In the quiet waters of Taylor Slough, once home to abundant communities of crayfish and small fish, silence has crept in. What should be a buffet for nesting wading birds has thinned into scarcity — and biologists know the culprit.
Swamp eels.
First detected in South Florida in the early 2000s, the sleek, snake-like fish — likely introduced through live food markets from Southeast Asia — now glide through the Everglades marshes as if they belong. Their appetite for crayfish and other small fish, the same prey wading birds rely on to feed their young, has reshaped the food web in unsettling ways, posing an unexpected threat to the multi-billion-dollar effort to restore the Everglades.
Nathan Dorn, aquatic ecologist and associate professor of biological sciences at FIU, has tracked changes in small animal communities in the Everglades for more than 18 years. His research began with water-level shifts caused by restoration projects, but startling data revealing the near disappearance of crayfish and other small fish in Taylor Slough kept pointing him in a different direction.
"These were numbers and densities of particularly the crayfish and a couple of fish that we'd never seen before," Dorn recalls. "They were so low, they were basically absent from the system."
Attention then turned to the eels.
Electrofishing surveys confirmed they had become a significant part of the large fish community — and a new predator in the system. In a recent study, Dorn and his colleague, FIU postdoctoral associate Matthew Pintar, uncovered how much of the prey base for seasonally nesting wading birds — an important indicator group for Everglades restoration — has been lost. According to their research, it accounts for an 80% loss in Taylor Slough.
Historically, the Everglades have been the epicenter of wading birds nesting in the southeastern United States. Species like the white ibis, great egret, little blue heron and tricolored heron depend on the wetlands to feed their chicks primarily with fish.
"If we've rerouted water in Florida to recapture even some of the functions of historical Everglades, we expect to see relatively high numbers of nesting birds on a semi-regular basis in Everglades National Park," Dorn said. "But the birds are fundamentally limited in their nesting efforts by food availability."
The effects reverberate through the marsh. With the loss of primary food for wading birds, feeding relationships have shifted, taking with them biodiversity and community structure.
From a scientific standpoint, this is called an ecosystem collapse — not a total breakdown, but a major shift in the food web.
"It's not that the Everglades is a completely collapsed ecosystem," Dorn said. "It's just a rearranged ecosystem in terms of its food web that is taking on a whole new characteristic just by a change in a top predator introduced unthinkingly."
In their native range, stretching from China to Indonesia and Bangladesh, swamp eels are known for resilience — a trait that gives them an advantage in Florida. Unlike any other fish in the Everglades, they withstand drought by sitting in mud and breathing air through modified lungs until the water level returns. When the marsh floods and baby crayfish and fish spill into the water, the eels are already lying in wait.
"That's not something that we think is a normal phenomenon," says Dorn. "It's a really novel predator with some novel traits and it's a new sort of loss factor for our fish in the system."
While Dorn thinks some of the native animals might adapt to the presence of this new predator, it is still unknown whether balance can be restored.
"They don't behave like any of our native fish, yet they control everything else. Now you're depending on the rest of the system to adapt to that animal, but it's probably not going to be the system or the community that we were anticipating we'd get," he said.
Plenty of species have slipped through the Everglades with little lasting harm, but swamp eels are proving far more disruptive.
"You almost have to take an invasive or introduced species as guilty until proven innocent, rather than the other way around, because you're not going to get them out without an incredible amount of effort."
Prevention needs to be the first line of defense, according to Dorn. If the system doesn't adapt, restoration goals would have to factor in the uninvited guests — so efforts align with where the Everglades is heading, not just where it has been.
Photo by Jamëg Mungen '23, biological technician
