HILO, HAWAII--Some of the most beloved critters of the desert, fondly known as horny toads, may become a casualty of invasions of Argentine ants. The coastal horned lizards feed almost exclusively on ants, but they much prefer to munch on the native varieties--exactly the ants that are driven out when Argentine ants move in. When that happens, the lizards disappear, a scientist announced here on 31 July at the Society for Conservation Biology meeting.
Coastal horned lizard populations have declined so much in southern California that the state's Department of Fish and Game considers the animal "a species of special concern." About 50% of its habitat has been modified by human activity, and those regions are now being invaded by exotic Argentine ants, which thrive in urban and agricultural areas and are becoming a scourge in California and much of the southern United States. The tiny Argentine ant forms supercolonies that overpower and eliminate native ant species, even though many native ants are much larger.
To find out whether ant invasions harm the lizards, conservation biologist Andrew Suarez of the University of California, Davis, and colleagues censused horned lizards over 3 years at 21 sites in four southern California counties. They found that as Argentine ants moved in, horned lizards disappeared. Feeding experiments showed that baby horned lizards can't survive on Argentine ants; the hatchlings lost weight so fast that the team terminated one experiment after 2 weeks. And field studies showed that adult lizards withered away when transplanted to territory controlled by Argentine ants.
"It appears habitats invaded by Argentine ants will never be suitable for horned lizards," Suarez says. Zoologist Gordon Rodda of the U.S. Geological Survey in Fort Collins, Colorado, agrees, pointing out that the study is "one more line of evidence that habitat destruction interacts with the introduced species, and the combination is devastating."