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July 15, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Small mammal predators invade Hawai`i

The islands of the Pacific are some of the most isolated in the world and have produced distinct floras and faunas. Endemism, whereby plants and animals are restricted to a single geographic area, is frequent. Hawai`i exemplifies this process. Formed entirely by volcanic action about 4,000 km (2,400 miles) from the nearest continental land mass, Hawai`i is the most isolated group of islands in the Pacific. Except for the Hawaiian bat, no terrestrial mammal naturally colonized the islands. Isolated from the enemies of their ancestors, Hawai`i's native plants and animals gradually lost their natural defenses against mammalian predators.

With human settlement of Hawai`i, and the introduction of six small mammals, native animals and plants were exposed to predation by these alien species. Carried on early sailing ships, three rats, one mouse and the domestic cat escaped to the islands. The Polynesian rat arrived with the first Hawaiian settlers and is now common in forests, agricultural and adjacent grassy areas, and wooded gulches. In the late 1700s, the Norway rat arrived aboard European sailing ships. It is associated mainly with human activity. The house mouse, arriving about 1816, occupies dry grasslands, scrublands, and forests from sea level to about 3,700 m (12,000 feet). The black rat arrived about 1870 but rapidly became the dominant rat within the islands. The black rat's ability to live in trees makes it dangerous to native forest birds.

Domestic cats probably arrived in Hawai`i soon after Captain Cook. Cats developed wild populations and now live in forests from sea level to high elevation on all the main islands.

In 1883, the mongoose was introduced to Hawaii in an unsuccessful attempt to control rats. Mongooses now occupy all habitats up to about 2,300 m (7,500 feet) on all main islands except Kaua`i and Lana`i.

These predators rapidly began to assault Hawai`i's native wildlife. Within the last 1,500 years, about 65% of Hawai`i's endemic bird life has gone extinct, and 30 of the remaining 48 species are endangered or threatened. Predation by rats, cats, and mongooses is considered a leading cause in the decline and extirpation of endemic Hawaiian birds. Habitat destruction and avian diseases are other important causes.

Many extinct Hawaiian birds, known only from fossil remains, nested on the ground and were susceptible to predation. The eggs and young of the nene (Hawaii's state bird) and the Pacific hawksbill sea turtle are eaten by feral cats and mongooses. Rats also prey on native Hawaiian tree snails and insect larvae and are suspected of competing for food with the 'alala (Hawaiian crow), oma`o (Hawaiian thrush), and some endemic insectivorous bird species. Bark stripping and seed predation by rats are known on many endemic Hawaiian plant species. Black rats eat the seeds, fruit, flowers, and bark of the endangered hau kuahiwi tree. Mice may compete with native species for plant and invertebrate food resources.

The alien mammals also brought diseases that are transmittable to humans. Black and Norway rats are carriers of fleas, which transmit plague. Plague was present in Hawai`i for 58 years (1899-1957). Rodents and mongooses carry leptospirosis, a bacterial disease transmittable to humans. Cats host a parasite that causes toxoplasmosis in humans and the 'alala.

Only two methods for controlling small mammals are available to land managers - trapping and 0.005% diphacinone bait placed in bait stations. Both methods, effective in small areas, are labor- and time-intensive and are impractical for large conservation areas. Scientists from Federal, State, and private organizations in Hawai`i are currently studying the ecology and biology of small mammal predators, and evaluating new control techniques, to develop management tools to lessen the impacts of these predators on native wildlife and plants.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes from the vent to the sea. Lava is entering the ocean near Kamokuna and enlarging the bench. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

A resident of Pahala felt an earthquake at 9:08 p.m. on Monday, July 12. The magnitude-2.5 temblor was located 6 km (3.6 mi) west of Pahala at a depth of 5.5 km (3.3 mi).

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Updated: 26 July 1999