Invasive weed removal harms native bird in California

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Invasive weed removal harms native bird in California
ENLARGE
brown and white, long-beaked shorebird stands on edge of water, spiky green plant bunch on either side of it
California clapper rail alongside invasive hybrid Spartina at Garretson Point in San Francisco Bay, California. The rail, once reliant on a native Spartina, changed its preference after the alien Spartina invaded its habitat. Wildlife managers hope to re-orient the bird to native Spartina before they finish eradicating the alien plant.
Image courtesy Robert Clark

Ecosystem restoration — the effort to return habitat close to its “natural” condition — often starts with the wholesale eradication of invasive plants, which can dramatically change ecosystems. But in 2011, removals of an invasive cordgrass in San Francisco Bay ran afoul of federal wildlife officials who were concerned that the plant was providing essential habitat for an endangered bird called the California clapper rail.

And because protection of the endangered rail is mandated by the Endangered Species Act, an eight-year eradication effort that had already removed about 90 percent of the invasive Spartina ground to a halt.

Since being introduced to the Bay by the Army Corps of Engineers in 1977, invasive Spartina has spread widely through the wetlands. Ecologists say both the invasive Spartina and the hybrid it forms with native Spartina are damaging the ecological integrity of the Bay, a body of water already noted for numerous invasive plants and animals.

A balancing act

Today, Science reports on a computer model aimed at meeting the conflicting goals of preserving the clapper rail while eradicating the invasive plants. The model aims to optimize ecological benefits while minimizing cost, and to balance the cost of damage from the invasion with the costs of eradication and restoration.

ENLARGE
red helicopter spraying low over wetlands. San Francisco Bay in background
A helicopter sprays herbicide on invasive Spartina cordgrass in San Francisco Bay, California. The eradication was halted because it was harming an endangered bird.
Courtesy Drew Kerr, Invasive Spartina Project

To simplify the calculations, the scientists assumed that Spartina will grow at an exponential rate, says Alan Hastings, professor of environmental science and policy at the University of California, Davis. Although exponential growth cannot continue forever, “At least early in the invasion, that’s not a bad assumption,” says Hastings, who oversaw the computer model’s development.

Before eradication began, “carpets” of invasive Spartina covered 800 acres along the bay shore.

ENLARGE
newly planted Spartina foliosa seedlings along a barren shoreline
Restoration planting of native Spartina foliosa as habitat for the California Clapper Rail in San Francisco Bay.
Image courtesy Whitney Thornton, Invasive Spartina Project

Invasive Spartina changes the ecosystem as sediment gathers on its roots and slows water flow. That can lead to floods and possibly dredging, and alter both the shoreline and views from bay front properties.

Invasive species commonly fail to feed or house native species, but the Spartina case is more complex, says first author Adam Lampert, a post-doctoral fellow at Davis. The invaders “provide habit for the endangered rail, but there are many other native species for which invasive Spartina does not provide suitable habitat.”

New light on invaders

“One idea of the study was to suggest a cost-effective management strategy,” says Lampert, who notes that the pause in eradication needed to save the rail could raise overall project costs.

In essence, the model suggests the best way to meet the multiple goals is to allow the replantings of native Spartina that began in 2012 to spread, and then finish off the invaders.

“You have to be patient to let the restored Spartina grow by itself,” Lampert says. “A combination of restoration and waiting is an effective compromise between the need to eradicate [the invasive plant] and the need to conserve the native bird.”

– David J. Tenenbaum

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Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer

Bibliography

  1. Optimal approaches for balancing invasive species eradication and endangered species management,” by Adam Lampert et al, Science, 30 May 2014.
  2. Hey, you calling me an invasive species?
  3. Nature Conservancy interactive maps of spread of invasive aquatic species across the U.S.
  4. Preventing the spread of invasive species in Wisconsin: boat transportation and bait laws