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Proliferation of Argentine Ants in California Linked to Declines in Coastal Horned Lizards

February 28, 2002
By: Kim McDonald

Close-up photo of  brown and red coastal horned lizard

Chris Brown, USGS

The pesky Argentine ant, which has proliferated throughout the coastal regions of California, invading homes and displacing native species of ants, is also contributing to a sharp decline in the state’s population of coastal horned lizards.

In separate papers published this month in the journals Conservation Biology and Ecological Applications, a team of San Diego biologists show that the severe decline in coastal horned lizards in southern California in recent years is intimately tied to the proliferation of Argentine ants, which have displaced many of the larger, native ant species on which the lizards feed.

“While habitat loss is often considered the leading reason for the decline of threatened species, the mechanisms responsible for their decline are often unknown,” says Andrew V. Suarez, who conducted the research on both studies while a graduate student at the University of California, San Diego. “These two studies are relatively unique in that we were able to identify a causal agent contributing to the decline of horned lizards—an invasive ant species. Moreover, this pattern was evident over a really large area ranging from the Mexican border to Los Angeles—a scale at which very few studies have been conducted.”

“Our work demonstrates that invasions can have community-wide effects,” adds Suarez, now a postdoctoral fellow at the University of California, Berkeley. “Essentially, the impacts of Argentine ants in California starts with the displacement of native ants and then cascades throughout the ecosystem.”

Close-up photo of Argentine ant latching onto a pencil tip

Andrew Suarez, UCSD

The tiny dark-brown and black 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,” says Ted J. Case, a professor of biology at UCSD and a co-author on the two papers. “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, Suarez and Case showed that baby horned lizards fed a diet of insects typical of a community after invasion of Argentine ants cannot grow and, in many cases, decline in weight. But when their diet is switched to insects typical of an uninvaded community, the scientists found that the baby horned lizards grow normally.

Dark grey, black and yellow-tipped coastal horned lizard

Chris Brown, USGS

“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.

“The diet of horned lizards changes as they get bigger, from smaller to larger ant species,” explains Suarez. “This indicates that ant diversity is important to preserve entire populations of lizards.”

Robert N. Fisher, a zoologist at the U.S. Geological Survey’s Western Ecological Research Center in San Diego and the first author of the paper in Conservation Biology, says 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 explains, 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,” notes Fisher, who has been conducting a comprehensive survey of reptiles and amphibians from Los Angeles to the Mexican border since 1995. “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. 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,” says Case. “That’s because they require water and cooler temperatures, which you get from our manipulation of the landscape.”

Another reason is the lack of genetic diversity among Argentine ants. In a paper published two years ago in the Proceedings of the National Academy of Sciences, Case, Suarez and Neil D. Tsutsui, a former UCSD graduate student, concluded that the ants’ reduced genetic diversity, from the small population of ants that initially invaded the U.S., allowed a giant “supercolony” of closely related ants to grow unchecked from San Diego to Ukiah, 100 miles north of San Francisco. In Argentina, by contrast, fighting among the more genetically dissimilar, territorial ants has managed to keep these insects in check and in smaller, much more sharply defined colonies than those in California.

Case says one important 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 don’t take care to avoid fragmenting the landscape, the natural fauna of the area won’t be preserved.

“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. 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,” points out 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,” says Suarez, “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.”