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Are invasive species really that terrible?

 

The West’s approach to managing invasive species has, for the most part, been a straightforward one: eradicate them swiftly and at all costs. Spray ‘em, poison ‘em, net ‘em, douse ‘em with fungus, and, when all else fails, eat ‘em – whatever the method, the important thing is that the invader is sent packing.

But as invasives embed themselves deeper into the fabric of the West, tearing them out becomes more challenging – and even, perhaps, damaging to ecosystems that have adjusted to the arrivistes. Is it time to rethink how we treat the aliens in our midst?

That’s the implication of a new study in the journal Science, which suggests that a scorched-earth policy can do as much harm as good. The study’s focus is Spartina alterniflora, or cordgrass, a salt-tolerant plant native to the East Coast that, in the 1970’s, was introduced into San Francisco Bay by the Army Corps of Engineers to stabilize marshlands. Inevitably, Spartina ran wild, displacing native grasses, choking waterways, and transforming rich mudflats into dense meadows. In 2005, the Invasive Spartina Project began eradicating the cordgrass with herbicide; by 2011, the group had wiped out 92 percent.

A native California clapper rail takes shelter in a stand of invasive Spartina alterniflora. Image courtesy of Robert Clark.

Everything was going according to plan until scientists made a disturbing discovery: populations of the California clapper rail, a federally-listed endangered bird native to the bay, had declined by nearly 50 percent during the Spartina removal program. The bird, it turned out, was using the cordgrass for nesting cover, just as it had once taken shelter in native grasses. Destroying Spartina was robbing the rail of its final stands of habitat. The U.S. Fish and Wildlife Service ordered a halt to the eradication.


The trick, then, is to somehow attack the Spartina without evicting the rail. But can the San Francisco Bay’s managers really walk such a fine line?

Alan Hastings, author of the Science study and professor of environmental science and policy at the University of California, Davis, says yes. Via a computer model, Hastings and his collaborators determined that the ideal approach to cordgrass would entail removing the invader only gradually, while simultaneously planting clumps of native grasses that, over time, would grow into suitable clapper rail habitat. Not until the native meadows had thoroughly established themselves would managers finish off the invasive Spartina.

“You have to go much more slowly than you would if you were thinking just about a single species,” Hastings explains. And that cautious approach isn’t just applicable to Spartina. Tamarisk, an invasive Eurasian tree that has overrun riparian areas throughout the Southwest, has come to provide shelter for endangered willow flycatchers, which once nested in native trees.

Still, these occasional examples don’t prove that we should flip the switch straight from knee-jerk eradication to unilateral tolerance. Plenty of invasives inflict severe ecological damage. Consider, for example, Yellowstone’s lake trout, an introduced fish that has decimated native cutthroat trout and set off a complex chain reaction resulting in protein-deprived grizzly bears devouring elk calves. In that instance and many others, beating back invasive species is mandatory if we’re going to keep native ecosystems healthy.

But that’s the whole point, adds Hastings: What’s most important isn’t a species’ place of origin, but rather the ecological role it plays. “An invasive species could provide habitat, flood protection, or another positive function,” he says. Hastily wiping out aliens without considering their place in altered, but oft-times still functional, ecosystems can easily lead to unintended consequences – as the clapper rail can attest.

Ben Goldfarb is an editorial intern at High Country News. He tweets @bengoldfarb13.