SANTA CRUZ — Lisa Cliggett expected a relaxing beach side escape when she stayed at a friend’s apartment in Santa Cruz in October.

Instead, the Kentucky woman found herself waging war on a freezer full of Argentine ants.

Thousands of them had set up shop inside the appliance’s insulation, and a steady stream of tiny bodies poured out of the cracks to forage in the kitchen. “There was a fortress within the freezer walls,” said Cliggett, who set out baits but still spent nearly an hour a day wiping up the fallen soldiers’ carcasses.

For millions of Californians, Argentine ants can be the stuff of nightmares. But their persistent presence in Northern California homes is merely one symptom of a larger problem — these invasive ants are out-competing native ant species and, in doing so, fundamentally altering California’s ecosystems. And by giving them shelter in our homes, we’re unwittingly helping them do it.

California yards and sidewalk cracks used to host a number of native ant species, such as winter ants and harvester ants. Sometimes they would venture inside, but rarely in great numbers. Then the Argentine ants arrived.

“Native ants are diverse; they’re beautiful; they interact with a lot of other organisms,” said David Holway, a UC San Diego ecologist. But “areas that are invaded by Argentine ants in California support very few native ant species.”

These tiny brown insects — just a few millimeters long — are native to the banks of South America’s Paraná River. In the past century, they’ve sneaked into shipping containers and surreptitiously made their way around the globe. Now they’re found on every continent except Antarctica, gaining strong footholds in areas with mild Mediterranean climates such as coastal California.

Argentine ant colonies are tricky to eradicate. Each colony has multiple queens. And, at least in California, they’re remarkably tolerant of neighboring Argentine colonies.

Deborah Gordon, a biologist at Stanford University, explained that ants recognize others in their colony by their scent, but Argentine ants don’t usually seem to differentiate other Argentine colonies from their own.

Toward other species, the foreign invaders can be ferocious: They’ll sometimes raid the nests of other ant species and rip them to bits. But their strategy for success in California might be a more subtle one. “They’re good at finding things,” Gordon said. They stake out food sources before other ants, who then back off.

“The irony is that it’s kind of hard to find Argentine ants in their native habitat,” Holway said. He hypothesizes that ant-on-ant aggression is the norm in their native ecosystem. Facing more competition for territory and resources from other species, they aren’t able to take over in the same way they can in California. He and Gordon both note, though, that ascribing a single cause to the success of any invasive species is an almost impossible task.

The loss of native ant species here knocks a carefully balanced web of ecological connections off-kilter. Insects and plants that depend on native ants to perform certain functions — from pollination to pest control — lose out, causing ripples of disruption to spread even further.

In some cases, invasive ants can cause irreparable damage. One of the most dramatic cases occurred on Christmas Island, a tiny dot in the Indian Ocean.

In the mid-1990s, yellow crazy ants worked in conjunction with mealybugs to create supercolonies that decimated the crab population on the island. Without the crabs there to remove leaf litter, the once-clear forests became dense with underbrush. Large trees died off from mealybug infestations.

The ecosystem-wide effects of invasive species are magnified on isolated islands.

Brian Fisher, an entomologist at the California Academy of Sciences in San Francisco who studies ant biodiversity around the world, says they’re trickier to trace in coastal California. Here, scientists are only starting to understand the extent of the Argentine ant’s influence.

The loss of native ant diversity is already having some concrete consequences, though: Horned lizard populations have decreased along the California coast because Argentine ants have displaced their food source, the larger native harvester ants.

In California, Fisher said, native ants are still lurking at the fringes. He suspects that if the Argentine ant were somehow eradicated in the Bay Area that the native ants would creep back in. We haven’t seen the ecological meltdown that’s happened on certain islands. “But that doesn’t mean it couldn’t happen in the future.”

We’re partially responsible for the Argentine ant’s success here, according to Gordon: Urban life offers the same comforts to ants that it does to humans.

“We give them water when it’s hot and dry, and we give them a warm refuge when it’s cold and wet,” she said. Even our more modest homes are five-star hotels compared to the ants’ shallow outdoor nests that easily flood during California’s wet winters and parch during drought-stricken summers.

The ants need moist soil to thrive, Gordon added. “We find that when they get far enough from sources of water and shelter from people that they actually don’t do that well against the native species,” she said.

Scientists are also concerned about the environmental impact of efforts to control the Argentine ant. Broad spraying of pesticides — the most common pest control strategy — has had little impact on the invasion.

“You couldn’t think of a better pest,” Holway said. “It essentially never goes away.”

They don’t sting like fire ants, and they don’t damage homes like carpenter ants. But their sheer numbers make them unwelcome houseguests. Kill the ants in one nest, and there’s another one waiting, poised to invade. The pesticides linger, washing into bays, where they can affect marine ecosystems.

Fisher suggested a different tactic: Researchers led by Neil Tsutsui at UC Berkeley are working to hijack the pheromone system that Argentine ants use to distinguish friends from foes. Treating infested areas with the right chemical message, they hope, could convince Argentine ants to turn against their Argentine neighbors instead of coexisting. Such a solution could someday spell relief for both native species and humans.

Cliggett, who battled the freezer infestation in Santa Cruz, said she’s used to coexisting with insects, but “to have them invade so completely into infrastructure and what we think of as sealed houses is really startling.”

Her battle ended only when her friend trashed the refrigerator. The new one is ant-free — for now.