It's a small, bumpy, flesh-colored gecko that hangs around houses. You probably have three in your broom closet. About a century ago the first of these geckos, natives of Turkey and Italy, appeared in the Florida Keys, possibly in nursery plants. They soon embarked on a whistle-stop campaign up the coast, thriving wherever they hopped off trucks.

Untold millions now cling to walls and windows throughout the South, hiding in crevices by day and eating bugs by night. They're happy in swamps and deserts, mountains and lowlands. On a parallel mission, they colonized Latin America in no time at all.

Like the Romans and Ottomans whose homes they once clung to, Hemidactylus turcicus are great invaders. But perhaps not as great as their cousins, Hemidactylus mabouia, African geckos that are also small and pinkish and tearing up the Americas. Or a handful of other Hemidactylus species, which some scientists dub the "cosmopolitan geckos" for their worldliness.

"Why are some species successful when you release them and some aren't?" said Michael Moulton, an ecologist at the University of Florida's Institute of Food and Agricultural Sciences. It's a question that has shaped his whole career.

Thick encyclopedias of invasive species come out annually, deeming one invader or another the next big thing. Once upon a time it was budgerigars, the parakeets sold in pet stores. "This species is rapidly becoming one of the most common birds in coastal Florida, with colonies of upward of 8,000 individuals," Moulton read aloud from a decades-old book. He closed it. "As far as I know, no one has seen a wild one in years."

Some scientists presume that the more of a species you introduce, the better its chances of taking up permanent residence. "But that's not true," Moulton said, citing the case of one Australian lark. "It's found in every type of habitat and climate in Australia -- jungle, desert, coast, even cities," he said. "But everywhere it's been introduced in the world it fails."

A few scientists believe in an all-or-nothing factor, where some species will make it no matter what, and others will fail no matter what. "It's a fascinating idea," said Moulton. "But I don't think there's an all or none pattern. I think there's a none or maybe pattern."

For a year, biologist Gomez-Zlatar worked with Moulton to find out. For her master's thesis she chased Hemidactylus turcicus around the University of Florida campus with a flashlight and infrared gun, checking 50 different walls and recording the geckos' size, location and temperature. She was occasionally mistaken for a thief.

This summer her findings were published: Turcicus like all kinds of walls, perch at all kinds of heights, seek food in the pitch dark and around lights. On a recent sultry evening in Gainesville, Gomez-Zlatar arrived in Moulton's office with three Mag-Lites. "It's the perfect temperature," she declared, for turcicus hunting.  Moulton and Gomez-Zlatar were barely out the door when they saw their first. "Here's one -- here's a juvie," said Gomez-Zlatar as the gecko wriggled toward shelter. "You can tell it's a turcicus because it's so bumpy." Within 20 minutes they spotted five more. The last took a heroic leap off the bottom of a staircase to avoid Gomez-Zlatar's hand.

But by observing different gecko populations, Klowden can try and figure out how the invaders interact with one another and compete with each other. "It's an ever-changing sort of puzzle," he said, noting that books on exotics quickly become dated. Sometimes Klowden will throw a few gecko species up on the same wall, and see who wins. Usually they disappear before he can find out.

virginia.smith@news-jrnl.com

DID YOU KNOW?

· The Giant Whiptail, a Central American lizard, made its debut in the 1980s. It lives off beetles, roaches and ants.

· The Cuban Tree Frog looks very much like native tree frogs, with one exception: It has much larger toe pads than its Floridian cousins.