Bidens wailele (Asteraceae: Coreopsideae): A New Critically Endangered Species from Kauai, Hawaiian Islands

Premise of research. The explosive diversification of Hawaiian Bidens is considered one of the premier examples of plant adaptive radiation in the native Hawaiian flora. Botanical exploration, especially in remote areas, continues and sometimes results in the discovery of new species. Most of these new discoveries are narrowly distributed endemics, and some are evaluated to be threatened or endangered, sometimes critically so, as is the case for Bidens wailele.Methodology. Bidens wailele K.R. Wood & Knope (Asteraceae/Compositae) is (1) described and illustrated from Kauai, Hawaiian Islands; we also (2) provide a diagnostic key distinguishing characters that separate B. wailele from all other native Kauai Bidens, (3) present a summary of its distribution, ecology, and threats; and (4) provide a formal Red List assessment utilizing the International Union for Conservation of Nature (IUCN) criteria for endangerment. This discovery is part of ongoing floristic research and exploration conducted by the National Tropical Botanical Garden Science and Conservation Department.Pivotal results. This new species occurs in extremely wet conditions around the waterfalls of Waialeale and Wainiha, central Kauai. Morphologically, it is most similar to Bidens valida, a Kauai endemic species naturally occurring around windswept ridges and cliffs of southeastern Kauai (as on Haupu, Hulua, and Kahili Mountains).Conclusions. This rare new species is presently known from only 700-800 individuals and restricted to three remote locations; B. wailele has been evaluated under the IUCN Red List criteria and proposed as critically endangered. A fourth population on a wet cliff where the new species historically occurred (i.e., Kamanu, Kauai) was destroyed by a landslide after its discovery in 2008, most likely caused by torrential rains and demonstrating the species' vulnerability for extinction.