Hawaii is home to several introduced rat species that pose ongoing challenges to the islands’ natural environment, agriculture, and public health. Understanding their presence and impact is important for managing these widespread rodents.
Rat Species in Hawaii
Three primary rat species are found across Hawaii: the Polynesian rat (Rattus exulans), the black rat (Rattus rattus), and the Norway rat (Rattus norvegicus). The Polynesian rat, also known as the Pacific rat, is the smallest, measuring 4 to 5 inches long with a tail of similar length. It has a slender body, pointed snout, and a ruddy brown back with a whitish belly.
The black rat, also called the roof rat, is medium-sized, 5 to 7 inches long, and known for its agile climbing. It has a tail longer than its head and body combined and can be black or various shades of gray to yellowish-dirty tan.
The Norway rat, also known as the brown rat or sewer rat, is the largest, weighing 10 to 18 ounces and measuring 8 to 10 inches long. It has a stout tail shorter than its head and body combined, and its fur ranges from reddish-brown to gray to black.
Arrival and Spread
The introduction of rats to Hawaii is linked to human voyaging, with different species arriving at various historical points. The Polynesian rat was the first, accompanying early Polynesian settlers around 1,000 years ago as accidental stowaways on their canoes. This species spread across the islands with the expansion of Polynesian communities.
Later, with European ships in the late 1700s, the black rat and Norway rat were introduced. These species also arrived as stowaways, rapidly establishing populations across the islands. All three species have since adapted to a wide range of habitats, from sea level to higher elevations, inhabiting urban, agricultural, and remote natural areas.
Ecological and Agricultural Impacts
Rats cause damage to Hawaii’s native flora and fauna, which evolved without ground predators. They prey on ground-nesting seabirds, forest birds, and their eggs and chicks, contributing to the decline of many native Hawaiian species. For example, their predation on palm seeds and seedlings has been linked to the loss of lowland palm forests. Rats also consume native tree snails, insects, and the fruits and seeds of rare endemic plants.
Beyond natural ecosystems, rats threaten Hawaii’s agricultural sector. They feed on and contaminate many crops, including sugarcane, macadamia nuts, pineapple, coffee, bananas, lychees, and various fruits and vegetables. This damage results in economic losses for farmers; for instance, roof rats cause an estimated 5-10% damage to the annual macadamia nut crop on the Big Island alone. Rats can also damage irrigation systems and storage areas, and their presence can deter agricultural workers.
Conservation Efforts
Managing rat populations in Hawaii involves strategies to protect biodiversity and agricultural interests. Trapping is a common method, using mechanical traps like snap traps and multi-kill devices. These traps are placed in strategic locations, sometimes within bait stations to prevent harm to non-target animals.
Baiting with rodenticides is another approach for controlling larger rat populations. These chemical baits are applied in controlled ways, such as within secure bait stations or through canopy baiting in trees. Habitat management is also a component of control, focusing on reducing food sources and eliminating nesting sites by cleaning up rubbish and maintaining vegetation. These ongoing efforts mitigate the negative effects of rats on Hawaii’s ecosystems and economy.