The snakes were former pets that were either released by their owners once they got too big or escaped after Hurricane Andrew wrecked their homes or pet shops in 1992.
A new report published in the Proceedings of the National Academy of Sciences says there has been a decline of as much as 99 percent in the number of medium-size mammal sightings in some areas where thousands of pythons are known to be slithering about.
"The magnitude of these declines underscores the apparent incredible density of pythons in Everglades National Park," said lead author Michael Dorcas of North Carolina’s Davidson College.
Researchers say foxes and bobcats could have disappeared as the snakes ate them, or they could have starved due to the disappearance of small prey like rabbits that were consumed by the snakes.
Efforts to remove the invaders have achieved only meager success.
Tens of thousands of Burmese pythons are believed to live in the Everglades, but only about 300 to 400 of them are being removed by hunters and wildlife officials each year.