It differed from other Hawaiian species of Amaranthus with its spineless leaf axils, linear leaves, and indehiscent fruits.

Past expeditions collected plant samples and seeds, but no specimens managed to survive ex situ conservation efforts outside of its native habitat.

Conservation and recovery plans for A. brownii were proposed by the United States Fish and Wildlife Service (FWS), which administers the island of Nihoa as part of the Hawaiian Islands National Wildlife Refuge in the Papahānaumokuākea Marine National Monument.

Following a lack of sightings for over 35 years despite intensive surveys, the species was classified as extinct on the IUCN Red List in 2018.

[3] Forest B. H. Brown, botanist of the Bayard Dominick Expedition to the Marquesas Islands (1921–1922),[4] helped provide descriptions and comments for some of the species described by Erling Christophersen and Caum.

[5] They named A. brownii after Brown in 1931[6] with the publication of their paper "Vascular plants of the Leeward Islands, Hawaii".

[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.

brasiliensis), Panicum torridum, naupaka (Scaevola sericea), Sicyos pachycarpus, ʻilima (Sida fallax), and Nelson's horsenettle (Solanum nelsonii).

[9] The plant grew during the moist, winter season from December through July in Nihoa's coastal dry shrubland habitat[12] in shallow soil on rocky outcrops in exposed areas between 120–215 m (394–705 ft).

[18] Inbreeding was a serious threat, as the small plant population must reproduce within its own circle resulting in genetic defects.

[11] A. brownii was also forced to compete with the non-native weed (Portulaca oleracea), the plant's main alien species threat.

[11] 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.

Endangered Species Act was originally submitted on June 16, 1976, but was withdrawn on December 10, 1979, as out of date and incomplete.

^ Wagner and Herbst list five naturalized species of Amaranthus in Hawaii in addition to the endemic A. brownii.

The authors note that the information may be both inaccurate and incomplete due to errors caused by a lost collection and lack of data.

[10] A. graecizans, A. retroflexus, and a third unknown species (possibly extinct or reclassified) have been proposed as additional naturalized candidates.

However, questions about good potable water sources and the fact that only six skeletons have been found cast doubt on this figure.