Portulaca oleracea
Prehistoric Polynesian habitation on Nihoa 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 wa"s 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". 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 Nihoa. All these threats may have contributed to its ultimate extinction.