The Big Squeeze: Pythons and Mammals in Everglades National Park
The wet, subtropical wilderness of Everglades National Park is home to a diversity of Floridian wildlife, but one invader is causing severe changes in these native animal populations. Many of the park’s mammals are declining dramatically as a result of invasive Burmese pythons, according to a recent study by U.S. Geological Survey scientists and partners. Mid-sized mammals such as foxes, rabbits, and raccoons that were previously populous in the Everglades are the most severely affected. USGS scientist and co-author Robert Reed to discusses the Burmese python situation and what these mammal declines mean for the Everglades ecosystem.
The Big Squeeze: Pythons and Mammals in Everglades National Park
Marisa Lubeck: Welcome and thanks for listening to this episode of CoreCast. I'm Marisa Lubeck.
The wet, subtropical wilderness of Everglades National Park is home to a diversity of Floridian wildlife, but one invader is causing severe changes in these native animal populations. Many of the park’s mammals are declining dramatically as a result of invasive Burmese pythons, according to a recent study by U.S. Geological Survey scientists and partners. Mid-sized mammals such as foxes, rabbits, and raccoons that were previously populous in the Everglades are the most severely affected.
I'm here with USGS scientist and co-author, Robert Reed, to discuss the Burmese Python situation and what these mammal declines mean for the Everglades ecosystem. Thanks for talking with me, Bob.
Robert Reed: Sure thing.
Marisa Lubeck: What's the extent of the problem and why are these mammals still vulnerable?
Robert Reed: Well, the general findings were that a number of mid-sized mammal species in the Everglades appear to have declined precipitously over the last eight years as compared to surveys done in the mid-1990s. Species that were declining the most were mid-sized animals like raccoons, opossums and marsh rabbits. And then some species, such as foxes and bobcats, weren't seen at all in recent years.
The spatial scale includes a large proportion of the Everglades and so we have two main aspects of the study. One was temporal or time-based. We repeated a study that was done in the mid-1990s that consisted of driving roads in the Everglades and just recording the number mammals we saw per kilometer driven. Then we also did a spatial aspect that includes looking at the heart of the area that's occupied by pythons in the southern Everglades.
Before and after showed that mammals had declined a lot since the 90s and the spatial showed that there are very few in the heart of the Everglades where the pythons have been there the longest. There are more on the edge of the python's range where they've only recently arrived and there are a lot more outside the area where we see pythons.
Marisa Lubeck: So does this mean that these animals are at risk of becoming endangered in the area?
Robert Reed: Well, the marsh rabbit is endangered in the lower Florida Keys, but because most of these species are common somewhere else, it's unlikely that they'd be formally classified as endangered under the Endangered Species Act. However, if you define the word endangered more generally as something that's at risk of local extinction, some of these species may qualify for that designation, especially in the southern Everglades.
Marisa Lubeck: Bob, what are the risks to the overall Everglades ecosystem? For example, could other species be indirectly imperiled?
Robert Reed: Everglades is a really complex ecosystem. The potential effects of removing some of these mid-sized mammals are largely unknown. Animals like marsh rabbits were formally really abundant. We've got records where people said they drove down a single levee in the morning and saw over a hundred marsh rabbits. And those same levees with rabbits are basically gone now. The rabbits were certainly an important part of the food lab for a variety of predators. Losing them could make it hard for the other predators to find food.
On the other hand, raccoons are notorious nest robbers. They're egg eaters. So we could see an increase in turtles or other egg-laying animals. We can take a hint on what might happen from the island of Guam where the invasive brown tree snake wiped out most of the island's birds. Many of the native trees on the island are now failing to replace themselves, because their seeds aren't being dispersed by birds.
And it's impossible to walk through the forest on Guam now, without ending up completely coated in spiderwebs from head to toe. Without birds to pick with spiders and to compete with the spiders for insect prey, big or weaver spiders are everywhere. And this is the sort of long-term effect of an invasive snake on the ecosystem that is really difficult to predict.
Marisa Lubeck: What sort of control measures could work in the park. For example, could people hunt and eat the pythons to keep their number sound, things like that?
Robert Reed: There are two main obstacles to trying to get rid of pythons at the scale of Everglades National Park. The first is that the park is a vast area. Thousands of square kilometers. And most of it is largely inaccessible to people. We only have access to a very small proportion of the pythons that are in the park. Most of them are encountered on roads or on levees. Those roads and levees only account for maybe a percent or two of the total area of the park.
The other problem is that pythons and snakes, in general, are generally hard to detect. I've been in the Everglades with three or four other people, standing in a circle that's maybe 6 or 8' in diameter, we're in a foot of water and between us there's a 12-foot Burmese python that has a radio transmitter inside it. We know it's there, but we cannot see it. So, there are obviously obstacles, but that doesn't mean that all hope is lost. We may be able to prevent their spread to the Florida Keys. We may be able to prevent their spread farther north and we may be able to protect locally important ecological resources.
For example, if we've got endangered wood storks that are nesting communally in a small area, perhaps we can deploy traps and other snake control tools to make sure that those birds are not being impacted by pythons. Local control may be possible.
Marisa Lubeck: Have any such control measures that instituted yet?
Robert Reed: Well most of the snakes that are currently being taken out of the Everglades are taken from roads and levees. And so, it's possible that we're at least having a local effect on population densities. About 1,500 pythons has been taken out of Everglades National Park and the areas around the park over the last decade alone.
Last year we conducted a trap trial where we put up 60 traps for about 60 days. Unfortunately, we didn't catch many pythons but we did catch some. And then suggest that if we continue to improve the traps, our success in controlling the snakes will continue to increase.
Marisa Lubeck: Speaking of remoteness, is it possible that the problem could actually be affecting more species than your findings indicate, given that it's so hard to get to most of the areas within the park to conduct your research?
Robert Reed: The results of our study only applied to mammals that could be seen on roads at night. There are also a number of mammals that rarely cross roads, especially the aquatic animals. And so, we don't detect them. And yet, animals like that might be at severe risk of being eaten by pythons. The round-tailed muskrat is perfect prey size for medium-sized python and the pythons spend a lot time in the water. Very few round-tailed muskrats were detected in recent mammal surveys in the Everglades.
But, they showed up with increasing frequency in python diets in the 2000s before they started decreasing in frequency in the python diets. That suggests that they may be declining as well, but we don't have good evidence based on observations or real population sizes yet. So, that's one of the main goals of future research is to start concentrating on some of those other species to determine what's going on with their populations.
Marisa Lubeck: Are human visitors to the park at risk?
Robert Reed: Chances of a human being attacked by a python in the park are extremely low. Even in their native range, Burmese pythons are not known to attack people very often. In the U.S., there have been deaths attributed to the Burmese Python, but all of those are from captive animals. There have been no attacks on people in the wild in the U.S.
Marisa Lubeck: These pythons are invasive, meaning they aren't native to the Everglades. Where did they come from?
Robert Reed: There's been a lot of controversy about the origin of the pythons that are in the Everglades. Some folks are convinced that hurricane Andrew in 1992 is the source of the population, because we know that there were python breeder facilities that were knocked over by the hurricane. On the other hand, the first pythons observed and the apparent founding population was way down in the southern Everglades, roughly in the area that's at the end of the main park road. That area's almost 50 kilometers from where are the import businesses were.
We will never know the answer to this question for sure. We do know that the snakes were ultimately brought in to the country, intentionally, for the pet trade. How they went from the pet trade into the wild is not as important. We know why they were brought in originally.
Marisa Lubeck: And when did the pythons start showing up in the Everglades?
Robert Reed: The snakes were probably established in the park prior to 1990. But, they probably took a while to build up their numbers and attain densities that allowed us to start seeing them regularly. Starting in the 80s, there were a couple found, in the 90s, there was a trickle of animals and then in the 2000s, that trickle turned into a flood.
Marisa Lubeck: Bob, as the scientist attempting to answer these questions about a complicated python problem in a very complicated ecosystem, what lessons have you learned along the way?
Robert Reed: One of the most depressing facts to those who are studying invasive reptiles around the world is that there has never been an established and wide-spread population of invasive reptile that has been eradicated. Eradication is really difficult and requires a lot of resources.
And that points to the need for prevention. If we can prevent these things from becoming established or becoming wide spread or abundant, then we may have a chance of preventing future scenarios like what we're seeing with Burmese pythons in the Everglades.
Marisa Lubeck: Thanks for your time, Bob.
Robert Reed: Sure thing, Marisa.
Marisa Lubeck: CoreCast is a product of the U.S Geological Survey Department of the Interior. I'm Marisa Lubeck. Thanks for listening.