The systematic surveys, called transects, don’t necessarily mean that the animals have disappeared completely. But the results are a measure of “relative abundance” and that’s considered a good indicator of trends across the entire system, Dorcas said.
BY CURTIS MORGAN | MCCLATCHY NEWSPAPERS
MIAMI — While Burmese pythons boomed in the Everglades, populations of key native animals headed in the opposite direction — their numbers crashing to near-zero in the case of bite-sized creatures such as raccoons, opossums and marsh rabbits.
Those are the findings of a sobering study published Monday, the first by scientists to assess the impact the giant constrictors have had on the complex food web of the Everglades. A word sums it up: carnage.
Scientists have firmly established what pythons eat, pulling the remains of just about everything that walks, crawls or flies in the Everglades from the bellies of captured snakes over the past decade. But just how much a snake population of unknown size has been eating has largely been a guess, at least until now.
According to the study published in the science journal Proceedings of the National Academy of Science, the probable answer is, once again, just about everything.
“When we actually did the calculations, we were astonished by the magnitude of the declines,” said Michael Dorcas, a biology professor at Davidson College in North Carolina and the study’s lead author.
The peer-reviewed study, based on nocturnal field surveys conducted before and after the python invasion, suggests a collapse of the park’s mid-sized mammal populations since the mid-1990s and points to the exotic snake as prime suspect.
Raccoons — once so abundant that park managers had to post signs warning campers to safeguard food from roaming hoards of the wily thieves — dropped 99.3 percent. Opossum sightings fell 98.9 percent. Observations of bobcats, foxes and deer also all fell precipitously, though researchers believe those declines could be less from snakes eating them and more from the indirect effect of pythons reducing the amount of prey to go around.
And marsh rabbits, small brown bunnies frequently seen foraging along roads in the park’s pre-python past, didn’t appear at all during the surveys.
Frank Mazzotti, a wildlife ecology professor at the University of Florida, likened the study to a grand jury investigation — a damning initial finding that needs more research to refine and confirm.
“We examined all the evidence and there is enough to indict pythons but we haven’t gone to trial yet,” said Mazzotti, one of 11 university and federal government researchers who co-authored the study.
Other factors, such as changes in Everglades water levels that can affect the food chain at microscopic levels may have contributed to the mammal decline as well, he said.
Still, the Obama administration pointed to the findings as more justification for the decision earlier this month to ban the import and interstate sale of Burmese pythons, two types of African rock pythons and yellow anacondas. The decision to declare them “injurious species” had been criticized by reptile breeders and collectors, as well as some Republican lawmakers who contend the measure would kill jobs in the cottage industry of constrictor breeding.
The study suggests a strong link between the rise of the snake and the fall of the bunnies and other mammals, finding little support for other possible causes. Between 2000 and 2010, a period when python captures soared from two to nearly 400 a year, no diseases swept the mammal population and there were no big losses of habitat or other major environmental changes that might explain the declines, the study found.
Besides the coincidental timing, researchers also found a pattern across the landscape, with the greatest losses in the southern portion of the park in and around Flamingo, where the python infestation has been heaviest. Mammal populations are strong on the park’s fringes or in adjacent areas.
“Our data are consistent in any number of ways with python as the primary reason or maybe the only reason, either directly or indirectly,” said Dorcas, author of the book “Invasive Pythons in the United States.”
Scientists can only guess at the population of Burmese pythons in the vast expanse of the Everglades, estimating the number in the tens of thousands, even after a record freeze in 2010 that may have knocked the population back by half or more. A month after the cold snap, biologists captured a 16-foot female in a nest with more than 100 eggs.
To assess the impact of pythons on wildlife, researchers counted live animals and road kill during sunset-to-sunrise surveys from 2003 to 2011, driving slowly along the main roads in Everglades National Park and counting the most common and easy-to-spot animals. They conducted 313 surveys, covering some 39,000 miles, as well as 26 surveys at sites along the park border and in adjacent federally protected lands. They compared the results to similar surveys conducted in 1996, before pythons had spread across much of the Everglades.
The changes in 15 years were startling. Before pythons, a raccoon would show up every 30 miles or so, according to the study. In more recent years, researchers would go nights without seeing a single one.
The systematic surveys, called transects, don’t necessarily mean that the animals have disappeared completely. But the results are a measure of “relative abundance” and that’s considered a good indicator of trends across the entire system, Dorcas said.