Amaranthus brownii

Description

Amaranthus brownii was the only endemic species of Hawaiian Amaranthus in the Hawaiian Islands.^[a] It was an herbaceous annual plant that grows to a height of 30–90 cm (0.98–2.95 ft) and had narrow, linear leaves, small green flowers, and fruit that held a single, dark red seed. A. brownii was monoecious; that is, the male and female flowers were found together on the same plant. It differed from other Hawaiian species of Amaranthus with its spineless leaf axils, linear leaves, and indehiscent fruits (fruit which does not open to release seeds when ripe). The fruits were ovoid and between 0.8–1 mm long and 0.6–0.8 mm wide. The plant is thought to have been anemophilous (pollinated by wind).

Distribution and habitat

Amaranthus brownii had a very limited range; it was found only on the 0.65 km² (0.25 sq mi) island of Nīhoa, located 275 km (171 mi) northwest of Kauai. It is thought that this endemic plant had probably always been rare and restricted to Nīhoa. Its former habitat is managed by the United States Fish and Wildlife Service and protected as part of the Hawaiian Islands National Wildlife Refuge in the Papahānaumokuākea Marine National Monument. A. brownii was one of three endemic and endangered species only found on Nīhoa, along with the Nīhoa fan palm (Pritchardia remota) and the Nīhoa carnation (Schiedea verticillata). At least nine other native plant species can be found in its habitat, including Hawaiian goosefoot (Chenopodium oahuense), lovegrass (Eragrostis variablis), koali ʻawa (Ipomoea indica), goat's foot (Ipomoea pes-caprae subsp. brasiliensis), Panicum torridum, naupaka (Scaevola sericea), Sicyos pachycarpus, ʻilima (Sida fallax), and Nelson's horsenettle (Solanum nelsonii).

The plant grew during the moist, winter season from December through July in Nīhoa's coastal dry shrubland habitat in shallow soil on rocky outcrops in exposed areas between 120–215 m (394–705 ft). At the time of its discovery in 1923, botanists first observed the plant growing in great quantity on the ridges towards Miller Peak and on eastern ridges of the island. Expeditions in the early and mid-1960s failed to identify any specimens, but in 1969, ethnobotanist Douglas E. Yen of the Bishop Museum collected specimens near Miller Peak. Derral R. Herbst and Wayne Takeuchi of the FWS collected the last known specimen^[c] on July 27, 1980. Carl C. Christensen also visited Nīhoa in 1980 to reevaluate endemic species last observed on the Tanager Expedition. Sheila Conant and Mark S. Collins visited Nīhoa in 1980 as well; Conant returned twice in 1981, first with Mark J. Rauzon and later with Audrey L. Newman. In 1983, Conant visited the island with Wayne C. Gagné. Conant found A. brownii growing on the island in 1981 and 1983, by which time only two populations of 35 plants were thought to exist: 23 plants were found near Miller Peak and 12 plants in Middle Valley. The two plant populations were separated by a distance of approximately 0.4 km (0.25 mi).

Prehistoric Polynesian habitation on Nīhoa^[d] may have initially led to a decrease in the plant population of A. brownii. Major threats to the plant included invasive species, fire, and hybridization with other Amaranthus species. Inbreeding was a serious threat, as the small plant population must reproduce within its own circle resulting in genetic defects. A. brownii was also forced to compete with the non-native weed (Portulaca oleracea), the plant's main alien species threat. In 2002 and 2004, the invasive gray bird grasshopper (Schistocerca nitens) presented an even larger threat to A. brownii.^[e] First recorded on the island in 1977, the increasing population density of gray bird grasshoppers led to massive defoliation on the island, leaving A. brownii at greater risk of predation. In 2004 alone, an estimated 400,000 gray bird grasshoppers destroyed almost 90% of the vegetation on Nīhoa. All these threats may have contributed to its ultimate extinction.

Conservation

According to zoologist and conservationist Sheila Conant, A. brownii was important due to its uniqueness in the Northwestern Hawaiian Islands as "the only Hawaiian endemic in this large genus which contains many economically and nutritionally important species." However, in more than a decade of field surveys on Nīhoa, no living plants were identified. Wildlife refuge staff visited the island during the dry season at least 21 times between 1983 and 1996. Originally, botanists thought the absence of the plant in field surveys might have been explained by the time of visits During the summer months, the stems of A. brownii dried up and could not be distinguished from other herbaceous plants. However, a seven-day visit to the island in April 2006 still did not find any specimens. Additional winter surveys were performed, but none were successful.

Ex situ conservation efforts to propagate A. brownii by seed in botanical gardens were also unsuccessful. During the 1981 expedition, A. bronwii seeds were collected by Sheila Conant and presented to the Waimea Arboretum on the Hawaiian island of Oahu and the Kew Gardens in London, England. Although the seeds at the Waimea Arboretum germinated and grew for a while, no plants survived beyond the stage of seedling development. Information about the outcome of the seeds sent to Kew Gardens is unavailable.

A proposal for listing A. brownii under the U.S. Endangered Species Act was originally submitted on June 16, 1976, but was withdrawn on December 10, 1979, as out of date and incomplete. It was proposed again on March 24, 1993, and was federally listed as an endangered species on August 21, 1996. On May 22, 2003, the FWS designated 171 acres (69 ha) on the island of Nīhoa as a critical habitat for A. brownii, as well as Pritchardia remota, Schiedea verticillata, and two other species also found on Nīhoa and other Hawaiian islands, ʻohai (Sesbania tomentosa) and Mariscus pennatiformis. Also in 2003, A. brownii was internationally classified as critically endangered on the IUCN Red List. As of 2010, A. brownii was one of 51 Hawaiian plant species listed as endangered or threatened under the Endangered Species Act.