Home > Species > Mammals SPECIES - MAMMALS Hawaiian Monk Seal - Photo: C Swenson SELECTED MAMMALS FOUND ON HAWAII’S OFFSHORE ISLETS The following list highlights the mammals found in Hawaii’s offshore islets. Although rabbits were recently removed from the last offshore islet in Hawaii, they are included below because of the biological damage they historically caused. People have historically reported seeing small rodents on certain offshore islets, which could be small rats or mice. While mice are possibly present on some islets, only rats have been positively identified in recent years. Therefore, mice are not included in the list below. Each species listed below is classified as either endemic or introduced. These terms are defined as follows: Endemic: Native only to Hawaii Introduced: Non-native species brought by humans either intentionally or accidentally Hawaiian Monk Seal or Ilio holo i ka uaua Monachus schauinslandi Endemic and Endangered species Hawaiian Monk Seal - Photo: C Swenson   The endemic Hawaiian monk seal is a highly endangered marine mammal that resides primarily in the Northwest Hawaiian Islands, with occasional sightings on the main islands of Hawaii. Their population is estimated at fewer than 1400 animals. They generally breed on islands that have little or no human presence, and encroachment by humans into their natural territories is one of the threats facing this endemic mammal. Entanglement in fishing nets and other debris is a major danger as well. The small numbers of seals in some of the populations, combined with a sex ratio heavily skewed towards males, has led to ‘mobbing’ of female seals by males competing to mate, causing severe injuries and some fatalities among female seals. Disease and predation by sharks have also taken their toll on the species. Recent declines in juvenile survivorship, possibly due to low prey availability, have not abated, especially in the French Frigate Shoals breeding colony. However, increasing numbers of monk seals have been sighted in the main Hawaiian Islands. Adult monk seals are sometimes seen resting on Lehua’s rocky shelves and on the sandy beaches of some islets along the windward Oahu coast. Rabbit or Lapaki Oryctolagus cuniculus Introduced invasive species European Rabbit - Photo: E VanderWerf   European rabbits were first documented in Hawaii in 1825, when they were introduced to Ford Island in Pearl Harbor, Oahu. Over the years, people released rabbits on Molokini, Manana, Lehua, Laysan, Lisianki, and Pearl and Hermes Atoll. Presumably they were introduced as a food source for visitors to these islands and, in the case of Laysan, with the idea of starting a commercial rabbit farm. Populations were eradicated several years ago from Pearl and Hermes, Laysan, and Manana, and disappeared naturally from Lisianki and Molokini. Lehua Island was the last offshore islet in Hawaii with rabbits up until 2005, when they were removed as part of the Offshore Islet Restoration Committee’s Lehua Ecosystem Restoration Project. Feral rabbits are still found on Hawaii Island, Oahu, and Kauai. Rabbits caused drastic impacts to the islands where they were released. On Lisianki Island, rabbits introduced in 1903 during guano mining operations completely removed all of the island’s living vegetation within 10 years, and the starved population died off some time before 1923. On Laysan Island, rabbits introduced at the same time extirpated 22 of the 26 plant species recorded on the island within 20 years of their release. By the time rabbits were eradicated from Laysan in 1936, at least three endemic land birds – the Laysan millerbird, the Laysan honeycreeper, and the Laysan rail – had gone extinct as a result of severe reductions in plant cover caused by rabbit herbivory. Rat or Iole Rattus species Introduced invasive species Polynesian Rat - Photo: G McCormack   Three species of rats have been introduced to Hawaii and other islands throughout the world. In order of decreasing body size they are: the Norway or Brown rat (R. norvegicus), the Ship or Black rat (R. rattus), and the Polynesian rat (R. exulans). R. exulans (see photo) was introduced to Hawaii by Polynesian settlers several hundred years ago, and R. norvegicus and R. rattus arrived in either the 18th or 19th century. They have different dietary preferences, distributions and histories of introduction. However, all three species are omnivorous, behaviorally plastic, have high reproductive rates, and can survive in a variety of habitats. One or more of these species occurs on an estimated 82% of all island groups worldwide. The most pronounced impact of introduced rodents on island ecosystems is the eradication of multiple native species preyed upon by rats. Introduced rats are responsible for an estimated 40-60% of all bird and reptile extinctions and have caused the extinction of endemic mammals, birds, plants, and invertebrates on islands throughout the world. All three species of introduced rats are present in Hawaii. R. rattus occupies all of the eight major islands, and R. exulans has been confirmed on all of the major islands but Niihau. R. norvegicus, the least abundant species, is absent on Kahoolawe and Niihau but present on the six other major islands. R. rattus was documented on Midway Atoll but has since been eradicated. R. exulans is currently found on Lehua and Mokapu Islands and is likely the species present on Kaula. Unidentified rodents have also been recently reported on Mokuauia, Alau, and Manana. Introduced rodents have wreaked havoc on seabirds in Hawaii. On Kure Atoll, R. exulans preyed on Wedge-tailed shearwaters, Laysan albatross, and Bonin Petrels until they were eradicated. Rats were also implicated as the cause of drastic declines in the population of Bulwer’s petrels on Popoia Island near Oahu. R. rattus had significant impacts on nest success in Bonin petrels on Midway Atoll, but reproductive success increased significantly after rats were eradicated from Midway. Wedge-tailed shearwater nesting success increased markedly following a recent eradication of rats from Mokolii. > Back to top of page Home  Islets  Species  Projects  Outreach  Links  About Us