Exotic Species vs Invasive Species: Importing One Outsider to Battle Another

Forget home-field advantage: When species roam, they can thrive. Free from their own predators and among other species that aren't used to competing with them, invasive species can monopolize resources and alter delicate ecological relationships. One way biocontrol experts have countered invasive-species threats is by bringing in other exotic species to target the invaders. As some past examples show, such tinkering can work, but it also can produce nasty unintended consequences if not done carefully.


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Salt Cedar vs Beetle

Invasive Species: Salt Cedars
Home: Mediterranean
How They Invaded: Brought intentionally to the southwestern U.S. to provide shade and wind breaks, and to stabilize eroding stream banks.
Foe: Mediterranean leaf-eating beetle

When it arrived in the southwestern part of the United States in the 1800s, the salt cedar tree's ability to adapt to dry climates made the plant an excellent windbreak, shade for grazing animals, and support to counter stream-bank erosion. But many of the characteristics that allowed the plant (also called tamarisk) to thrive in these warm areas also allowed it to become a pest, pushing out endemic plants such as screwbean mesquite.

"Perhaps its biggest advantage is that it's extremely salt-tolerant," says Joseph DiTomaso, a weed ecologist at the University of California, Davis. Not only that, but the plant uses salt as a weapon. Salt cedar leaves, DiTomaso says, have glands often coated in white salt that the plant removes from the soil. When it rains, the salt washes onto the soil's surface and makes it difficult for competing plants to grow. "In essence, it's creating a habitat which gives it an advantage," DiTomaso says.

Salt cedar infestations in the 1920s covered about 10,000 acres, DiTomaso says. By 1970, the plant had invaded more than 130,000 acres in the southwestern U.S. and parts of Mexico. But in the late 1990s and early 2000s, after testing natural salt cedar enemies, researchers with the U.S. Department of Agriculture found their champion: They released the leaf-eating beetle Diorhabda elongata, which kills the plant through several years of attacking its leaves.

The beetles worked... but nothing in nature is ever so simple. In 2010, the USDA issued a moratorium on the beetle's release. The reason: An endangered bird called the willow flycatcher decided to nest in the plant. For most tree-dwelling animals, the salt cedar's success meant a reduction in habitat; they preferred the trees that the plant displaced. But the salt cedar's branch architecture, which has some vertical branches similar to the willows, made an inviting abode for certain doves and for the flycatcher.

The beetle defoliates the salt cedar and makes the endangered birds' nests more vulnerable to hawks and other predators. When biocontrol experts first released the beetle, there was an agreement that they would avoid flycatcher habitats, but DiTomaso says the release continued to be controversial because you can never be sure. "The insect is pretty mobile," he says, "and it just took a few years to end up everywhere, even the willow flycatcher habitat."