Liliwai

Acaena exigua

Description

Liliwai (Acaena exigua ) is a perennial herb of the Rosaceae family with leafy stems 0.4-1.6 in (1-4.1 cm) long. The flowering stems of this plant are 2-6 in (5.1-15.3 cm) long. The leaves, about 0.4-1 in (1-2.5 cm) long, are comprised of six to 17 oval leaflets, which are glossy above and whitish beneath. The petalless flowers are in short, dense spikes 0.2-0.4 in (0.5-1 cm) long. The receptacle is urn-shaped and encloses the fruit, which is a pale brown, cone-shaped achene.

Habitat

Liliwai grows in the Metrosideros ('ohi'a) montane bog community, characterized by thick peat overlaying an impervious clay substrate with hummocks of sedges and grasses, stunted trees, and shrubs. The vegetation type features a high diversity of plants with an abundant understory growing in rich soil that is well-watered by annual rainfalls of 100 in (254 cm). Associated species include Deschampsia nubigena, Dichanthelium spp., Oreobolus furcatus, Metrosideros polymorpha, and Vaccinium.

Distribution

A. exigua declined sometime after 1920 and throughout the remainder of the twentieth century for unknown reasons. No individuals of A. exigua were known to exist in 2000. Historically, this species occurred at Puu Kukui on West Maui and Mount Waialeale on Kauai. A. exigua was last collected on Kauai by Heinrich Wawra in 1869-70 but was never seen there in the twentieth century. The species has not been collected from West Maui since 1957. Botanists sought this plant unsuccessfully for many years, although several other rare plant associates were found.

Threats

Chronic impacts of feral pigs and increased cover of alien plant species in the montane bogs of Kauai occurred long after the decline of this species. Therefore the reasons for the disappearance of this species are not known, and although impact from herbivory and rooting by pigs has been an assumption, feral pigs became established at Waialeale only during the final decades of the twentieth century. Additionally, the West Maui habitat of Puu Kukui has always been pig-free. Consumption of vegetative or floral parts of this species by alien slugs or rats, although undocumented, could have been a factor in its decline. This manner of predation could still be a critical limiting factor for A. exigua. Other factors that might have severely limited this plant are an alien pathogen such as a disease, fungus, or nematode; loss of pollinators; and some as-yet-undetected micro-environmental change.

A. exigua, if it still exists, is threatened by its own tiny population base, human impacts from collecting, and possible predation by insects or rodents. The lack of genetic diversity could depress the reproductive vigor or adaptability of the species. Its extremely low population levels in such a restricted area leave the species vulnerable to a prolonged drought or other single severe environmental disturbance, which could result in the ultimate extinction of A. exigua.

The trampling of associated native plant species and the introduction from other areas of invasive alien plant species into its montane bog habitat are threats to A. exigua caused by excessive human visitation.

Potential future threats could include feral pigs and alien plant species. Habitat degradation by feral pigs on Kauai is currently a primary threat to the native montane bog plant communities there, which comprise potential habitat for A. exigua. Puu Kukui on West Maui was pig-free in 2000; however, it is posible that feral pigs, if not adequately controlled, could reach the summit of Puu Kukui and cause serious degradation of the montane bog habitat. A substantial loss of native plant cover, dramatic increases in bare ground, and the progressive invasion of the rush Juncus planifolius and other alien plants has accompanied feral pig activity on Waialeale.

Conservation and Recovery

The quite limited former habitat of this species needs to be methodically, thoroughly, and completely searched. The plant is diminutive and easily overlooked; however, its leaf and rosette morphology are so distinctive as to be unmistakable. If A. exigua plants still exist, a vigorous search should find them. If found, a great amount of research needs to be initiated to definitely establish the cause of its decline. It will be necessary also to determine what might be done to protect the species in the future as the West Maui bog complexes of Puu Kukui and Eke are virtually pristine, having never been damaged by feral pigs. There is also no reason to believe that the bogs have been chemically altered, as evidenced by the continued presence of other bog-dependent plant associates.

Researchers should investigate the subtle and little-understood threats of lack of pollinators, alien slug and rodent predation, and disease that are likely to have caused the decline of the species in the first place and are likely to be limiting factors to its survival. If this species cannot be relocated after extensive searches, delisting due to extinction may be proposed.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

Senior Resident Agent Office
U.S. Fish and Wildlife Service
300 Ala Moana Boulevard, Room 7-235
P.O. Box 50223
Honolulu, Hawaii 96850-5000
Telephone: (808) 541-2681
Fax: (808) 541-3062

Reference

U.S. Fish and Wildlife Service. 15 May 1992. "Endangered and Threatened Wildlife and Plants; Determination of Endangered or Threatened Status for 15 Plants from the Island of Maui, Hawaii." Federal Register 57 (95): 20772-20787.