a photo of rose-ringed parakeets in London
In London's St. James' Park, rose-ringed parakeets are a colorful, if noisy, new addition. Toby Melville/Reuters

Madrid has pledged to cull its population of monk parakeets. But once non-native fauna and flora gain a foothold in the city, they can be hard to remove.   

Earlier this month, the city of Madrid announced that it would begin an “ethical” cull of up to 12,000 monk parakeets that had taken up residence in the Spanish capital. Native to South America, the chattersome, bright-green birds construct huge communal nests, often on utility poles, where they can cause fires and power outages. Their nests are so heavy that if they tumble to the ground, they risk injuring those below. The culling program—a combination of hunting with traps and egg sterilization—will cost €100,000.

A similar parakeet eradication campaign was waged in London earlier this decade; culls of rose-ringed parakeets, which are now common in much of Europe, are afoot in France. Meanwhile, in Berlin, it’s a North American transplant—the swamp crayfish—that is causing problems: Authorities announced this month that they had removed 22,000 of the crustaceans from the city’s many lakes this year, where they’d been seen as a threat to local fauna due to their extremely fast reproduction and lack of local predators.

Is Europe in the midst of an invasive species panic? Stories of cities targeting non-native fauna and flora certainly seem to be coming thick and fast. This summer, Britain’s government “declared war on ‘alien’ invaders,” including plants such as skin-inflaming giant hogweed and Himalayan balsam, a pink-flowered annual with a fondness for polluted, nitrogen-rich air that has supercharged its urban proliferation.

A stag in London’s Richmond Park stand among bracken, a native species that has become invasive. (Henry Nicholls/Reuters)

Such non-natives must be driven out, the argument goes, because the competitive advantage they possess over natives can lead to the latter being squeezed out. Sound familiar? At times, the rhetoric around invasive species carries eerie echoes of the language used to demonize human migrants. In Britain, the Himalayan balsam has been labelled a “10-foot thug” by the Daily Mail, a newspaper that not uncommonly uses very similar language to describe asylum seekers. (Events where volunteers slash back the plant from vulnerable riversides are referred to as “balsam bashing.”) This might not hurt the plant’s feelings, but othering something that’s been living in Britain since the 1830s as a menacing interloper still muddies the waters of public discussion.

What’s more, using emotive terms like “invasion” to describe  suddenly flourishing species can misrepresent the true process by which they have arrived in our midst, says Ian Rotherham, a specialist in invasion biology and environmental history at the U.K.’s Sheffield Hallam University. As climate change exerts its will—and once-exotic species relocate to new habitats—potential conflict between struggling natives and resourceful newcomers will intensify. But rather than being protagonists in environmental change, new species are often merely filling a vacuum created by human activity. It’s a situation perhaps too complex to be managed by short-term efforts like culling and clearing.

“In many cases, native species have been removed before invaders arrive to fill the vacant space” says Rotherham. And if those new species flourish as once-well-established ones falter, it can be because we have changed the environment to make it more like their native habitat.

“The landscape is full of nitrogen fertilizer and atmospheric pollution, so non-native species are not coming into a pristine environment, but one we’ve already skewed by climate change and other factors. With urban heat islands, that can create a double whammy.” Meaningfully committing to protecting local environments might thus be less about trying to exterminate a particular species than combating the environmental damage that made their expansion possible.

Rotherham also stressed than the terms invasive and non-native are not synonymous. The number of non-native species that become invasive—spreading at the speed and intensity of an epidemic—is minuscule: “In Britain, we estimate that only 1 percent of non-native species introduced naturalize”—that is, survive and spread without direct human assistance—“while only 1 percent of those naturalized species become invasive.”

After causing problems in the United States, the South American rodents called nutria have started to naturalize in parts of Europe, such as this albino nutria in a river near Frankfurt, Germany. (Michael Probst/AP)

So while the manifestation of tropical transplants like monk parakeets in cities like London and Chicago may attract a lot of notice (especially since they are annoyingly loud), their proliferation is in fact an extremely rare occurrence.

At the same time, native species can also turn invasive, even if they have lived in the area for long periods of time. Take the heathland fern bracken, for example, which grows across a huge area of the world. It’s indigenous to Britain, but has only recently become a problem in the country (and many others) because it is no longer harvested as animal bedding or, as was once common in the Scottish Highlands, to use as thatching. Bracken has thus abruptly graduated from being a supporting presence in British landscapes to an invasive one, even though it’s been around for centuries.

All the above doesn’t mean there’s anything wrong with human-led efforts to clear or cull genuinely harmful plans and animals: A single invasive species can indeed be “capable of destroying economies,” Rotherham says, something that happened as recently as 2015 in Punjab, where a whitefly infestation destroyed two-thirds of the cotton crop. “People can even starve because invasions are not dealt with. It’s not a trivial phenomenon, and after climate change it’s probably one of the biggest issues we face.”

These disasters mainly afflict subsistence agriculture, so they’re not very useful templates for understanding the effects of invasive species in urban areas. Still, there may be plenty of other reasons for trying to control overly assertive species and create space for others. There are up to 3 million meme-inspiring feral hogs roaming Texas, a “Malthusian nightmare” tearing up Dallas parks and golf courses. In New Orleans and throughout the U.S. Gulf states, nutria—chunky water rodents released from fur farms—have damaged levees; now they’ve reached Europe. In the mountains of Wales, rhododendron bushes have crowded out native species and become a host for non-indigenous pests, even if they do look beautiful when in flower. Meanwhile on the island of Guam, the brown tree snake has come close to destroying local bird populations.

The problem is that it can be extremely difficult to eradicate any species that has managed to get a foothold in an ecosystem. “Once the cat’s out of the bag,” says Rotherham, “it’s not going to get back in.” Some species can be controlled effectively, but that process may prove both expensive and essentially never-ending. “If you have to control, you may need to be prepared as a society to pay for this forever.”

Berlin fisherman Matthias Engels poses with “Berlin Lobster”—invasive North American crayfish removed from the city’s lakes and rivers. (Fabrizio Bensch/Reuters)

That could mean that Madrid’s dream of a completely parakeet-free city will prove illusory, and these beautiful and charismatic birds will have to be killed en masse at regular intervals—and at some expense—from now on, unless Madrileños learn how to make peace with their chatty new guests.

For other, more edible species, commercial harvesting can be part of the invasive solution: The North American crayfish Berlin removes in its annual cull have become a seasonal local delicacy—“Berlin Lobster”—which at least means the species now has an indigenous predator in the form of humans.

In other cases, there is faith to be taken in the resilience of nature. A cleaner, less-polluted urban environment can sometimes make its own remedies when faced with large-scale species disruptions. “There is a common pattern in which an invasive species grows exponentially and quite catastrophically, but then its presence subsequently flattens, and the species is then absorbed into the new recombinant ecosystem,” says Rotherham. “That’s a pattern we see time and time again, and cities are where that happens.”

Rotherham’s hometown of Sheffield is already showing this process with a notorious invasive pest known as Japanese knotweed, whose arrival in North America has inspired much hand-wringing. Knotweed installed itself this century along the industrial city’s then-heavily polluted urban rivers. Now that those waterways are cleaner, a long-lost and much-beloved animal has returned: otters. “Otters love Japanese knotweed and [also intrusive] rhododendron. They’re not bothered whether they are native or not—they are just good fodder.”

So now both organisms are thriving, and the new plant is also triggering some other happy biological feedback, Rotherham says. “Under the knotweed, we’re now getting all sorts of ancient woodland species growing under what is a pseudo-woodland canopy.”

Like many an invader before it, knotweed was met with fear and hatred. But given some time and clean place to live, nature seems to be finding a way to put it in a proper, manageable place.

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