In the menagerie of human mythology, mute swans occupy a special place. Across millennia they've symbolized transformation and devotion, light and beauty. Now a plan to eradicate the birds from New York has made them symbols of something else: a bitter and very modern environmental controversy.

The debate swirls around a host of prickly questions. Are mute swans rapacious destroyers of wetlands, or unfairly demonized because they're not native to this area? Are some species in a given place more valuable than others, and why? Which deserves more protection, the animals that inhabit our landscapes, or the ones that might thrive in their absence? The answers depend on whom you ask.

"Mute swans always attract controversy, and tend to polarize people. And, as with most things, the truth probably lies somewhere in the middle," said waterfowl researcher Chris Elphick of the University of Connecticut. "The real issue is that there are no simple answers."

Mute swans are not native to North America. New York's population descended from escapees imported for ornamental gardens in the late 1800s. Weighing up to 40 pounds apiece, they can eat 10 pounds of aquatic vegetation daily. In their absence, that food might be eaten by native wildlife. Mute swans are also aggressive during nesting season, and have been blamed for attacking ducks and pushing out other waterfowl.

Late in January, New York’s Department of Environmental Conservation issued a draft of a plan to reduce New York’s wild mute swan population to zero by 2025. Nests and eggs would be destroyed; a few adults might be sterilized or permitted to live on in captivity, but the rest would be killed.

"They are large, destructive feeding birds, and as much as they are beautiful, they can wreak havoc on the underwater habitat that a lot of other fish and wildlife depend on," said Bryan Swift, a DEC waterfowl specialist and lead author of the plan. "We have an obligation to sustain native species. The question then is, 'At what level?' But in the case of introduced species, I don't think we have that same obligation."

That's one way of looking at it. "We have so little opportunity to experience wildlife in New York City," said David Karopkin, director of animal advocacy group GooseWatch NYC, "and now they're targeting the most beautiful animals that we do have. The fact of the matter is, they're part of our community."

Were mute swans not so beautiful, the plan might not have caused much outcry. But unlike other animals tagged as invasive and pestilential — like Burmese pythons, feral hogs, and snakehead fish — mute swans are widely beloved. For people who live near wetlands around Long Island and the Hudson Valley, where most of New York's mute swans live, they're also a part of everyday life.

To their defenders, the fact that mute swans are non-native carries little weight. As one Queens resident told the New York Times, “If they were born here, they should be considered native by now.” And some think the swans' heritage is being held arbitrarily against them.

"If Mute Swans were native to North America, they would not be viewed negatively by state wildlife agencies," said ornithologist Don Heintzelman, an author of bird-watching field guides who is working with Friends of Animals, a New York-based animal advocacy group, to oppose the plan. Complaints that that mute swans harm other wildlife "are greatly overblown," he said.

Over the last few years, some scientists have argued that non-native species are unfairly persecuted, their negative effects too frequently assumed rather than conclusively demonstrated. In fact, the evidence for ecological harm by mute swans is somewhat mixed.

In a review of mute swan impacts on wetlands published this month, French ecologists said the swans sometimes attacked other birds — but sometimes they did not. Likewise, their feeding habits sometimes damaged aquatic plant communities, but not always.

"The consequences of mute swan presence may strongly differ from an ecological context to another, so that no simple rule of thumb can be provided," wrote the researchers. They did, however, say that the risk of mute swans reproducing prolifically and out-competing other species could justify their eradication from North America "as a safety measure."

Swan defenders say the need for a safety measure is far from clear. In Maryland's Chesapeake Bay, where a controversial mute swan eradication program was enacted in 2003 to help restore aquatic vegetation, the U.S. Fish and Wildlife Service previously said that swans had likely done little harm to seagrass beds.

"That's a good example of how the science on this is incomplete," said Brian Shapiro, New York state director of the Humane Society of the United States. Whatever effects New York's 2,200 swans may have, he said, is a drop in the bucket compared to human-generated pollution and habitat destruction.

But scientists argue that the effects would be much more severe if mute swan populations grow as anticipated, and more difficult to control. "The dilemma wildlife managers face is that if they wait until there is no doubt that native species are being adversely affected, it will be much, much harder to do anything about it," said Elphick. And although human impacts on habitat and native wildlife are undoubtedly greater, mute swans could consume a disproportionate amount of resources.

"Introduced species are a major cause of extinction and biodiversity loss, and the concern is that if they're not controlled then we will see the world's biota become much more homogenous," Elphick said. "Mute swans are just one example of many."

Mute swans' lives are not the only ones that matter, said Michael Schummer, a waterfowl ecologist at the State University of New York at Oswego. People care so deeply about mute swans because they're aware of them — but if they paid more attention to birds they impact, they might care about those as well.

A tundra swan in Ulmstead Point, Maryland. Image: Emily Carter Mitchell/Flickr

Migratory tundra swans, said Schummer, which fly every year between the Arctic and southeastern United States, rely on those same wetlands. If they land after a 1,000-mile flight and can't find a meal, it's a disaster. "But maybe people don't recognize that, or even realize that there's a native swan that migrates," Schummer said.

Swift noted that two dozen other duck species share the mute swans' habitat, yet people are frequently unaware of them. And unlike migratory birds, mute swans stay in the same locale year-round, feeding through the growing season. "It's like pulling up corn every time it sprouts," Swift said. "It has a much more lasting and damaging effect."

"I value the long-term stability of the ecosystem for the individuals that live within it," said Schummer, who supports the DEC plan. "When that is threatened by one species, and the well-being of the collective is at risk, then something needs to be done."

Given these competing and deeply-entrenched views, a compromise seems unlikely. Yet there may yet be a middle ground, says behavioral ecologist Marc Bekoff, a pioneer of an emerging discipline called "compassionate conservation," which tries to balance species- and population-level considerations with the well-being of individual animals.

"The take of compassionate conservation is that individuals count. Our motto is, 'Do no harm.' In this case, it would be worth pursuing every single possible non-lethal alternative," said Bekoff. Rather than killing mute swans outright, as is happening in Maryland, wildlife managers could try to prevent eggs from hatching by shaking them or spraying them with oil. Swans might also be sterilized.

These are just possibilities, said Bekoff. What matters is that people try to find alternatives to killing. In the end, non-lethal measures may prove to be the only control palatable to a public that's come to adore swans, whatever their impacts.

Of course, non-lethal methods tend to be more expensive than killing. Volunteer assistance from the public may be required. Friends of Animals director Edita Birnkrant said her group would oppose such measures, while Shapiro said the Humane Society would be open to helping.

"We want there to be a focus on non-lethal management," Shapiro said, noting that Humane Society volunteers have helped to non-lethally control Canada geese by scaring them with dogs or shaking their eggs. "We'd like to have a dialogue on this."

Swift said he's open to such a discussion, and would welcome the help. "There's definitely room to work on this issue," he said. Following public outcry, he's also considered treating New York's mute swan populations differently. Those around Lake Ontario, which are growing by about 13 percent every year, might be eliminated, while Long Island's swans could be managed to balance their impacts with public sentiment.

"I can't speak for the whole agency," Swift said, "but I'm certainly going to entertain that idea and discuss it with other people involved in management."

New York's DEC is accepting public comment on the plan until February 21st. If things go their way, those non-native swans might just become fully naturalized ecological citizens.