Invasive coqui frogs are subtly altering the ecosystem of Hawaii island as they gobble up mites, ants and other bugs and leave behind droppings that attract flies, scientists conclude.

While the tiny frogs, native of Puerto Rico, are known for their loud and often maddening cries, they also are taking a toll on the Hawaii island environment, according to Ryan Choi and Karen Beard, with the Department of Wildland Resources at Utah State.

Researchers with headlamps fanned out across the island over four months in 2009, collecting frogs, and analyzed their stomach contents.

They also collected “leaf litter” at sites where coquis are abundant and also where they are not. Dead leaves are where many of the bugs that coquis eat reside, but the teams also collected bugs that live in foliage.

The results, published in the May issue of the journal Biological Invasions: Where coqui populations are dense, the number of leaf litter bugs fell by 27 percent and mites alone by 36 percent.

The number of flies was nearly one-fifth greater where coquis are prevalent, Choi and Beard found.

The research teams collected leaf litter in 10-inch-by-10-inch patches and extracted the bugs, which they then identified using a dissecting microscope. They also used vacuums to suck up the bugs for analysis. 

Flies were collected with sticky traps.

In all, they collected 21,382 invertebrates (not all of them insects) at the coqui-populous sites and 28,184 from the largely coqui-free sites.

“Across 15 sites on the island of Hawaii, we found that coqui frogs were associated with a reduction in the total number of leaf-litter invertebrates, primarily Acari,” the scientists reported, referring to the group of arachnids that includes mites and ticks. 

They said the increase in flies can be attributable to the presence of coqui excrement — and their carcasses when they die.