page_title: Houtman Abrolhos
section_title: Flora
hierarchical_section_title: Houtman Abrolhos / Terrestrial flora / Flora
caption_attribution_description: Nitraria billardierei (nitre bush) in fruit. On a causeway, St Kilda, Adelaide, South Australia
context_page_description: The Houtman Abrolhos is a chain of 122 islands, and associated coral reefs, in the Indian Ocean off the west coast of Australia. Nominally located at 28°43′S 113°47′E, it lies about eighty kilometres west of Geraldton, Western Australia. It is the southernmost true coral reef in the Indian Ocean, and one of the highest latitude reef systems in the world. It is one of the world's most important seabird breeding sites, and is the centre of Western Australia's largest single-species fishery, the western rock lobster fishery. It has a small seasonal population of fishermen, and a limited number of tourists are permitted for day trips, but most of the land area is off limits as conservation habitat. It is well known as the site of numerous shipwrecks, the most famous being Dutch ships Batavia, which was wrecked in 1629, and Zeewijk, wrecked in 1727.
  In July 2019 the Houtman Abrolhos was declared a national park by the Western Australian government.
context_section_description: The vascular flora of the Houtman Abrolhos has been thoroughly surveyed, and species lists have been published for 119 islands. As of 2001, these lists totalled 239 species from 68 families. A further six species have been collected in the Houtman Abrolhos, but cannot be allocated to islands because insufficient location information was recorded. There have also been collections of mosses, liverworts and lichens, but no information has been published on these non-vascular groups.
  The islands with the greatest floristic diversity are East and West Wallabi Islands, with 124 and 97 species respectively. 54 species occur in all three island groups. The most widely distributed species are Nitraria billardierei (nitre bush), which has been recorded on 106 islands; the exotic Mesembryanthemum crystallinum (iceplant), on 88 islands; Threlkeldia diffusa (coast bonefruit), on 72 islands; and Atriplex cinerea (grey saltbush), on 70 islands. On the other hand, Eucalyptus oraria (ooragmandee) and Acacia didyma occur only on East Wallabi Island.
  As of 2001, five species of priority flora occurred on the islands. One, Acacia didyma, is no longer considered a priority species. The remaining priority species are  Chthonocephalus tomentellus, which is rated Priority Two under the Department of Environment and Conservation's Declared Rare and Priority Flora List; Calocephalus aervoides and Galium migrans, both Priority Three; and Lepidium puberulum, Priority Four.
  Ninety-five exotic species from 29 families have been recorded. In general, islands that have or had human settlements are the weediest. Of greatest concern is the noxious weed Lycium ferocissimum (African boxthorn), which has long spines that can trap birds. This weed was recorded on the islands as early as 1970. Efforts to eradicate it began in 1990; there was a lull in eradication in the late 1990s, but the program was later reinstated, and in July 2007, the Department of Environment and Conservation reported that the species had been eradicated from 14 of the 18 islands on which it had been recorded. Other noxious weeds include Opuntia stricta (prickly pear), Verbesina encelioides and Echium plantagineum (Paterson's curse).
caption: Nitraria billardierei (nitre bush), pictured here in Adelaide, South Australia, is the most widely distributed plant species in the Houtman Abrolhos
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