CAIMITO, Puerto Rico (AP) – The resounding calls of “ko-KEE, ko-KEE” ring out in a cadence that brings music to Puerto Rican nights.

The tiny frog that produces the melody is named coqui after its distinctive two-note call and, because it is native only to Puerto Rico, has become a symbol for the island, its culture and its people.

“It’s like a part of us, a part of our family,” says schoolteacher Lourdes Colon, who hears the frogs each night outside her house in tree-covered hills south of San Juan.

Some Puerto Ricans lament that the sound is becoming less common as parking lots and subdivisions eat away at green spaces, and scientists say the coqui now faces serious threats to its survival.

Roadside billboards aim to raise awareness, proclaiming “May the coqui keep singing.” Coquis are printed on towels and T-shirts. Their chirp echoes in TV commercials, and their name advertises everything from cafeterias to handmade soaps.

The coqui has inspired folk songs and poems. Even the Caribbean island’s earliest inhabitants, the Taino people, used the frog’s unmistakable form in rock carvings.

When Colon returns home, she often lies in a hammock and is calmed by “the orchestra of nature” – the frog song that lulls some islanders to sleep.

Coquis are as small as the tip of a thumb and difficult to spot. Nearby islands have related species, but they don’t make the same sound, so parents have for generations passed on the myth that coquis can survive only in Puerto Rico.

In recent years, though, large numbers appeared in distant Hawaii, probably in shipments of tropical plants.

Many Hawaiians, unaccustomed to frog calls that can reach 100 decibels, say they are noisy pests. US officials plan to start eradicating the frogs there on Oct 1, likely using pesticides.

“The populations are very high and in some areas very, very, very noisy,” says Lyle Wong of Hawaii’s Agriculture Department. “They’re just changing the character of our evening hours.”

While Puerto Rico’s government has urged Hawaii to find an alternative, biologists largely agree coquis have upset Hawaii’s natural balance, preying on native insects and boosting the populations of predators like rats and mongooses.

But in Puerto Rico, the frogs are under assault from the destruction of their habitats and pollution.

Three of 16 named species in this US territory are believed extinct. All belong to the genus Eleutherodactylus, from those with the traditional “ko-KEE” call to others with differing songs.

Amid the concrete of San Juan, the frog song persists from gardens moist enough to sustain them. But scientists have documented declines.

Richard Thomas, a reptile and amphibian expert from the University of Puerto Rico, estimates frog populations in the rain forest of El Yunque, where there can be thousands in one acre (0.4 hectare).

Coquis stop singing as he nears, but Thomas has perfected a whistle to restart their chirping.

Searching in the foliage, he whispers “Right here!” and illuminates a frog as its vocal sac expands and constricts.

Only males make the “ko-KEE” sound, marking their territory with the “ko” and calling females with the “KEE.” The song intensifies after rain and on dark nights.

Thomas has studied coquis since the 1960s, when he discovered the common coqui was a unique species and named it. Today, he worries other species could fade like the golden coqui, which he hasn’t seen since the 1970s.

“I like the animals and I hate to see them go,” he says. But, he adds, “as we build more and more malls and resorts along the coast, and grind up more of the limestone hills, who knows what’s in store?”