Invasive Argentine ant is no picnic.

Ant problems are the number one reason people call in pest controllers, and the most troublesome ants are exotic species that have hitchhiked into and around the United States. For other insects, exotic ants are more than a pest. Native ants commonly disappear when the aggressive, competitive newcomers move in.

Researchers are now documenting the effects of the invading ants higher up the food chain. At last week's meeting of the Ecological Society of America in Albuquerque, Andrew Suarez reported that the Argentine ant is forcing a dramatic change in the diet of the homed lizard, once common but now declining in Southern California.

In its natural habitat, the homed lizard feeds almost exclusively on native harvester ants. Suarez and Ted Case, biologists at the University of California, San Diego in La Jolla, set out to see whether the Argentine ant's steady expansion from urban lots and farms into natural areas is affecting the lizard. Since its arrival in New Orleans at the turn of the century, the ant (Linepithema humile) has traveled to points east, west, and beyond (SN: 11/7/92, p. 314).

The researchers studied lizards both in the lab and in the field. They offered caged lizards a choice of native or Argentine ants. From three field sites, they collected droppings for evidence of what the lizards were eating.

The Argentine ants are either unappetizing or too small to be worth much effort. "We've never found homed lizards eating them in the wild," says Suarez, "and in laboratory conditions, homed lizards always prefer native ants over exotic wits."

In areas where Argentine ants have displaced natives, the lizards eat mainly beetles. Suarez says that switch probably contributes to the decline of the lizard, which is also losing ground to development. "When diet changes that drastically, it has to have a negative effect."

Ecologist Allison Alberts of the San Diego Zoo agrees. Understanding such "cascade effects" helps in the management of remaining natural areas and is "extremely important," she says--though the findings are usually grim.

"Working with invasion biology is very depressing," says Suarez. "That's why we have so many happy hours."