PHOENIX - The thousands of undocumented immigrants who cross the border from Mexico into the United States daily could eventually take a toll on wildlife habitats and animals in southern Arizona, experts say.

While studies haven't been done to show the effects of border crossings on wildlife, biologists say that trails used by illegal immigrants would be most detrimental to animals.

Because many of these trails follow streams, the constant foot traffic keeps animals from frequenting their usual water sources, and in some cases, their home range.

"They're forced to move away from areas with accessible water all the time," said Wayne Schifflett, manager of the Buenos Aires National Wildlife Refuge. "Less-preferred habitats would jeopardize their security. There are less trees, less water and more predation to the animals."

Schifflett said up to 1,000 border crossers come through the refuge southwest of Tucson daily. And it's just one of the corridors that illegal immigrants use to get into Arizona, one of the hottest illegal crossing points on the U.S.-Mexico border.

Other ecologically sensitive crossing points in the state include the Organ Pipe Cactus National Monument and the Cabeza Prieta National Wildlife Refuge.

Experts say animals most at risk include deer, antelope and javelina, which roam the forests and wildlife refuges at lower elevations where most undocumented immigrants cross.

Threatened species could also suffer, such as the lesser long-nose bats whose caves have been used by illegal immigrants for shelter, according to the U.S. Fish and Wildlife Service.

In the Buenos Aires refuge, illegal traffic has cut about 100 miles of new roads into wildlife areas.

Since 1999, problems associated with undocumented immigrants increased significantly, Schifflett said.

"In 1985, I never saw (an illegal immigrant) coming through the refuge," he said. "Today, 75 percent of my time is spent on problems on the borders; it used to be 100 percent wildlife."

Another problem with the heavy traffic are the large amounts of empty water jugs, old clothes and food scraps that are left behind by crossers. Some border areas look like city dumps.

While the trash itself may not pose an imminent danger to animals, biologists worry about long-term effects, such as soil erosion and water contamination.

"All that plastic, all that glass, all that metal burned and crushed, could start fires," said Thetis Gamberg, a biologist for the Fish and Wildlife Service in Tucson.

"Ingestion of trash probably kills wildlife," Gamberg said. "The erosion process gets worse because of hundreds of people stomping around. All of these things are small compared to global impacts, but nobody knows because nobody does the research."

Though unlikely, some wildlife also could approach people for food if they can't find anything to eat, said Doug Duncan, a Fish and Wildlife Service biologist.

"It highly depends on the situation. If there's a drought, certain animals are going to be more desperate to find food and more willing to do things they wouldn't normally do," Duncan said.

This summer, temperatures and rainfall were close to normal in southern Arizona, though the region was still in the midst of a long-term drought. The number of reported encounters between animals and people statewide have also decreased in the last two years, according to the Arizona Game and Fish Department.

However, some officials are trying to ease the potential burden of border crossers on both wildlife and people.

The Bureau of Land Management has posted bilingual signs that ask people not to litter on their territories, which includes the San Pedro Riparian National Conservation Area and the Ironwood Forest National Monument.

"We're letting (illegal immigrants) know they're entering a protected resource," said Bill Childress, manager for the San Pedro riparian area. "We're hoping they will treat our protected areas with respect."