Pythons are killing off predators in Everglades, leaving rats to thrive

FORT LAUDERDALE, Fla. — If you don’t like rats, you might have more in common with invasive Burmese pythons than you think. A new study by the University of Florida shows that pythons in the Everglades are killing off predatory mammals such as foxes and bobcats and otters, but not depleting ample cotton rat populations.

Pythons, brought to Florida via the exotic pet trade in the 1970s, have thrived in the wild, establishing breeding populations, growing to as large at 18 feet, and wreaking havoc on the Everglades ecosystem. They’re also expanding up to Lake Okeechobee.

You’d think that adding a lethal predator like the python to the mix would bring down rat populations, but it hasn’t. In some instances, rat populations have actually increased.

“Mammal communities in python-invaded portions of the Greater Everglades Ecosystem are increasingly dominated by cotton rats and other rodents,” the study stated.

University of Florida mammalogist Robert McCleery, one of the researchers on the study, explained that cotton rats “are always a large part of any grassland system in the southeast, but in the Everglades, cotton rats and other small mammals … are now practically the entire mammal community.”

This can harm both ecosystems and people. The study states that “cotton rats are reservoirs for zoonotic viruses such as the Everglades Virus (EVEV) and hantaviruses.” Huntaviruses can lead to a severe, sometimes fatal, respiratory disease, and is typically passed from rodent to human, not human to human, the Centers for Disease Control and Prevention say. With less mammal diversity in South Florida, cotton rats can “amplify the presence of these diseases,” the study says.

Researchers from the University of Florida set out into the grassy swamplands of South Florida to find out how the invasive serpents were making room for rats. To do so they released radio-collared cotton rats into two wilderness Everglades areas — one, near Homestead, with lots of pythons and one at the Fran Reich Preserve west of Boca Raton, with almost none.

When they compared what killed the rats, the results were stark.

They then tracked the rats until they died — each collar had a mortality detector that signaled when the animals stopped moving for a long period of time. Then they located the collars to decipher the rats’ fate.

“Sometimes the snakes regurgitate [the collar], sometimes it’s in the poop, other times we actually find the snake,” McCleery said. “We caught, over two years, eight or nine pythons just with cotton rats [wearing trackers].”

Usually the researchers had to do a bit of poop forensics. If they found the collar in snake poop, they used DNA testing to determine it was from a python.

“An owl or red shouldered hawk or something like that — they kind of rip off the fur. So you’ll see the fur strewn about, and then you see some pecks on the collar, you see a feather or two and some whitewash [bird poop].”

McCleery said mammals will eat the whole rat and there will be bite marks on the collar once the bobcat or fox poops it out.

On one occasion, things got weird. “We knew the tracker was in a snake,” McCleery said, “but we couldn’t catch the snake — it wasn’t a python, and then the snake was eaten by an alligator and we saw it swimming down the canal.”

Predatory mammals have gone missing

McCleery and his team found that when pythons show up, foxes, bobcats and coyotes lose. In the area with low python populations, mammal carnivores accounted for 35.7% of the rat deaths, but in the area with a high python population, mammals accounted for only 10.8% of the rat deaths.

Pythons killed none of the rats in the low-python area, and 16.2% in the high-python area.

Birds, such as hawks and owls, killed the rats at about the same rate as mammals in the low python area (35.7%), but picked up the rat slack in the high python area, killing almost half the rats.

Alligators and native snakes didn’t seem as affected by the pythons, and ate around 25% of the rat kills in each spot.

The takeaway, says the study, is that the rats survive at about the same rate regardless of pythons, but that pythons change who’s around to kill them. “We found that cotton rats had little risk of predation from mammalian carnivores on our site with high python occurrence,” said the study. In other words, there just aren’t many mammals left to eat the rats, but hawks and owls pick up a bit of the slack.

Though not part of the study analysis, some experts say that the pythons prefer larger mammals as prey because they offer more calories. Sure they’ll eat 10 rats if they have to, but would prefer to eat one big meal, like a fox, bobcat, possum or baby deer.

Researchers at the Conservancy of Southwest Florida, which tracks pythons, said that hatchlings eat mice and rats, but quickly move up the food chain. They found a 10-pound python with a 6-pound white tailed deer fawn in its stomach, and routinely find adult deer remains inside pythons of 100 pounds or more. White tailed deer, they said, are the primary prey base for the endangered Florida panther.

Previously, researchers had wondered what caused fox, bobcat and coyote populations to plummet in the Everglades — was it because pythons ate their food, or ate them? This study and others indicated that cotton rats’ high birth rate means there’s plenty of food for foxes and bobcats, so the cause of their demise is likely pythons.

Consequences

McCleery said there are two significant consequences to pythons promoting rat dominance.

One is that because cotton rats are always present, there’s always going to be something for the pythons to eat — they won’t eat themselves out of a prey base. The second thing, he said, is that globally we are replacing larger animals with smaller ones. “There’s all these things that [larger] mammals do — they do seed dispersal and nutrient cycling and soil aeration, all these things, and now all we have are these cotton rats. They don’t do all the functions that we need, that are lost.”

The one positive, McCleery said, is that the rats have proven to be so resilient to the invasive pythons that if we can ever recover mammals such as foxes, bobcats and mink, they’ll have plenty of food.