C) were
recorded in Miami and Naples on 10 January 2010
(NOAA 2010). On the morning of 11 January,
monitoring stations across south Florida recorded air
temperatures ranging from
-4to0°C. West Palm
Beach and Miami both set record lows (0.5 and 2.2°C,
respectively) for 11 January. Between 2 and 11
January, West Palm Beach and Naples set records
for number of days (10) with air temperature lows at or
below 7.2°C (NOAA 2010). A combination of dura-
tion and extremes of this historic cold period resulted
in extensive press coverage of mortality of native and
alien wildlife including manatees, sea turtles, croco-
diles, numerous species of fish, iguanas, and pythons
(Fantz 2010; Quinlan 2010; Waters 2010).
Our objectives were to report on behavior, thermal
biology, and fate of telemetered pythons (n = 10),
fate of non-telemetered pythons (n = 104), and
habitat and environmental conditions experienced
by pythons during and immediately after the historic
cold spell in southern Florida.
Methods
Adult Burmese pythons used for telemetry (n = 10)
were obtained from ENP. For each python we
measured snout-vent length (SVL), total length (TL),
tail girth, and mass at the time of transmitter implan-
tation. Each python was implanted intraperitoneally
(Reinert and Cundall 1982; Hardy and Greene 1999,
2000) with two VHF radio transmitters obtained from
Holohil Systems Ltd. Small transmitters (11 g,
40 9 11 mm) were used for snakes less than 16 kg
and larger transmitters (25 g, 45 9 11 mm) were used
in larger snakes. Transmitter weights were less than
0.5% of each snake’s body mass. We also inserted one
temperature-recording data logger into each snake.
Pythons less than 16 kg received a smaller (3 g,
16 9 6 mm) data logger (Maxim Integrated Products)
and larger snakes received a larger (30 g, 30 9 40 9
10 mm) data logger (Onset Computer Corporation).
144 F. J. Mazzotti et al.
123
Each data logger was programmed to record body
temperature (Tb) every 30 min for one year. Simul-
taneously, we deployed a temperature data logger in
each of two biophysical snake models constructed of
105 9 5 cm copper pipe painted black. Biophysical
snake models have similar thermal properties to live
snakes and allow detailed interpretation of thermal
data (see Peterson et al. 1993 for explanation). Most
pythons were acclimated to the wild well before the
onset of these record cold temperatures and had the
transmitters surgically implanted 2 weeks to 9 months
before this cold event. The two most recent surgeries
were conducted 2 weeks and four and a half weeks
prior to the cold event.
As part of an ongoing python radio-tracking
project, we located each telemetered snake weekly
using fixed-wing aircraft flying at an altitude of
150 m and a speed of 175 kph. During the cold spell,
we supplemented our weekly fixed-wing flights with
helicopter flights and/or on the ground tracking of
each snake 1–2 times per week until confirmation of
death or survival. We ‘walked in’ on all snakes that
had been located from the air for visual identification
and to obtain a GPS location. We accessed snakes on
the ground either via helicopter or by foot from the
nearest vehicle access point. When we sighted a
snake we recorded details on surrounding habitat
(e.g., tree island, marsh, hammock, road, levee),
position, and health. We took in situ pictures of each
python and recovered carcasses of dead snakes for
necropsy and to remove temperature data loggers.
Python Tb’s were compared to model temperatures
using linear regression.
To search for other Burmese pythons throughout
the Everglades landscape, we surveyed hammocks,
ponds, tree islands, canals, levees, roads, and trails by
air, vehicle, boat, and foot between 2 January 2010
and 4 February 2010 (Fig. 1); we also solicited
Fig. 1 Locations in south Florida of python search efforts, python captures (live and dead), and environmental data stations
Cold-induced mortality of invasive Burmese pythons in south Florida 145
123
observations from colleagues and ENP staff. Addi-
tional pythons were encountered while radio-tracking
snakes on the ground.
To determine correlates with python mortalities,
environmental data (air temperature, surface water
temperature, and rainfall) were obtained from ENP
weather station records. No single weather station in
ENP recorded all three environmental variables;
hence, we obtained data from the closest station that
recorded each variable. We used air temperature
recorded at Royal Palm, surface water temperature
from station P35, and rainfall from NP44.
Results
Nine of 10 telemetered pythons (90%; all 8 females
and 1 of 2 males) died during the cold period of 2–11
January 2010. All 10 telemetered pythons were found
on the surface of the ground or in vegetation. The
lone survivor was found in a hardwood hammock
(forest). Six of the nine dead pythons were found on
tree islands, one in a marl prairie, one along an
ecotone between mangrove forest and sawgrass
marsh, and one in a hardwood hammock. Four of
the nine dead pythons were partially covered by
vegetation and one had small bite marks on its body,
presumably from a rodent. Two of the pythons
appeared dead when found but then partially revived
after being transferred to a laboratory maintained at
23°C; one of these died within 24 h, and the second
never fully recovered and was subsequently
euthanized.
One hundred and four non-telemetered pythons
were found between 2 January and 4 February 2010.
Among those 99 had a date and fate (alive or dead)
associated with them. Fifty-nine (60%) were found
alive and 40 (40%) were found dead. Among the dead
were 13 individuals whose death could not be
connected to the cold snap; these snakes were found
after being run over by a mower (n = 2), while being
carried around by an alligator (n = 3), dead on
roadways (n = 3), or killed by humans (n = 5)
(Fig. 2). One hundred and one non-telemetered
pythons were documented as being associated with
a specific habitat type. Among these, 84 (83%) were
found in artificial habitats such as levees, canals, and
roads; of these, 32 (38%) were found dead. Seventeen
(17%) non-telemetered pythons were observed in
natural habitats, and 9 (53%) of these were dead.
Overall, 52 (87%) of 60 surviving non-telemetered
pythons were found associated with artificial habitats.
Sex was determined for 50 of the 104 non-tele-
metered snakes (48%); of the 12 females identified 2
were dead, and of the 38 males identified 4 were
dead. Recovered dead snakes averaged 260.4 cm
total length (TL) (53.7 SD) with a range of 167–
433 cm TL, representing juveniles and adults of both
sexes.
Air and surface water temperatures in ENP for the
record cold spell in January 2010 are presented in
Fig. 3. During a 14-day period starting on 2 January
2010, daily minimum air temperatures fell below
10°C for 12 days and below 15°C for all 14 days.
Maximum daily water temperatures remained below
15°C for most of the period. Both air and water
temperatures fell to their lowest on 11 January
(Fig. 3), and ice was observed on the surface of
shallow water south of Florida City adjacent to
southeastern ENP.
Fig. 2 Summary of
Burmese pythons found
alive and dead between 2
January and 4 February
2010. Cause of death for
pythons summarized here
were as a result of the cold
snap and other
circumstances (snakes
found after being run over
by a mower, while being
carried around by an
alligator or found dead on
roadways)
146 F. J. Mazzotti et al.
123
We recovered temperature data loggers from eight
dead telemetered pythons. Among these individuals,
Tb fluctuated daily with minimum temperatures
below 10°C and maximum temperatures above
30°C prior to 9 January (Fig. 3), similar to data
collected from telemetered pythons in similar habitats
over previous winters and indicative of active ther-
moregulation. The maximal daily python tempera-
tures reflect increased Tb likely from basking
behavior (i.e., Tb matched snake model tempera-
tures). Body temperatures declined substantially
during the coldest period (9–11 January) so that
maximum snake temperatures were \10°C and in
some snakes \5°C on the morning of 11 January.
After 11 January, Tb of the dead individuals tracked
more closely with air temperature, indicating a lack
of active thermoregulation (Fig. 3). Comparison of
Tb of dead pythons and model temperatures between
1,000 and 1,700 h when snakes typically actively
thermoregulate for 4 days before and 4 days after the
cold snap indicated active thermoregulation before
the cold snap (R
2
= 0.277) and a lack of thermoreg-
ulation afterwards (R
2
= 0.025). Although the exact
time snakes died is unknown, a lack of thermoreg-
ulatory behavior after 11 January indicates that the
snakes were either dead or incapacitated at that time.
Fig. 3 Summary of air temperature (Royal Palm), along with
hourly python body temperature (Tb) (a and b), surface water
temperature (P35), rainfall (NP44), and snake model and mean
python Tb from within Everglades National Park from 2
January until 16 January 2010 (c). No one station recorded all
variables, so the three closest stations were used for this
summary. Note that although air temperature rarely exceeded
20°C before 9 Jan, snakes frequently maximized body
temperature during the daytime (i.e., snake temperature
matches snake model temperature). From 9 to 11 Jan, snake
temperatures dropped precipitously as environmental temper-
atures decreased to below 0°C on the morning of 11 January.
Snake thermal patterns began changing on 10 January (arrows)
and after 11 January snake thermal patterns changed so that
maximal snake temperatures more closely matched air
temperature indicating a lack of behavioral thermoregulation
by the snakes. Presumably, most of the snakes died sometime
between 10 and 13 January
Cold-induced mortality of invasive Burmese pythons in south Florida 147
123
Discussion
Unusually cold temperatures in January 2010 were
associated with mortality of Burmese pythons in the
Florida Everglades, and it is possible that the mortality
observed among telemetered pythons was typical of
pythons in natural habitats within ENP. However,
mortality may have been lower in artificial habitats, as
discussed below. It is unclear whether python mortal-
ity was exacerbated by the duration of the cold event,
the extremely cold temperatures at the end of the
period, the sequence of events including a long
persistent rain prior to plunging temperatures, or some
combination of these factors. However, both the lack
of active thermoregulation in telemetered pythons
during and immediately after 11 January (Fig. 3) and
the increase in sightings of dead pythons after 11
January (Fig. 2) suggest that mortality or incapacita-
tion of pythons occurred during this time period.
While it is indisputable that large numbers of pythons
died, attempts to interpret these deaths with respect to
python ecology are complicated by a number of
factors, including behavior, availability of refuges,
and unknown variation in detection probabilities.
Telemetered pythons appeared to exhibit maladap-
tive behaviors during the cold spell, as evidenced by
observations that all 10 individuals were found on the
surface rather than in sheltered refugia. Evidence
from other snakes (and crocodilians) suggests that a
major difference between tropical and temperate
species is their thermoregulatory behavior (Lang
1987; Shine and Madsen 1996). This difference in
behavior between temperate and tropical species has
been well described for crocodilians. Temperate
species such as American alligators (Alligator mis-
sissippiensis) are more likely to deliberately seek heat
and avoid cold than are tropical species such as
American crocodiles (Crocodylus acutus) or specta-
cled caimans (Caiman crocodilus) (Lang 1987;
Brandt and Mazzotti 1990). Lang (1987) described
both alligators and crocodiles basking to maintain Tb
on cool days; however, when temperatures cooled
further tropical crocodiles continued to bask despite
cold air temperatures, whereas warm-temperate alli-
gators avoided basking and instead retreated to
warmer refugia in the water. Brandt and Mazzotti
(1990) confirmed this observation for alligators and
caimans placed in an outdoor enclosure at the
Savannah River Ecology Laboratory in Aiken, South
Carolina. In that study, both species basked during
cool weather but alligators retreated to the water
during freezing temperatures whereas caimans did
not; the latter died as a result. Maladaptive behavior
of basking during freezing temperatures appeared to
be responsible for the deaths of at least some of the
Burmese pythons documented here. Avery et al.
(2010) observed similar results for Burmese pythons
maintained in an outdoor enclosure with thermal
refugia provided in Gainesville, Florida during the
same time period. Avery et al. (2010) ascribed
mortality of 7 out of 9 pythons to cold temperatures.
They also suggested that this cold related mortality
was related to maladaptive behavior of the captive
pythons that neither avoided cold temperatures nor
sought available warm temperatures.
If inappropriate behaviors or intolerance to cold
contributed to cold-induced python mortality, an
obvious question is whether either thermoregulatory
behavior during cold weather or physiological toler-
ance to cold is genetically based. If so, and if there is
heritable variation in behavior or physiology within
the Florida population of Burmese pythons, then the
cold event might have exerted selective pressure on
the population in favor of individuals with greater
physiological cold tolerances or appropriate thermo-
regulatory behavior (i.e., refuge-seeking). The native
range of Burmese pythons includes regions with
winter lows that regularly drop below freezing (Zhao
and Adler 1993; Schleich and Ka
¨
stle 2002), indicat-
ing that populations of this species can persist in
areas cooler than south Florida. However, most
Burmese pythons in the international live animal
trade are sourced from Southeast Asia (Groombridge
and Luxmoore 1991). If pythons from tropical areas
exhibit reduced tolerance to cold as opposed to more
temperate populations, and if Florida pythons are
sourced only from tropical areas, then the Florida
population may have a limited ability to spread
northward (Rodda et al. 2009).
In some snakes, thermoregulatory and other
behavioral tactics appear to be set early in life, and
exposure to novel thermal or habitat conditions later
in ontogeny can provoke maladaptive behaviors
(Kingsbury and Attum 2009; Aubret and Shine
2009). The python population in south Florida has
probably not previously experienced such cold tem-
peratures as those in early 2010; thus the population
may be thermally naı
¨
ve, providing an alternative
148 F. J. Mazzotti et al.
123
hypothesis for observations of cold-induced mortal-
ity. Because no young-of-the-year pythons (live or
dead) have been found since the cold event, it
remains to be seen whether recent experience with
cold weather will affect future behavior of pythons.
All populations of large-bodied pythons and boa
constrictors inhabiting areas with cool winters,
including northern populations of Burmese pythons
in their native range, appear to rely on use of refugia
to escape winter temperatures (Bhupathy and Vijayan
1989; Chiaraviglio et al. 2003; Alexander 2007;
Waller et al. 2007). These refugia are usually burrows
or other subterranean retreats, but deeper water may
also be used. All of the 10 radio-telemetered pythons
were resident in natural habitats of the Everglades
ecosystem, an area characterized by perennially high
water tables and seasonal flooding. Although holes in
the karst limestone underlying much of this area are
abundant, most such holes remain flooded (Loftus
et al. 2001). Dry refugia in ENP tend to be limited to
uprooted trees and dense clumps of grass, two types
of thermal refugia that could easily have their
capacity to insulate reduced by precipitation during
a cold persistent rainfall. The apparently inappropri-
ate behaviors exhibited by some pythons, including
remaining on the surface during inclement weather,
may be the result of a lack of suitable thermal refugia
in Everglades habitats: the sole surviving telemetered
python was found in a large hardwood hammock with
thick vegetation and leaf litter, a habitat that may
have been drier than the small tree islands and marl
prairie used by most pythons that died. In contrast,
artificial habitats, especially raised levees associated
with canals and roads, have abundant refugia (bur-
rows, erosional holes, etc.) that would be more likely
to remain dry and thermally secure. If availability of
refugia in areas with high water tables limits ability to
escape from cold, then we can anticipate occasional
python population reductions in future severe cold
snaps in the Everglades and similar habitats. How-
ever, we would expect higher survival in drier natural
areas with burrows and large tree hollows and in
artificial habitats as described above; paradoxically,
this could allow relatively higher python survival in
areas outside the Everglades which are mostly
located farther to the north. Our observations of
non-telemetered pythons were especially helpful in
illustrating this point: most pythons known to have
survived the cold event were found in elevated areas,
and all 6 pythons found in higher areas to the
northwest of ENP (Collier-Seminole State Park, Big
Cypress National Preserve) were alive. Using radio-
telemetric data from Everglades National Park alone
would likely have provided a skewed view of overall
mortality rates. Data from the non-telemetered
pythons are also likely to be skewed, as a result of
differences in detection probabilities among live and
dead snakes; however, these data provide valuable
insight on habitat-based variability in mortality rates.
If mortality of telemetered pythons is due to
maladaptive behaviors or genetically fixed cold intol-
erance rather than unavailability of suitable refugia,
then range expansion hypotheses for Burmese pythons
in the United States may warrant re-evaluation. The
inappropriate behavior by Burmese pythons during the
cold spell was more like that of a tropical reptile whose
geographic range extends into warm temperate or
subtropical areas than that of a warm temperate reptile
whose geographic range extends into subtropical or
tropical areas (e.g., American alligators). Therefore
we hypothesize that Burmese pythons are not likely to
reach the distributional limits of alligators in the US,
with the caveat that Burmese pythons from a different
genetic background may respond differently. Consis-
tent with this hypothesis, Rodda et al. (2009) stated
that Burmese pythons found in temperate areas of the
native range appear to hibernate in the winter. Our
results suggest that at least some Burmese pythons in
southern Florida did not seek refuge during the cold
spell, but that others appeared to use refugia. We could
not distinguish between python use of refugia for
thermal or other purposes.
Adult female pythons died during the cold event,
likely reducing overall recruitment in the population
in 2010. Removal of reproductively mature individ-
uals from the population via direct mortality or
reduced capacity to reproduce will suppress popula-
tion growth rates (Caswell 1982; Heppell 1998), and
population growth may be further reduced if there is
significant juvenile mortality. Future population
modeling efforts to predict impacts of extreme cold
events should consider the effect of such unantici-
pated removal of individuals on survival and fecun-
dity. Such modeling efforts should also include
stochastic freeze conditions at a rate equivalent to
the historic rate of freezes in south Florida (Storey
and Gudger 1936; Storey 1937), with the caveat that
climate change scenarios should also be incorporated.
Cold-induced mortality of invasive Burmese pythons in south Florida 149
123
Knowledge of detectability of an invasive species is
important for planning control and eradication pro-
grams (Christy et al. 2010). Pythons are less detectable
in natural areas than in artificial habitats, but are more
detectable in both areas after a cold spell. That pythons
are apparently more detectable in artificial habitats is
likely a result of a combination of accessibility and
visibility. Pythons may also have been more visible to
human researchers during this study because the
snakes appeared to increase the amount of time spent
basking after the cold event. Because maximizing
removal rates is an important component of invasive
species control (Christy et al. 2010), timing rapid
responses (Stanford and Rodda 2007) in suitable
habitats during and after unusual climatic events might
increase removal rates of pythons.
Acknowledgments This research was supported by the US
Geological Survey Priority Ecosystems Science program, the
US National Park Service Critical Ecosystems Studies
Initiative, and the South Florida Water Management District.
We thank T. Kiechkefer and T. Hill for tracking and collecting
pythons, J. Vinci for making figures, S. Williams for formatting
the manuscript, and R. Harvey for editing the manuscript.
Everglades National Park agents, park staff, park partners, and
visitors assisted by reporting observations and helping to
recover pythons. We are especially indebted to B. Hill of the
South Florida Water Management District for his reports. This
manuscript was greatly improved by comments from H.
Waddle, P. Schofield and both anonymous reviewers. Permits
and approvals required for this research were obtained from the
US National Park Service and the Animal Research Committee
at the University of Florida. References to non-USGS products
and services are provided for information only and do not
constitute endorsement or warranty, expressed or implied, by
the US Government, as to their suitability, content, usefulness,
functioning, completeness, or accuracy.
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which
permits any noncommercial use, distribution, and reproduction
in any medium, provided the original author(s) and source are
credited.
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... Warm and stable temperatures were found to be the most important climatic suitability drivers for L. californiae, which is consistent with variables explaining reptile distributions (Aragón et al., 2010;McCain, 2010;Qian, 2010) and their establishment and spread into novel areas (Mazzotti et al., 2011;Dawson et al., 2017;Lin et al., 2019), usually reflecting their thermal preferences (Rodda et al., 2009;Rödder and Lötters, 2010). For instance, cold intolerance restricts the distribution of the invasive P. molurus (Jacobson et al., 2012), and cold winter temperatures prevent elevational expansion of the invasion of many-lined sun skinks (Eutropis multifasciata) (Lin et al., 2019). ...
... The general importance of temperature on understanding reptile distributions is especially relevant in the context of invasion biology and future global warming (IPCC, 2013;Li et al., 2016;Trisos et al., 2020). Global warming may lead to more favorable conditions for invasive reptiles, fueling their expansion to novel areas (Rodda et al., 2009;Silva-Rocha et al., 2015) and augmenting their fitness and impacts in ecosystems already invaded (Hellmann et al., 2008;Mazzotti et al., 2011). Climate change may also lead to increasing temperatures and unstable climatic conditions (IPCC, 2013), which could impair invasive reptiles' expansion and success (Mazzotti et al., 2011;Winter et al., 2016). ...
... Global warming may lead to more favorable conditions for invasive reptiles, fueling their expansion to novel areas (Rodda et al., 2009;Silva-Rocha et al., 2015) and augmenting their fitness and impacts in ecosystems already invaded (Hellmann et al., 2008;Mazzotti et al., 2011). Climate change may also lead to increasing temperatures and unstable climatic conditions (IPCC, 2013), which could impair invasive reptiles' expansion and success (Mazzotti et al., 2011;Winter et al., 2016). In both cases, physiological tolerance and phenotypic plasticity may play a prominent role in shaping the response of invasive reptiles to changing climatic conditions (Urban et al., 2014;Card et al., 2018). ...
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The interaction between climate change and biological invasions is a global conservation challenge with major consequences for invasive species management. However, our understanding of this interaction has substantial knowledge gaps; this is particularly relevant for invasive snakes on islands because they can be a serious threat to island ecosystems. Here we evaluated the potential influence of climate change on the distribution of invasive snakes on islands, using the invasion of the California kingsnake (Lampropeltis californiae) in Gran Canaria. We analysed the potential distribution of L. californiae under current and future climatic conditions in the Canary Islands, with the underlying hypothesis that the archipelago might be suitable for the species under these climate scenarios. Our results indicate that the Canary Islands are currently highly suitable for the invasive snake, with increased suitability under the climate change scenarios tested here. This study supports the idea that invasive reptiles represent a substantial threat to near-tropical regions, and builds on previous studies suggesting that the menace of invasive reptiles may persist or even be exacerbated by climate change. We suggest future research should continue to fill the knowledge gap regarding invasive reptiles, in particular snakes, to clarify their potential future impacts on global biodiversity.
... McEachern et al., 2015;Sanders et al., 2015). Ultimately and despite behavioural thermal buffers, survival of ectotherms depends on physiological thermal tolerance, as demonstrated by Burmese python (Python bivittatus Kuhl, 1820) deaths in cold weather despite selective basking and sheltering (Dorcas et al., 2011), iguanas (Iguana iguana Linneaus, 1758) falling from trees during cold snaps in South Florida (Campbell, 2011;Chappell, 2020), and green sea turtle (Chelonia mydas Linneaus, 1758) strandings and cold-stunnings (Roberts et al., 2014). ...
... Gorman & Hillman, 1977). Notably, a record cold spell in 2010 caused deaths of many native and non-native reptiles in southern Florida (Mazzotti et al., 2011(Mazzotti et al., , 2016. Both L. carinatus populations in this study probably became established well before 2010 and within 10 years of one other (Duquesnel, 1998;Krysko & King, 2002;Krysko et al., 2005). ...
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Native-range thermal constraints may not reflect the geographical distributions of species introduced from native island ranges in part due to rapid physiological adaptation in species introduced to new environments. Correlative ecological niche models may thus underestimate potential invasive distributions of species from islands. The northern curly-tailed lizard (Leiocephalus carinatus) is established in Florida, including populations north of its native range. Competing hypotheses may explain this distribution: Thermal Matching (distribution reflects thermal conditions of the native range), Thermal Potential (species tolerates thermal extremes absent in the native range) and/or Thermal Flexibility (thermal tolerance reflects local thermal extremes). We rejected the Thermal Matching hypothesis by comparing ecological niche models developed from native vs. native plus invasive distributions; L. carinatus exists in areas of low suitability in Florida as predicted by the native-distribution model. We then compared critical thermal limits of L. carinatus from two non-native populations to evaluate the Thermal Potential and Flexibility hypotheses: one matching native range latitudes, and another 160 km north of the native range that experiences more frequent cold weather events. Critical thermal minima in the northern population were lower than in the south, supporting the Thermal Flexibility hypothesis, whereas critical thermal maxima did not differ.
... Though the likelihood of pythons spreading outside of Florida to neighboring U.S. states is low (Avery et al., 2010;, the degree to which they continue to spread remains uncertain (Rodda et al., 2009). Given their susceptibility to cold winter temperatures, climate is likely to be a primary factor limiting the northward spread of pythons outside of Florida (Michael E. Dorcas et al., 2011;Mazzotti et al., 2011). Additionally, the availability and connectivity of wetland habitats used by pythons may also limit their ability to spread north of Lake Okeechobee (Mutascio et al., 2018). ...
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Invasive predators have caused catastrophic declines in native wildlife across the globe. Though research has focused on the initial establishment, rapid growth, and spread of invasive predators, our understanding of prey resilience to established invasive predators remains limited. As a direct result of invasive Burmese pythons (Python molurus bivittatus), medium- to large-bodied native mammals decreased drastically across much of southern Florida as early as 2003. By 2014, most of these mammal species were exceedingly rare within the core invasion area, while pythons expanded outward to newly invaded areas. We used python observations to delineate the core python invasion area from the more recently invaded invasion front, and we compared changes in mammal occurrence from 2014 to 2019 between these two areas. We surveyed mammal communities using camera traps and scat surveys and used these observations to quantify the changes in occurrence among mammal species. As expected, occurrence of medium- and large-bodied mammals declined within the invasion front. However, contrary to our expectation, we observed little evidence of resilience among mammals within the invasion core. Of the 15 species detected in 2019, invasive black rats were the only species to increase in occurrence within the invasion core. Additionally, we observed declines in occurrence among native rodents within the invasion core, which were previously thought to be resistant to the effects of pythons. The continued presence of invasive pythons appears to be shifting the diverse mammal communities of southern Florida to one primarily composed of invasive species.
... Rapid in situ physiological adaptation of this nature has already been reported for several non-native lizard species in Florida (Stroud et al., 2020). Furthermore, since its initial establishment in Florida in the 1990s, P. grandis has been exposed to extreme coldweather events that have caused substantial cold-induced mortality in multiple non-native squamate species (Campbell, 2011;Fieldsend & Krysko, 2019a;Mazzotti et al., 2011Mazzotti et al., , 2016, illustrating how powerful selective forces might drive rapid population-level adaptation. ...
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Aim To investigate whether the frequently advocated climate-matching species distribution modeling approach could predict the well-characterized colonization of Florida by the Madagascar giant day gecko Phelsuma grandis. Location Madagascar and Florida, USA. Methods To determine the climatic conditions associated with the native range of P. grandis, we used native-range presence-only records and Bioclim climatic data to build a Maxent species distribution model and projected the climatic thresholds of the native range onto Florida. We then built an analogous model using Florida presence-only data and projected it onto Madagascar. We constructed a third model using native-range presences for both P. grandis and the closely related parapatric species P. kochi. Results Despite performing well within the native range, our Madagascar Bioclim model failed to identify suitable climatic habitat currently occupied by P. grandis in Florida. The model constructed using Florida presences also failed to reflect the distribution in Madagascar by overpredicting distribution, especially in western areas occupied by P. kochi. The model built using the combined P. kochi/P. grandis dataset modestly improved the prediction of the range of P. grandis in Florida, thereby implying competitive exclusion of P. grandis by P. kochi from habitat within the former's fundamental niche. These findings thus suggest ecological release of P. grandis in Florida. However, because ecological release cannot fully explain the divergent occupied niches of P. grandis in Madagascar versus Florida, our findings also demonstrate some degree of in situ adaptation in Florida. Main conclusions Our models suggest that the discrepancy between the predicted and observed range of P. grandis in Florida is attributable to either in situ adaptation by P. grandis within Florida, or a combination of such in situ adaptation and competition with P. kochi in Madagascar. Our study demonstrates that climate-matching species distribution models can severely underpredict the establishment risk posed by non-native herpetofauna.
... The literature is full of examples of invasive species establishing in centers of human activity (reviewed in Lockwood et al. 2013) and examples of extreme weather events influencing population establishment and persistence (e.g., Frederiksen et al. 2008;Mazzotti et al. 2011;Meshaka 1993;Tinsley et al. 2015). Studies of Anolis lizards are prime examples: several Anolis species thrive in urban environments (e.g., Latella et al. 2011;Kolbe et al. 2016;Winchell et al. 2016;Tiatragul et al. 2019), and in some cases, rely on human structures (Hulbert et al. 2020). ...
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Human activity causes major changes in natural landscapes via introduction of non-native species, development on natural habitat, and alteration of local weather patterns. These factors contribute to global change and may interact to affect local populations of plants and animals. We studied a viable, non-native lizard population (Anolis sagrei) in southeast Alabama, USA that has depended upon thermal conditions inside a greenhouse nursery during the winter for at least 10 years. Using Capture-Mark-Recapture surveys, we compared population parameters and movement patterns of this introduced A. sagrei population to a native lizard population (Sceloporus undulatus) that also inhabits our study site. The population size of both species fluctuated over time, but that of A. sagrei was considerably larger than S. undulatus. Anolis sagrei was relatively philopatric and confined within the greenhouse and its immediate vicinity, whereas the S. undulatus population extended into the surrounding forest habitat. The thermal landscape within the greenhouse was substantially altered after the roof was removed due to winds from a tropical storm. Indeed, temperatures of all microhabitats commonly used by lizards frequently dropped below the critical thermal minimum for A. sagrei and below freezing during winter. Post-winter surveys revealed that no A. sagrei individuals survived, indicating that the temperature change in the greenhouse resulted in extinction. The native S. undulatus population, however, was still present after winter. Our study provides rare documentation of an extinction of an established introduced population and illustrates the role that human-made structures and natural weather events play in the process of biological invasion.
... Similarly, climate PCA1 explained more than 66% of the suitable habitat distribution of nocturnal G. japonicus. Previous studies observed Fig. 2 a, b Present (1960~1990) and c-f future (2016-2080) distribution of the suitable habitats of Gekko japonicus over its entire distribution range according to climate change scenarios of RCP 4.5 (c, d) and 8.5 (e, f), which predicted from the MaxEnt species distribution modeling that low winter temperature reduced the survivorship of introduced Burmese pythons (Python bivittatus) in Florida (Mazzotti et al. 2011), and limited expansion of introduced many-lined sun skinks (Eutropis multifasciata) to lower altitudes in Taiwan (Lin et al. 2019). Second, the importance of low altitude seems to be related to the nocturnal lifestyle and interspecific competition of G. japonicus. ...
Article
Full-text available
Background Understanding the geographical distribution of a species is a key component of studying its ecology, evolution, and conservation. Although Schlegel’s Japanese gecko ( Gekko japonicus ) is widely distributed in Northeast Asia, its distribution has not been studied in detail. We predicted the present and future distribution of G. japonicus across China, Japan, and Korea based on 19 climatic and 5 environmental variables using the maximum entropy (MaxEnt) species distribution model. Results Present time major suitable habitats for G. japonicus , having greater than 0.55 probability of presence (threshold based on the average predicted probability of the presence records), are located at coastal and inland cities of China; western, southern, and northern coasts of Kyushu and Honshu in Japan; and southern coastal cities of Korea. Japan contained 69.3% of the suitable habitats, followed by China (27.1%) and Korea (4.2%). Temperature seasonality (66.5% of permutation importance) was the most important predictor of the distribution. Future distributions according to two climate change scenarios predicted that by 2070, and overall suitable habitats would decrease compared to the present habitats by 18.4% (scenario RCP 4.5) and 10.4% (scenario RCP 8.5). In contrast to these overall trends, range expansions are expected in inland areas of China and southern parts of Korea. Conclusions Suitable habitats predicted for G. japonicus are currently located in coastal cities of Japan, China, and Korea, as well as in isolated patches of inland China. Due to climate change, suitable habitats are expected to shrink along coastlines, particularly at the coastal-edge of climate change zones. Overall, our results provide essential distribution range information for future ecological studies of G. japonicus across its distribution range.
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Tropicalization is a term used to describe the transformation of temperate ecosystems by poleward‐moving tropical organisms in response to warming temperatures. In North America, decreases in the frequency and intensity of extreme winter cold events are expected to allow the poleward range expansion of many cold‐sensitive tropical organisms, sometimes at the expense of temperate organisms. Although ecologists have long noted the critical ecological role of winter cold temperature extremes in tropical‐temperate transition zones, the ecological effects of extreme cold events have been understudied, and the influence of warming winter temperatures has too often been left out of climate change vulnerability assessments. Here, we examine the influence of extreme cold events on the northward range limits of a diverse group of tropical organisms, including terrestrial plants, coastal wetland plants, coastal fishes, sea turtles, terrestrial reptiles, amphibians, manatees, and insects. For these organisms, extreme cold events can lead to major physiological damage or landscape‐scale mass mortality. Conversely, the absence of extreme cold events can foster population growth, range expansion, and ecological regime shifts. We discuss the effects of warming winters on species and ecosystems in tropical‐temperate transition zones. In the twenty‐first century, climate change‐induced decreases in the frequency and intensity of extreme cold events are expected to facilitate the poleward range expansion of many tropical species. Our review highlights critical knowledge gaps for advancing understanding of the ecological implications of the tropicalization of temperate ecosystems in North America.
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Animal movement and resource use are tightly linked. Investigating these links to understand how animals utilize space and select habitats is especially relevant in areas that have been affected by habitat fragmentation and agricultural conversion. We set out to explore the space use and habitat selection of Burmese pythons ( Python bivittatus ) in a patchy land use matrix dominated by agricultural crops and human settlements. We used radio telemetry to record daily locations of seven Burmese pythons over the course of our study period of approximately 22 months. We created dynamic Brownian Bridge Movement Models (dBBMMs) for all individuals, using occurrence distributions to estimate extent of movements and motion variance to reveal temporal patterns. Then we used integrated step selection functions to determine whether individual movements were associated with particular landscape features (aquatic agriculture, forest, roads, settlements, terrestrial agriculture, water), and whether there were consistent associations at the population level. Our dBBMM estimates suggested that Burmese pythons made use of small areas (98.97 ± 35.42 ha), with low mean individual motion variance characterized by infrequent moves and long periods at a single location. At both the individual and population level, Burmese pythons in the agricultural matrix were associated with aquatic environments. Only one individual showed a strong avoidance for human settlements which is troublesome from a human-wildlife conflict angle, especially as Burmese pythons have been observed entering human settlements and consuming livestock in our study site. Our study is one of the first to contribute to the knowledge of Burmese python ecology in their native range as the majority of studies have focused on invasive populations.
Article
Aim The use of species distribution models (SDMs) to predict biological invasions is a rapidly developing area of ecology. However, most studies investigating SDMs typically ignore prediction errors and instead focus on regions where native distributions correctly predict invaded ranges. We investigated the ecological significance of prediction errors using reciprocal comparisons between the predicted invaded and native range of the red imported fire ant (Solenopsis invicta) (hereafter called the fire ant). We questioned whether fire ants occupy similar environments in their native and introduced range, how the environments that fire ants occupy in their introduced range changed through time relative to their native range, and where fire ant propagules are likely to have originated. Location We developed models for South America and the conterminous United States (US) of America. Methods We developed models using the Genetic Algorithm for Rule-set Prediction (GARP) and 12 environmental layers. Occurrence data from the native range in South America were used to predict the introduced range in the US and vice versa. Further, time-series data recording the invasion of fire ants in the US were used to predict the native range. Results Native range occurrences under-predicted the invasive potential of fire ants, whereas occurrence data from the US over-predicted the southern boundary of the native range. Secondly, introduced fire ants initially established in environments similar to those in their native range, but subsequently invaded harsher environments. Time-series data suggest that fire ant propagules originated near the southern limit of their native range. Conclusions Our findings suggest that fire ants from a peripheral native population established in an environment similar to their native environment, and then ultimately expanded into environments in which they are not found in their native range. We argue that reciprocal comparisons between predicted native and invaded ranges will facilitate a better understanding of the biogeography of invasive and native species and of the role of SDMs in predicting future distributions.
Article
Life history traits are often used to distinguish equilibrium from nonequilibrium populations. This is invalid, both within the framework of r- and K-selection theory and within a demographic model which takes age structure into account. In both cases, the patterns favored in an equilibrium population are even more intensely favored in a nonequilibrium population which happens to spend most of its history in a state of population decline. The only characteristic which seems to distinguish equilibrium populations is a lower absolute magnitude of selective pressure on some traits. The traits favored in declining populations include long lifespan, slow development, delayed reproduction, iteroparity, low degree of senescence, and perhaps a relatively high degree of investment in whatever offspring are produced. The traits favored in increasing nonequilibrium populations are largely the opposite: short life, fast development, and early reproduction, semelparity, senescence, and perhaps less investment in individual offspring. Over evolutionary time, nonequilibrium populations will diverge in one direction or the other. This model predicts the existence of suites of species with dinstinctly 'declining' (K-selected) characteristics, and other groups with distinctly 'increasing' (r-selected) characteristics. Because the same traits that increase the proportion of its history that a population spends in decline also increase the time required for extinction, the former group will occupy relatively stable habitats, the latter group more disturbed habitats. This requires no equilibrium assumptions; all of the species involved may be local losers. The predictions of the model are supported by an analysis of the relation between competitive tolerance, age at first reproduction, and life span in North American trees. -Author