The Everglades seldom benefit from invasive species. Burmese pythons have unraveled food webs for decades. Asian swamp eels threaten wading birds. Green iguanas damage landscapes and infrastructure. But now, as non-native species of apple snails disappear, the endangered Everglades bird, the snail kite, is paying the price.
Named for its specialized diet of almost exclusively freshwater apple snails, the snail kite is especially vulnerable when that single food source is threatened. After years of drought in the Everglades, the ecosystem's population of the only type of apple snail native to Florida failed to recover, pushing the birds to rely on hardier, non-native species. Now prolonged dry conditions are killing off those invasive snails as well, leaving the kites with little to eat and sending their numbers down again.
That strain is already visible in this year's nesting season.
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"The University of Florida thinks there are only 30 successful kite nests in the whole state of Florida," said Paul Gray, a science coordinator for Audubon's Everglades Restoration Program, a nonprofit conservation organization. "And when you have a population of 3,000 you're trying to sustain — 30 nests is not enough."
The bird species has long ridden dramatic population swings. About 3,000 snail kites lived in Florida in 2001, but a severe drought that year devastated the Everglades and Lake Okeechobee, cutting the population in half. Another drought in 2007 reduced it again to about 750, putting the bird on the brink of extinction.
An unlikely lifeline followed: invasive apple snails. Larger, more prolific and able to survive harsh conditions, they provided a steady food source that helped the population rebound to roughly 3,000 by 2019.
But even those resilient snails are now declining as drought intensifies.
"So just as we thought the kites were going to be okay living off this exotic snail, there are signs they're not necessarily okay," Gray said. "The population estimates were down to about 2,000."
Unlike native apple snails, the invasive species grow to the size of a tennis ball, lay up to 1,000 eggs at a time, reproduce year-round and can survive more than a year buried in mud. Still, they depend on just the right water conditions — a fragile balance, earning them the nickname the "Goldilocks species" from Gray.
Water's central role in bird conservation
The South Florida wetland dynamic is central to the snail kite's survival.
"It's the whole story of South Florida, down here we work on water," said Gray. "I work with a bird conservation organization — what is most of my work? Water. Because that's what supports the birds."
Seasonal cycles of wet and dry periods create the conditions needed for apple snails to thrive, which in turn feeds the birds. But human-built canals, levees and drainage projects have disrupted these cycles, reducing water in critical areas during dry periods.
Gray is also warning that much of the water meant for the Everglades is being diverted away, leaving the wetlands too dry.
"When we drained Florida and built those big canals to the east and west coast out of Lake Okeechobee, that water used to go to the Everglades, and now it's going out to the coast," Gray said. "We're wasting it."
Dependent on wetlands for survival, the snail kite has become a measure of the Everglades' health.
"They are used as an indicator species for how healthy those wetlands are, and for lots of wetland restoration activities that are going across the state," said Ellen Robertson, an assistant professor at the University of Florida in the Wildlife Ecology and Conservation department.
Water levels shape nearly every part of the bird's life cycle, from where it feeds to whether it attempts to breed at all. During dry years, kites often have to abandon nests and search for better conditions elsewhere.
"If conditions are bad in one place, they can switch to another spot," Robertson said. "But if there's widespread drought, like we've seen over the past year, they can't just move to the next wetland, because all of the wetlands nearby are dried out. They are more likely to not reproduce."
“We could lose kites in Florida and we would still be here. But we would also have lost a very special part of Florida.”
Each spring, Robertson is a part of a team that travels by airboat through miles of marsh, wading deep water to monitor nests and using mirrors on poles to check eggs and chicks.
"It's hard work, but it's lots of fun," Robertson said. "It's a beautiful way to see the wetlands and snail kites, and do important work to make sure the population is staying healthy."
With another dry year underway, much of the work is about finding answers.
Scientists are trying to determine how many snail kites remain in Florida — results they expect within the next six months — and why native apple snail populations still haven't recovered after years of decline.
"People come from all over the world to see our birds and water. And the kites are one of those Christmas ornaments that are on your tree," said Gray. "We could lose kites in Florida and we would still be here. But we would also have lost a very special part of Florida."
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