Argentine Ants Threaten Horned Lizards

By Cat Lazaroff

SAN DIEGO, California, February 26, 2002 (ENS) - Argentine ants, which have infiltrated the coastal regions of California, invading homes and displacing native species of ants, are also contributing to a sharp decline in the state's population of coastal horned lizards. California biologists have learned that the invasive ants are exacerbating the problems the lizards already face in their declining habitat.

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Argentine ants are believed to have arrived in the U.S. in the 1890s (Photo courtesy Andrew Suarez, UCSD)
The tiny dark brown and black Argentine ants, which are about two millimeters in length, are thought to have entered the United States aboard ships carrying coffee or sugar from Argentina during the 1890s, then expanded throughout California and the southern parts of the United States. In the Southeast and much of the South, their proliferation is limited to some extent by the introduction of fire ants.

But in California, where those competitors are largely absent, the ants thrive in the temperate and damp coastal regions, killing and displacing native ants, many of which are 10 times larger in size. Their smaller size appears to be one main reason why populations of coastal horned lizards, which prefer to feed on the larger native ants rather than on other, harder to capture insects, have declined by 50 percent or more in areas where Argentine ants have invaded.

"Although the biomass of Argentine ants is greater than the biomass of the native ants they've displaced, the horned lizards don't seem to want to eat these introduced ants," said Ted Case, a professor of biology at the University of California at San Diego (UCSD) and a coauthor on two studies released this month. "Even in the laboratory, when we feed them Argentine ants and nothing else, the growing lizards can't maintain their weight. They're not getting enough nutrition. They don't seem to want to eat these ants."

In a series of laboratory experiments, Case and Andrew Suarez, a former graduate student at UCSD, showed that baby horned lizards fed a diet of insects typical of a community after invasion of Argentine ants can not grow and, in many cases, decline in weight. When their diet is switched to insects typical of an uninvaded community, the scientists found that the baby horned lizards grow normally.

"A comparison of diets among age classes of coastal horned lizards suggests a diversity of ants is necessary to support lizard populations," write Suarez and Case in their paper in the February issue of "Ecological Applications."

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Populations of coastal horned lizards have plummeted in recent years (Photo courtesy Chris Brown, USGS)
"The diet of horned lizards changes as they get bigger, from smaller to larger ant species," said Suarez, now a post-doctoral fellow at the University of California, Berkeley. "This indicates that ant diversity is important to preserve entire populations of lizards."

Robert Fisher, a zoologist at the U.S. Geological Survey's Western Ecological Research Center in San Diego and the first author of the second paper, published in "Conservation Biology," said another important factor in the decline of the coastal horned lizards is "their preference for open sand, such as coastal sand dunes and thick chaparral habitats."

Many of these once pristine, sandy coastal areas have become fragmented by developments, he explained, while chaparral habitats at low elevations, where these lizards are present, have been given low priority for preservation by land managers, because chaparral habitats at higher elevations are so much more abundant.

The lizards, which are designated as a species of concern in California, are candidates for federal and state listing because of their sharp declines, according to the scientists.

"Horned lizards may be one of the few 'charismatic' reptiles people might care about in California," noted Fisher, who has been conducting a comprehensive survey of reptiles and amphibians from Los Angeles to the Mexican border since 1995.

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Coastal horned lizards appear to prefer larger native ants over the invasive Argentine ants (Photo courtesy Chris Brown, USGS)
"Many people remember finding horned lizards growing up in Los Angeles, Anaheim, San Bernardino or San Diego because their habitat was abundant in the large river bottoms," noted Fisher. "This is not a legacy we are passing on because most of these places are now flood channels and have been destroyed. These lizards are hanging on in localized patches now within the reserves and will need monitoring and management into the future."

In the paper published in the February issue of "Conservation Biology," Fisher, Suarez and Case report that in their survey of 21 sites in four counties in southern California, they found that where Argentine ants had invaded, populations of coastal horned lizards were either very low or non-existent. Their survey also showed that inland areas where Argentine ants had invaded and where coastal horned lizards were either absent or declining were close to urban or suburban developments.

"Argentine ants got into southern California 100 years ago, but they've only become problematic as we've become more urbanized," said Case. "That's because they require water and cooler temperatures, which you get from our manipulation of the landscape."

One lesson learned from the decline of the coastal horned lizard is that land managers attempting to preserve open space around new suburban or urban developments may succeed in preserving natural vegetation, but if they do not take care to avoid fragmenting the landscape, the natural fauna of the area will not be preserved, Case said.

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Biologist Andrew Suarez examines ants in the field (Photo courtesy UCSD)
"Even though we're setting aside open spaces as we develop in southern California, that open space may not have all of the natural characteristics that we had hoped to preserve," said Case. "It may look the same. The major plant species may be the same. But many of the fauna - lizards, insects, nocturnal species that are hidden from view - are heavily influenced by their proximity to people."

"The next nightmare for the lizard may be the red imported fire ants, which have been spreading through urban habitats in southern California since 1998," added Fisher. "They have destroyed small vertebrate communities through the southeast and may do the same here."

"If we want to preserve populations of this beautiful lizard in the future, we not only have to make sure that enough habitat is put aside, we also have to make sure that the remaining habitat is monitored to prevent the invasions of exotic ants," Suarez concluded. "Essentially, the impacts of Argentine ants in California starts with the displacement of native ants and then cascades throughout the ecosystem."