Four rodent species, the house mouse, the Norway rat, the Polynesian rat and the black rat, inhabit most islands of Hawai'i. In cities, rats are one of Hawai'i's biggest pest problems.

Rats are common and know there is food to be found wherever there are humans. Anything edible is fair game. These nocturnal, disease-carrying scavengers often seek shelter in crawlspaces, rafters under floors, bathroom walls and attics where it's dry and dark with easy access to food and water.

For most Hawai'i residents, a rodent problem is solved with over-the-counter bait traps and poison or a phone call to a pest control expert. But what about rats we don't see? What damage are they doing?

The natural environment is everybody's business in Hawaii. What we don't see, will hurt us. Even the most remote ecosystems in Hawai'i are under attack by rats that ravage native plants and animals. Polynesian rats were Hawai'i's first invasive species, arriving with the Polynesians but spreading out before even the earliest settlers, damaging native flora and fauna. The Norway and black rats introduced from Western ships added to the destruction of native species and spread human diseases, such as the plague and leptospirosis.

Offshore islands, many of them Hawai'i State Seabird Sanctuaries, host thousands of magnificent seabirds. Rats attack nesting seabirds, their eggs and their fledglings, birds that include the wedge-tailed shearwater ('ua'u kani), brown booby ('a), Bulwer's petrel ('ou), white-tailed tropicbird (koa'e kea) and the red-tailed tropicbird (koe'a 'ula).

On several offshore islets, grow some of Hawai'i's most diverse and intact coastal plant communities. Rats are a problem for them as well. They feed on young plants and on seeds of rare and vulnerable native plants, such as the lama tree, loulu palm and pua ala shrub.

With the goal to restore habitats and aid the recovery of species damaged by rats, state and federal agencies have used diphacinone-based rodenticides in bait stations to help conserve natural areas and watersheds since 1994. Diphacinone is most toxic to mammals and relatively harmless to birds, fish and invertebrates. Diphacinone has been one of the most widely used rodenticides in the world since the 1950s.

Examples of the use of diphacinone in bait stations to reduce rat populations to benefit native birds include the O'ahu  'elepaio, Maui parrotbill, puaiohi, palila, po'ouli, Hawai'i creeper and  'akepa. The elepaio has been found to be particularly vulnerable to rat predation, with nests and incubating and brooding females being frequently taken by rats.  Although males and females attend nests, only the female incubates or broods at night the time when rats are most active.  Control of rats from 1996 to 2000 resulted in an increase in reproduction from 0.33 to 0.70 fledglings per pair and an increase in annual survival of adult females from 0.50 to 0.83 (only the female attends nests).