HAIKU, Hawaii - Fritz and Thorunn Bathelt designed their house in the hills overlooking the ocean here to tread lightly on the landscape, leaving much of the lush native plants to grow at will, installing solar panels that allow them to live off the grid and tapping their water supply from 80 feet below the ground.
But some evenings, when the skies darken over this remote stretch of Maui's north shore, you can find Fritz Bathelt prowling the grounds with murderous intent. He will stalk his prey through the palm trees and around the ferns, alert for a pair of beady eyes, the only part of the animal that doesn't blend into its surroundings. When he spots one, he takes matters into his own hands.
"I squish him," Bathelt says triumphantly, his own eyes glinting coldly and his fists clenching in a death grip.
What has incurred the wrath of this and many otherwise nature-loving Hawaiians is a pale-colored tree frog about the size of a quarter. The coqui frog would barely attract notice in this state of exotic flora and fauna but for its one distinctive quality: the male's high-pitched and incessant mating call that pierces through the night air.
Ko-kee! The two-note whistle, which gave the tree frog its name in its native Caribbean, can sound charming when heard the first time, and in solo. But the coqui doesn't call just once, he calls all night long. And there is rarely just one coqui; often, there can be thousands.
Coquis arrived in Hawaii about 10 years ago, apparently aboard plants imported from the Caribbean, to become yet another in a series of invasive species to descend on the island state. As relative newcomers to Hawaii, coquis have yet to develop natural predators. As a result, their numbers are growing - much to the dismay of those being kept awake by their shrill nocturnal whistling.
In one park on the Big Island of Hawaii, the number of coquis has been estimated at 10,000 an acre, and their calls have been measured at 90 decibels, comparable to the racket a lawnmower makes.
Unbearable, in other words, say those who have unwittingly played host to coquis.
"I was considering selling the place," says Thorunn Bathelt. "It was so intense, I was at the end of the rope. There is no sleeping with the frogs. There is no living with the frogs."
The Bathelts heard their first coqui about seven years ago and believe the frogs arrived on their property in bromeliads bought from a local nursery. Coquis eggs resemble fertilizer pellets, a perfect camouflage that has allowed them to spread.
The couple has been battling the frogs ever since. They've allowed state agents to test various frog-killing schemes on their property and even enlisted those traditional predators: little boys. "We put a bounty on them," Fritz Bathelt says of the $5- to $10-a-head price he has paid local boys for every frog they caught on his property.
Hundreds of alien species have invaded the islands in recent years, disrupting its unique ecosystems and in some cases contributing to the demise of native animals and plants.
While other areas of the world are troubled by non-native species, Hawaii faces additional problems because of its isolation. As a result, native flora and fauna are often ill-equipped to compete with invasive species because they never had to develop such survival techniques. But as global trade and traffic have increased, Hawaiian species are finding their turf overrun with aliens such as the coqui frogs. "There are no native amphibians or reptiles in Hawaii," says Bill Mautz, a biologist at the University of Hawaii in Hilo. "They were all introduced by humans."
Hilo, on the Big Island of Hawaii, is among the areas where coquis have become established. To date, they have been found in pockets of four islands - Hawaii, Maui, Oahu and Kauai. Just southeast of Hilo is the most dense population of coquis, in Lava Tree State Park.
Officials believe the frogs were introduced there when the state planted ornamental shrubbery and trees around the parking lot. After dark, the park is a cacophony of coqui calls, the sound seeming to echo and intensify under the canopy of the trees.
The park should serve as warning, Mautz says, for how the frog population could quickly get out of hand. "We're still in the early stage of the invasion," he says. "In another five years, it will be hopeless."
The frogs have the potential of upsetting the existing ecosystem, he says, both by what they eat - insects and spiders - and by their becoming food for a new predator that could add yet another disruption.