208 Endangered Plants with rapid development and land-use change, leads to special problems for the western states that wish to conserve their floral heritage Worldwide, Australia and New Zealand have attracted much attention Official listings show 1931 endangered plant species in Australia and 22 species in New Zealand (Stermer, 1995) Australia has already lost 117 plant species to extinction in modern times (Davis et al., 1986), and has 3329 plant species that are considered rare or threatened (6.2% of the flora) An estimated 993 species are at risk of extinction in the next 50 years (Cropper, 1993) Some plant families and genera are more prone to extinction than others Twenty vascular plant families have at least 50% of their species threatened (e.g., Brunelliaceae, 91.9%; Zamiaceae, 88.9%; Araucariaceae, 78.9%; Taxaceae, 75%; Limnanthaceae, 72.7%) In Australia, species-rich genera such as Acacia, Eucalyptus, and Grevillia contain a proportionally greater number of rare species than species-poor genera Gymnosperms, containing relatively ancient species, may be less adapted to today’s rapidly changing environment (IUCN, 1998) Cacti in the United States are particularly prone to extinction: 72 of 268 native species are very rare (Benson, 1982) In California, shrubs and subshrubs of the species-rich genera Arctostaphylos, Ceanothus, Eriogonum, and Ribes have a higher proportion of threatened and endangered plants relative to their proportion in the flora (Cody, 1986) Curiously, some rare plant habitats may have human origins Some moorlands, or blanket bogs, in western Europe were created around 7700 years ago by human activities of burning and grazing These heathlands and grasslands today support many endangered plant species Causes of Endangerment There are several natural causes of rarity that lead to endangerment Ancient taxa, like cycads, are thought to be prone to extinction as they cling to increasingly shrinking relict habitats New taxa, resulting from recent speciation, are thought to be susceptible to extinction because they have not had time to spread in distribution to reduce risk Rare species generally lack an ability to rapidly colonize areas and they are often poor competitors Many rare species are edaphically restricted to specific soils or geology However, based on recent rates of extinction, natural causes of rarity (individually or combined) are no match for human-related causes of endangerment Species usually become endangered because of multiple, human-related causes The overwhelming cause of plant species endangerment is habitat loss, directly linked to agriculture, forestry, and urbanization Remaining small populations have a higher risk of extirpation (local extinction) or extinction Coinciding with habitat loss is reduced habitat quality related to invasive species, grazing, and other land-use changes (Flather et al., 1994) There are many examples of habitat loss to choose from In the wetter forested Sierra Madres of Mexico, Guatemalan fir (Abies guatemalensis) or Pinabete trees are considered endangered from over-exploitation of old-growth forests, land-use change, and livestock grazing (Burton, 1991) Persea theobromifola, once an important timber species in western Ecuador, has been reduced to fewer than 12 trees at Rio Palenque (Gentry, 1986) In the United States Burton (1991) reported that only 1% of the 1,036,000 km2 (400,000 mi2) of tallgrass prairie remains, now existing in isolated patches and small nature reserves Over half of all continental U.S wetland and aquatic habitats have been destroyed Fire suppression and succession, invasive exotic plants, and habitat fragmentation will make it increasingly difficult for rare species to persist Causes of rarity vary by region Though habitat loss was generalized as an overall problem, agricultural development was specifically noted in the southern Appalachian region, whereas urban development and forest clearing were specifically noted in Florida In the Gulf Coast areas, shoreline modification and development were specifically noted in endangerment patterns (Flather et al., 1994) Commercial exploitation is also a problem Cacti in the southwestern United States are a prime example Legislation regulating the collection of cacti in Arizona and California date from 1929 Yet, in a single year, over half a million small cacti were illegally collected for sale in the United States, Japan, and Europe (Burton, 1991) One of the two colonies of the 2.5-cmdiameter Nellie Cory cactus (Coryphantha minima) was completely eliminated by thieves in the 1960s The rarest species demand the highest prices from collectors Rare orchids are plucked one by one by various ‘‘collectors.’’ Some threatened orchids in India, such as Paphiopedilum druri, Dendrobium pauciflorum, Dendrobium nobile, and Diplomeris hirsuta, are used for medicinal purposes As such, they are facing increasing pressure from growing and aging human populations Natural environmental change can both cause and maintain rarity Cropper (1993) reported that some rare species, such as Leptorhychos gatesii in Australia, are observed only following natural fire A riparian herb, Collomia rawsoniana, required periodic high and low streamflows to persist Likewise, disruption of the natural hydrology of Lake Okeechobee, the largest lake in Florida, has led to the endangered listing of Cucurbita okeechobeensis Land management decisions can obviously upset required disturbance patterns Fire suppression activities and flood control will likely increase the rarity of such species The introduction of exotic grazers such as goats, pigs, rabbits, and sheep often leads to devastated native floras For example, 47 of the 49 native plant species on St Helena Island in the south Atlantic Ocean are now rare or threatened The island was once dominated by beautiful forests of St Helena ebony (Trochetia melanoxylon) Goats were introduced on the island in 1513, and the goat population skyrocketed by 1588 Goats ate ebony seedlings and humans used mature trees as fuel, so by 1810 the forests were destroyed and the ebony was extinct About 260 naturalized exotic plant species were also introduced to the island (Davis et al., 1986) Introduced rabbits have greatly threatened Acacia carnie in arid western New South Wales, Australia (Cropper, 1993), and nearly decimated Dudleya traskiae (ironically called the Santa Barbara live-forever) on Santa Barbara Island off California (Benseler, 1987) Hawaii’s beautiful Haleakala silversword (Argyroxiphium sandwicense ssp macrocephalum), with a flower stalk up to m tall, almost went extinct in the 1920s due to vandalism and grazing by introduced cattle and goats Like many endangered plant species, several additional threats must be held at bay A major effort in Hawaii is under way to