Thursday 26 March 2015

Experimental evidence suggests Burmese Pythons are responsible for the rapid decline of Mammals in the Everglades National Park.

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.

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.

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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