Burmese Pythons, Python molurus bivittatus or Python bivittatus, are large predatory
Snakes from Southeast Asia. They are thought to have been introduced to the
Florida Everglades some decades ago, either by deliberate release or unintended
escapes from the pet trade and private owners, with occasional sightings
reported throughout the 1980s and 1990s. From 2000 onwards a sharp increase in
numbers was reported from the Everglades National Park, at the southern end of
the Greater Everglades Ecosystem, combined with a sudden and rapid drop in the
population numbers for almost all Mammal species in the same area. As a result
many people concluded that the Pythons were responsible for this rapid loss of
Mammal species, but if this is the case then it is a novel event, as while introduced
Snakes have previously been held responsible for rapid declines in multiple
species on small islands, no Snake, or any other animal except Man, has ever
been linked to a rapid decline in multiple Mammal species with very different
ecological roles in a continental ecosystem.
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Burmese Python in the Everglades National Park. Susan Jewell/U.S. Fish and Wildlife Service/Wikimedia Commons.
In a paper published in the
Proceedings of the Royal Society Series B: Biological Sciences on 18 March 2015,
Robert McCleery and Adia Sovie of the Department of Wildlife Ecology and Conservation at the University of Florida, Robert Reed of the United States Geological Survey’s Fort Collins Science Center, Mark Cunningham of the Florida Fish and Wildlife Conservation Commission and Margaret Hunter and Kristen Hart
of the United States Geological Survey’s Southeast Ecological Science Center,
discuss the results of an experiment in which Marsh Rabbits, Sylvilagus palustris, were
experimentally re-introduced to two areas within the Everglades National Park.
March Rabbits are small (up to 1
kg) Lagomorphs found in wetlands throughout the southeatern United States. They
reproduce extremely quickly, producing up to six litters of three-to-five young
per season; species with reproduction rates this high generally have population
levels determined by the availability of food rather than predation, making
them good test models for the study; if the Pythons were capable of supressing
Marsh Rabbit populations then they should be capable of doing the same for species
which reproduce more slowly.
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Marsh Rabbit on Sanibel Island, Florida. Jean-Lou Justine/Wikimedia Commons.
Rabbits were captured at several
sites in the northern Greater Everglades Ecosystem where Burmese Pythons have
not yet become established, and were fitted with radio transmitters and released
at two sites in the Everglades National Park, Coastal Prairie Trail, a coastal
marsh wetland, and East Taylor Slough, a freshwater wetland, as well as a
control site at the Arthur R. Marshall Loxahatchee National Wildlife Refuge,
which is about 100 km to the northeast of the Everglades National Park, and
where no Pythons have ever been recorded within 10 km. At a second control site
in the Fakahatchee Strand Preserve State Park Rabbits were captured, tagged and
immediately released.
Location of study sites in south Florida, United States. Relative
frequency of Burmese python observations were based on 2008–2013 records in the
Early Detection and Distribution Mapping System. ETS, East Taylor Slough; CPT,
Coastal Prairie Trail; FAK, Fakahatchee Strand Preserve State Park; LOX, Arthur
R. Marshall Loxahatchee National Wildlife Refuge; STA, Storm Water Treatment
Area. McLeery et al. (2015).
Rabbits that died within ten days
of being released were excluded from the study, to exclude deaths caused by the
stress of capture and transporting. In other instances where a mortality signal
was given by the transmitter or where a Rabbit appeared to stop moving the
cause of death was investigated. Deaths were attributed to predation by Birds
if the kill site had Bird droppings or feathers, eyes had been removed, the
carcass had beak marks, or the fur and internal organs were scattered found it
in a circle measuring 20-50 cm in diameter. They were classed as Mammal kills
if Mammal droppings or prints were found at the site, the body cavity had been
opened and the internal organs removed, the skin of the Rabbit had been pulled
over its legs, there were bite marks to the head, one or more of the long bones
had been broken or the body had been buried. Unlike Birds or Mammals, Reptilian
predators tend to swallow prey items whole, so investigating the death of a
Rabbit eaten by a Reptile typically took the researchers straight to the
predator.
Of nineteen Rabbit deaths within
the Everglades National Park for which a cause could be determined, seventeen
were the result of predation by Burmese Pythons, while Bird and Mammal
predators were responsible for one death each. The cause of 36 Rabbit deaths in
the two control areas was also established, with eight attributed to Birds and
23 to Mammals; five deaths were attributed to Reptiles, three to Rattlesnakes, Crotalus adamanteus, and two to American
Alligators, Alligator mississippiensis. At the end of the
monitoring period, in February 2013, four of the original Rabbits were still
alive, and droppings of both adult and immature Rabbits could be found,
suggesting that they had begun to breed. However a follow-up survey in December
2013 could find no droppings or any other signs of Rabbits, suggesting that by
this point they had all been consumed.
While the Rabbits suffered very
high levels of predation from Pythons in areas where they were present, they underwent
lower levels of predation by other predators, particularly larger Mammals. This
suggests that either these had also been consumed, or that they had been
competitively excluded by the removal of their food source. The latter is quite
plausible as Pythons are able to take a wide range of prey, and of going for
long periods of time without eating, enabling them to remain present at
relatively high population levels even when prey numbers are very low. In
either event it seems unlikely that efforts to re-introduce the missing Mammals
to the park will succeed while the Pythons are still present, and there is a
high risk that the Pythons will remove Mammals from other areas as they expand
their range further.
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