Invasive snails are helping an endangered bird make a comeback in Florida

Ever since the island apple snail invaded the Florida Everglades, an endangered bird species known as the snail kite has recovered from the threat of extinction.

It’s a rare case of a destructive invasive species having a positive impact, said Robert Fletcher, a professor of landscape ecology at the University of Florida who also directs the snail kite monitoring program.

“There’s a lot of concern that these birds are already endangered. Then these non-native snails come in,” Fletcher said. As it happens host Nil Koksal.

Everglade snail kites are eagle-like raptors that depend on wetland ecosystems to feed exclusively on Florida apple snails.

In the early 2000s, a severe drought in the Everglades caused the population of this local apple snail to disappear. And because snail kites rely on them as their sole food source, their numbers have dropped from more than 3,000 birds in the late 90s to about 700 in 2009, according to a 2022 report by conservation organization Audubon Florida.

Birds adapt to new seasons

Cue the non-native apple snail.

After they appeared in the southeastern United States, the snail kite population in Florida rebounded to about 3,000 birds today.

But when the first non-native snails were discovered in 2004, Fletcher said people were concerned that “it would increase the risk of extinction. [the snail kite] and actually push them closer to extinction.”

The main concern is that these non-native snails are up to five times larger than those used by birds to hold their claws and release their bills.

“Scientists quickly noticed snail kites trying to forage on these non-native snails, but generally unsuccessfully. They will drop the snails quite often,” said Fletcher.

Once the non-native snails had spread through much of the Everglades and overtaken the native ones, however, the snail kite population began to rise.

And birds bred in wetlands with invasive snails fared far better than birds forced to live without them as a food source, said Fletcher.

“It’s amazing,” he said. “We don’t know how the birds handle the big snails.”

He and his students have been watching snail kites for years, circling them as they fly from their nests to measure many things, including the size of their bills.

Collected data shows their bills are getting bigger every year to accommodate the baseball-sized snails.

What it means for the local ecosystem

Samuel Chan, an invasive species expert at Oregon State University, said island apple snails from South America may have disappeared and settled in Florida through the pet or aquarium trade. Because this species of snail is more resistant to drought conditions, it can outcompete the native species and take over.

While the recovery of snail kites is good news, it appears to be based almost entirely on the occurrence of exotic snails, Chan said. The bad news, he added, is that native snails haven’t made a comeback.

“I think the birds tell us something about what they don’t have in their habitat,” he said. “The reality is that these invasive species fill the hole, but it will have consequences in other layers of the ecosystem.”

Chan calls non-native apple snails a “substitute” food source for snail kites, which have adapted to feed on them as a survival strategy.

“But the surrogate host is not the solution. It doesn’t just save the snail kite; it also does other things.

Fletcher admits it’s a complicated situation and he also has concerns about the species’ potential long-term impact on the Everglades.

While the island’s apple snails may save snail kites, they are known to destroy them, Fletcher said. It has decimated agricultural fields and feasted on aquatic vegetation in many areas of the world.

“This is difficult,” he said, “because depending on the situation, there can be many consequences when a new species comes to the area – but sometimes there are benefits.”

Fletcher and Chan also cite Florida’s apple snail population as a barometer for the health of local ecosystems.

For the story of the return of the snail kite to have a happy ending, Chan says, indigenous diversity and resilience are needed. If endangered birds continue to rely solely on non-native invaders, they may have a devastating effect on the ecosystem.

“If you want to return native snail kites, you must return native snails,” he said.