206 Endangered Plants concludes with the management and social implications of protecting endangered plants • Kinds of Rarity There are many published definitions of rarity In an extreme example, DuMond (1973) stated nine criteria for rarity, including species that are: (1) found out of its expected context; (2) particularly subject to extinction or severe reduction in total population size by human activities that have already caused a significant population reduction; (3) found only in a very specific habitat of limited occurrence; (4) thought to be a relict of a no-longer extant vegetation association; (5) an indicator of a unique extant vegetation association; (6) recognized as an example of a wide, disjunction pattern; (7) at its natural distribution limits within the area in question; (8) known to be introduced and has become naturalized only on a very small scale; or (9) does not consistently occur as a member of any particular natural plant community Gaston (1997) described rarity in another way, noting that rare species can be delimited on the basis of one, two, or at most a few of the following variables: abundance, range size, habitat specificity (habitat occupancy), temporal persistence (e.g., taxon age), threat (probability of, or time to, extinction), gene flow, genetic diversity, endemism, and taxonomic distinctness Rare species in the IUCN Red Book (1998) are described in terms of population size rather than an assessment of extinction risk, which is reserved for threatened and endangered status The Nature Conservancy and associated network of Natural Heritage Programs rate species endangerment based on five levels of global rarity (i.e., G1, G2, G3, etc.), national rarity (N1, N2, etc.), and state rarity (S1, S2, etc.) The kinds of rarity that concern conservation biologists relate specifically to the potential vulnerability to extinction Commonly recognized kinds of rarity (Rabinowitz, 1981; Cropper, 1993) include species with: • • Small populations, large geographic range, and occurrences in several habitats For example, American chaffseed (Schwalbea americana) had a historical distribution from Mississippi to Massachusetts It resides now in 20 relatively small populations in five states, with the greatest populations in South Carolina It can survive in several vegetation types as long as there is enough light in the understory Fire suppression, which allows for succession and canopy closure, threatens several Schwalbea populations Another example is Hypochoeris maculata, which has small populations over a wide range in Great Britain Large populations, large geographic range, but occurrences in specific habitats For example, several grassland species of Calochortus (lilies) in California have large populations following wildfires They were thought to be far more widespread prior to a century of fire suppression and encroachment of shrubs into grassland habitats Sparse populations in a large geographic range but in specific habitats For example, Psilotum nudum grows only on rocky outcrops and always in sparse populations, but it occurs in Australia, New Zealand, and Easter Island (Cropper, 1993) • • • Small geographic range, but locally abundant in specific habitats For example, the 48 endemic species of Bignonaceae in Amazonia have a total home range of only a few thousand square kilometers, but some of these species are locally abundant in restricted habitats (Gentry, 1986) A specific example in Scotland is Primula scotica, which has a tiny range with a few large populations Small geographic range, and sparse in specific habitats This may be the most critical type of rarity, for small populations restricted to small and specific habitats are highly vulnerable to extinction For example, the scouring of a new stone quarry in South Africa ended life in the wild for Moraea loubseri, a small, sparse-population iris (Stermer, 1995) Small populations of concern in any region despite populations elsewhere For example, many floristically poor northern European countries typically have 200 or more plant species listed as threatened or endangered However, many of these species invaded broadly since the last Ice Age, and they are buffered from extinction with widely scattered populations in several countries (Davis et al., 1986) Small populations of new species For example, a newly discovered species of Calyptranthes from El Yunque, Puerto Rico, has a population of four trees, and Auerodendron pauciflorum from Quebradillas and Calyptranthes luquillensis from the Luquillo Mountains have only five individuals each (Cody, 1986) Generally, species may be ranked for protection based on overall rarity, magnitude of potential threats to populations, and the immediacy of the threats Thus, a monotypic genus with high-magnitude and imminent threats might receive a higher priority ranking than a subspecies with moderate or nonimminent potential threats Endangerment Patterns Slightly over 90% of threatened plants are single-country endemics Species with restricted ranges face the greatest threat of extinction It follows that endemic plant species on islands appear to be the most highly endangered About 95% of the plant species on the Canary Islands are endemic, and 50% are considered endangered (Cody, 1986) Senecio heritieri, for example, is restricted to a small area of rocky slopes on the south coast of the island of Tenerife, one of the Canary Islands On Crete, two-thirds of the 155 endemic species are endangered (Cody, 1986) Compare these numbers to those of the entire United States, where there are about 20,000 plant species of which 2050 species are rare and threatened About half the plant species in Mediterranean climate areas (parts of California, South Africa, Australia, and the Mediterranean basin) are narrow endemics that dominate the threatened and endangered species lists in their various countries (Figure 1) (Davis et al., 1986) California, for example, contains 669 endemic species of the 2050 species on the U.S rare and threatened list (Davis et al., 1986) Chile has 50 forest tree species; 47 of these species are endemic and 38 species are listed as endangered, vulnerable, or rare Over half of the rare and endangered plants of the continental United States grow within the borders of 12 western