Wilkesia is a small genus of two perennial flowering plants in the family Asteraceae (tribe Madieae), both endemic to the island of Kauaʻi in the Hawaiian archipelago. The genus belongs to the silversword alliance, a celebrated adaptive radiation of roughly 34 species in the subtribe Madiinae that descended from a single Pacific Coast tarweed ancestor that reached Hawaiʻi approximately 5.1 million years ago. The three genera of the alliance — Wilkesia, Argyroxiphium, and Dubautia — together represent one of the most striking examples of rapid plant diversification known to science.
Both Wilkesia species are shrubby rosette-forming plants. Their leaves are long, strap-shaped to linear with longitudinal venation, and juvenile leaves tend to be larger and less firm than mature ones. Like other silversword alliance members, Wilkesia produces composite inflorescences (capitula) with large flower heads whose bracts are covered in sticky, glandular trichomes — a trait shared with their Californian tarweed relatives. Unlike many Asteraceae, Wilkesia and Dubautia lack ray florets, the petal-like outer flowers present in sunflowers and daisies. Wilkesia plants are polycarpic, meaning they flower multiple times over their lifespan. All members of the silversword alliance are tetraploid (2n = 14 chromosomes), which distinguishes them from their diploid North American relatives.
The two species are Wilkesia gymnoxiphium (iliau) and Wilkesia hobdyi (dwarf iliau). Both are restricted to dry scrub and woodland habitats on Kauaʻi with relatively low annual rainfall. Both are listed as endangered or critically endangered on the IUCN Red List, with major threats from introduced browsing ungulates, invasive plant species, and pollinator decline.
Etymology
The genus name Wilkesia honors Captain Charles Wilkes, commander of the United States Exploring Expedition (1838–1842), which collected and documented many Pacific island plant species.
Distribution
Both species of Wilkesia are narrowly endemic to the island of Kauaʻi, Hawaiʻi, where they grow in dry scrub and woodland habitats with relatively low annual rainfall. No other naturally occurring populations exist outside Kauaʻi.
Ecology
Wilkesia species grow in dry scrub and woodland habitats on Kauaʻi and can occur sympatrically with some Dubautia species. Like all silversword alliance members, they are perennials that take several years to flower. Their composite inflorescences attract native pollinators; pollinator loss is identified as a contributing factor in their population decline.
Conservation
Both Wilkesia species are listed as endangered or critically endangered on the IUCN Red List. Key threats include browsing by introduced feral ungulates, competition from invasive plant species, and loss of native pollinators. Wilkesia hobdyi is tracked by the Center for Plant Conservation's National Collection.
