In the last several decades, Burmese pythons have been blamed for the precipitous decline of native mammals in the wetlands of Florida. But the evidence so far has either been indirect or suggested that there may be perpetrators as well. Well, now there’s little doubt left: These invasive pythons (mostly former pets) caused more than three-fourths of all marsh rabbit mortalities during a year-long study. The findings were published in Proceedings of the Royal Society B this week.
In Everglades National Park, sightings and removals of Burmese pythons (Python molurus bivittatus) have been sporadic in the 1980s and 1990s, though they increased sharply in the early 2000s. Researchers analyzing python gut contents found that mammals made up 75 percent of their diet, and these ranged from muskrats and raccoons to bobcats.
To establish a direct link between non-native pythons and mammal declines, a team led by Robert McCleery from the University of Florida, trapped marsh rabbits (Sylvilagus palustris) and then fitted 95 healthy adults with a 28-gram radiotransmitter that has a built-in mortality sensor. They released the small, brown rabbits to one of three sites: two areas in the park (where pythons are established) and a control site outside the park where pythons haven’t been observed.
"We specifically chose the marsh rabbits to study because it's a critter that breeds all the time, and you shouldn't be able to eat enough of it to wipe it out," McCleery tells the Los Angeles Times. That makes its disappearance particularly distressing. These rabbits should be common throughout the southeastern U.S.
The team radiotracked the rabbits at least once every two days using handheld receivers, and they examined each carcass or mortality site for the cause of death. Predation by pythons and other reptiles were distinctive since they eat the rabbits whole, and the transmitter would then be inside the snake or alligator. They also tested for Burmese python DNA on the fur of two predated rabbits.
By the end of the year-long experiment, two of the rabbits survived (and likely created some bunnies), and the team was able to document 68 rabbit mortalities. In the park, pythons accounted for 77 percent of the deaths. For comparison, in the control site outside of the park (below, b), no rabbits fell victim to pythons: 71 percent of the mortalities were classified as mammal predations. Researchers came to that conclusion based on scats and tracks, bite marks, bone breaks, opening of the body, and the removal of organs.
In 2012, the U.S. Fish and Wildlife Service banned the import of four pythons, including the Burmese python. "A lot of really smart people have been trying to figure out how to solve this problem and there are still plenty of things we can try," McCleery adds. "We are not done yet."
Images: U.S. National Park Service (top, middle), R.A. McCleery et al., Proc. R. Soc. B: 2015 (bottom)