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INVASIVE SPECIES
Non-native, invasive species of plants and animals are a huge problem for the
United States. With impacts on the environment, the economy, and human health, it is
estimated the cost of invasives is over $138 billion a year. It is estimated that over
50,000 non-native species of plants and animals have been released into the United States.
There are many different reasons animals get released ranging from accidental, biotic
enhancement, and pest control. Upon recognition of this problem the federal government
implemented the National Invasive Species Act of 1996 [http://thomas.loc.gov/cgibin/query/z?c104:H.R.4283.ENR:] to try and manage invasive species.
The Fort Collins Science Center [www.fort.usgs.gov] has an Invasive Species
Science Branch [www.fort.usgs.gov/ISS/] that provides research and technical assistance
to invasive species management concerns. They are developing the Invasive Species
Information Node of the National Biological Information Infrastructure (NBII)
[http://invasivespecies.nbii.gov], which is a comprehensive, web-accessible database of
invasive plant and animal species and disease agents. They have also teamed up with
Colorado State University [www.colostate.edu], NASA
[http://nasascience.nasa.gov/about-us] and other organizations to construct models to
better understand and predict invasive species behavior.
[link to Invasive Reptiles]
INVASIVE REPTILES
There are at least 53 invasive species of reptiles in the United States, the majority
being in Florida and Hawaii. One of the most problematic invasive reptiles, however, has
been in Guam. The Brown Tree Snake
[http://www.fort.usgs.gov/resources/education/bts/bioeco/btsnake.asp] was accidentally
let into Guam and is responsible for eradicating nearly all native bird species and most of
the bats and lizards on Guam. Its damages total 1 to 4 million dollars per year. It is
frequently the cause of power outages. This snake also poses a threat to humans as it can
be aggressive when threatened and is venomous.
Picture: It is estimated that up to 13,000 snakes per square mile occur in some forested areas in Guam
In the 1950’s the South American Cane toad was introduced to control sugar cane
pests. Now it preys and competes with native species and is highly toxic to predators.
Recently, Florida has been having problems with invasive snakes. Giant
constrictors, believe to have been released by pet owners, are establishing themselves in
the Everglades National Park. [www.nps.gov/ever/] There are concerns for the threatened
and endangered wildlife that reside in the park as well as the possible spread of these
snakes to other parts of Florida and other areas of the United States.
[link to Burmese python page]
THE BASIC BIOLOGY OF THE
BURMESE PYTHON
The Burmese python, Python molurus bivittatus, is a subspecies of the Indian
python. Recently this python has established itself as an invasive species in southern
Florida. A viable population has inhabited Everglades National Park
[www.nps.gov/ever/], and the species continues to expand its range outside the park.
This population is thought to have resulted from the illegal release of unwanted pets.
There is now concern over the spread of this species through much of Florida and
possibly to other parts of the United States.
NATIVE RANGE AND HABITAT:
The Burmese python is native to India, Burma, Indonesia, Vietnam, southern
China, and parts of Thailand. The species is reported to be declining or rare over some of
this range. Climate may be a factor that limits the python’s distribution. The western
edge of this range follows deserts and relatively dry areas, showing that aridity is likely
the limiting condition of its western habitat. The southern and eastern limits follow the
coast of Asia although it has a disjunctive range in Indonesia and Malaysia. The northern
limits of the python’s reach are met by the high altitude, high latitude and cold
temperatures of the Himalayan Mountains and by cold areas in lower elevations in China.
The Burmese python is found in a number of different habitats and is considered
to be a habitat generalist. It can inhabit estuarine mangrove forests, marshes, swamps,
arid scrub jungles, rainforests, river valleys, grasslands, and rocky foothills. They do
make use of larger undisturbed areas to hunt and hide in but are also present in human
occupied areas. Pythons have been spotted in rocky clefts and caves, abandoned
mammal burrows, hollow trees, dense water reeds, drainage pipes, and mangrove
thickets. Pythons are also very good climbers and juveniles are often spotted in trees.
A permanent water source is beneficial to these snakes. Pythons are quite at
home in the water and are often times considered semi-aquatic. They are excellent
swimmers and can hold their breath for at least 30 minutes. The water is often a suitable
means for foraging and hunting.
Pythons can be both nocturnal and diurnal. This can depend largely on the time
of the year. They appear to be diurnal from October through April and primarily
nocturnal from June through August in their native range.
TO EAT OR BE EATEN:
In addition to being a habitat generalist, the python is considered to be a generalist
predator. It will prey on a number of different animals. It is known to feed on mammals,
birds, reptiles including monitor lizards and, occasionally, other snakes. In some cases it
was reported that insects and some fruits were found in the stomachs of the python.
Adults chiefly prey on large mammals and birds to a lesser extent. One record has shown
that an 18 foot long python killed and ate a four foot long leopard. Predation of humans
is rare but a few reports of attacks have surfaced. Many of these cases were from pet
snakes in the United States. Pythons are, however, quite capable of killing an adult
human. Children are at an even greater risk of attacks.
Burmese pythons use a number of different tactics to find food. They have the
ability to ambush their prey. They make use of this “lie in wait” approach as well as
actively forage for food. They are a powerful constrictor and use this to overcome their
prey.
There are a few natural predators of the python. In its native range, otters and
tigers are known to kill pythons. Diseases such as tuberculosis, ticks, and other parasites
have been known to infect these snakes. In Florida potential predators include alligators,
the American crocodile, eastern indigo snakes, turtles, raptors, and medium to large sized
mammals. Automobiles, farm equipment, and lawnmowers also kill significant numbers
of Burmese pythons in Florida.
REPRODUCTION, DEVELOPMENT, AND SIZE OF THE PYTHON:
Burmese pythons become sexually mature at around 2-3 years old in their native
range. Copulation normally occurs during hibernation and cooler times of the year. In
Florida they have been found breeding in the open swamps and on levees. There have
been reports of mating balls forming, where a number of males are found with one large
female. Females have the ability to store sperm until conditions are ideal to lay eggs.
The typical number of eggs laid by a python is 15-54 (average 30), but up to 107 have
been recorded. Hatching occurs after 2-3 months. The female python will remain with
the eggs and will not feed during this time. The females incubate the eggs, and by
shivering are capable of raising their body temperature 6-8° C to aide in the incubation.
This gives the snake the ability to reproduce in a more temperate climate than other
snakes.
Hatchlings measure 43-79 cm and can grow extremely quickly. They have the
potential of growing 170% in the first year. Female pythons generally grow longer than
males (typically 5-6 feet longer). They have the potential to grow to great lengths (5.8-7
m) and up to 91 kg in weight. In captivity, pythons have been known to live for over 30
years (their longevity in the wild is unknown).
FLORIDA’S POPULATION OF PYTHONS:
Reports of Burmese python sightings are growing in Florida, especially in
Everglades National Park. [www.nps.gov/ever/] Recently they have been spotted in
Collier-Seminole State Park [www.floridastateparks.org/collier-seminole/] and Big
Cypress National Preserve. [www.nps.gov/bicy/] Many of these sightings were on roads,
road shoulders, slopes of canal levees and other higher grounds where they are easily
visible to humans on foot. However, radio telemetry has shown that snakes are in no way
dependent on these human-altered areas, and some observations were made in remote
areas such as mangrove creeks, creek banks and marshes.
From 1979 to July 2005, there were 248 python observations recorded. The
numbers of observations has increased dramatically in the Everglades National Park
[www.nps.gov/ever/] since 2001. In 2007 alone, 250 pythons were found in the park.
They are now being recorded farther north and east. Officials suspect that most of the
Burmese python introductions were unwanted pets that became too large to handle and
were released illegally into the park.
As evidenced by the discovery of large numbers of juveniles and several females
brooding eggs, Burmese pythons are reproducing in southern Florida. Authorities
suspect that the population is large and increasing in the Florida Everglades and has the
potential to spread from this area. Pythons have the potential to become an important
predator on native wildlife populations in Florida, having the ability to cause harm to the
ecosystem. They will compete for the same prey as alligators and indigo snakes but will
become prey to these animals as well.
[link to Species at Risk]
[link to Risk Assessment]
[link to Related Resources] (my bibliography)
SPECIES IN FLORIDA AT RISK TO THE
BURMESE PYTHON
The Everglades National Park [www.nps.gov/ever/] is home to many threatened
and endangered animals. In many instances, this is the only remaining habitat where the
species is found. There are 16 species listed as endangered and 6 species listed as
threatened in the Everglades. The following is a list of vertebrates most likely to be
impacted by an established population of boas or pythons. This list was compiled by
comparing the geographical ranges of these animals and where the pythons are believed
to have inhabited.
Lower Keys Marsh Rabbit, Sylvilagus palustris hefneri: This
rabbit is listed as endangered and its range is limited to the Florida Keys. Its habitat is
similar to the python’s habitat: salt or freshwater marshes, mangroves and shrubby edges
of wetlands.
Florida Mastiff Bat, Eumops glaucinus floridanus: This bat is listed
as endangered by the state of Florida. Its range is limited to the southern tip of Florida
where the python has been found. This bat would be especially vulnerable to juvenile
pythons.
Snail Kite, Rostrhamus sociabilis plumbeus: This Falcon is listed as
endangered. Its range overlaps with what is expected of the python. It is dependent upon
the same habitat for hunting as the python.
Key Largo Woodrat, Neotoma floridana smalli: Also listed as
endangered, this woodrat is susceptible to predation throughout its natural range.
Cape Sable Seaside Sparrow, Ammodramus maritimus mirabilis: This bird
is listed as endangered and only found on the southern tip of Florida. It is a permanent
resident of the freshwater interior marshes of the Everglades.
Key Deer, Odocoileus virginianus clavium: This endangered small deer
resides in the Florida Keys. Its habitat is similar to the python, and it could easily be
taken down by an adult python.
There are also a few listed species that could suffer because of competition for food and
habitat with pythons and boas.
American Crocodile, Crocodylus acutus: Found only in
the southern portion of Florida, these reptiles are listed as endangered. They share a
common habitat with the python and would be in direct competition for food and space.
There is also a large threat of predation upon juvenile crocodiles.
Eastern Indigo Snake, Drymarchon corais couperi:
Currently listed as threatened, this species’ geographic range is throughout the
southeastern United States. This snake could easily be impacted by the presence of the
python because of the shared habitat and competition for prey. There is also the threat of
exposure to different pathogens from the introduced pythons.
This is just a brief list of threatened and endangered wildlife that could be affected
by a viable population of Burmese pythons and other constrictor snakes. For more
threatened or endangered species at risk visit [www.fnai.org] or
[www.floridainvaders.org].
A BRIEF RISK ASSESSMENT OF THE
BURMESE PYTHON IN FLORIDA
The Fort Collins Science Center [www.fort.usgs.gov] is looking into the potential
impacts of introduced snakes to Florida as well as the rest of the United States. There are
different species of giant constrictor snakes that have the potential to alter the community
of life in the Everglades National Park [www.nps.gov/ever/] as well as other parts of the
southern United States. The most common of these snakes is the Burmese python
(Python molurus bivittatus). Several hundreds of these pythons have been collected from
The Everglades. The other species include: Python reticulatus, Boa constrictor,
Eunectes murinus, Python sebae, Python natalensis, Eunectes notaeus, Eunectes
beniensis, and Eunectes deschauenseei.
In 2006 the South Florida Water Management District petitioned the US Fish and
Wildlife Service to add many of these giant constrictors to the list of Injurious Wildlife.
This list limits private ownership of these snakes. Other regulatory actions have also
been taken or proposed to combat this issue of invasive snakes. In 2006 the Florida
Wildlife Commission proposed that private ownership be regulated. This could be done
by tagging pet snakes to facilitate tracking of lost or released individuals.
At this time the USGS has proposed a risk assessment to look at the potential
impact of these snakes. They plan to conduct comprehensive literature reviews to gather
information for each of the above snake species. Using this information, they will project
which animals are likely to fall prey to these snakes.
In February 2008, the USGS released its findings from a study looking at “What
parts of the US mainland are climatically suitable for invasive alien pythons spreading
from Everglades National Park” [link to study and/or news release]. It projects that the
Burmese python could find “comfortable climatic conditions” in close to a third of the
United States, mostly in the south.
This study looked at the climate of the python’s native range in Asia and matched
it to that of the United States. It takes into account rainfall and temperature statistics of
an area including Pakistan east to China and south to Indonesia. The climatically
matched areas of the United States “ranged up the coasts and across the south from
Delaware to Oregon, and included most of California, Texas, Oklahoma, Arkansas,
Louisiana, Mississippi, Alabama, Florida, Georgia, and South and North Carolina.”
Present Climate suitability
Also in this study the USGS projected the distribution of the python by the year 2100 to
be even larger when looking at predicted climate maps. These projections “extend
northward beyond the current limit to include parts of the states of Washington, Colorado,
Illinois, Indiana, Ohio, West Virginia, Pennsylvania, New Jersey, and New York.”
Year 2100 projected climate suitability based on global warming models.
The USGS and Everglades National Park [www.nps.gov/ever/] are looking more
into the requirements for the survival of the Burmese python. They are hoping to be able
to predict where these snakes are going and if they will develop into a viable population.
The USGS is exploring other risks of this invasive species such as their potential for
social, economic, and other environmental impacts. Both agencies are also looking into
developing tools to control these snake populations and/or prevent their spread.
These climate maps showing the potential range of the Burmese python along
with the basic biology of the snake demonstrates the urgency for action. These snakes
have the ability to cause a lot of damage to the Everglades ecosystem as well as to a good
portion of the southern United States.