Endangered Plants

ENDANGERED PLANTS Thomas J. Stohlgren Colorado State University I. Introduction THE GROWING PUBLIC CONCERN about endangered II. Kinds of Rarity plants is well founded. It is estimated that there are III. Endangerment Patterns between 235,000 (Raven et al., 1986) and 270,000 IV. Causes of Endangerment (IUCN, 1998) species of vascular plants (including V. Consequences of Rarity ferns and fern allies, gymnosperms, and flowering VI. Legal Mandates to Protect Endangered Plants plants) in the world today; two-thirds of these species VII. Selected Examples of Endangered Plants are found in the tropics (Raven et al., 1986). The World VIII. Implications Conservation Union (IUCN, 1998) currently lists 33,798 species (12.5% of the world’s flora) as threatened, and 6,522 species of plants (2.4% of the world’s flora) as endangered. Threatened plants are found GLOSSARY among 369 plant families in 200 countries. The U.S. Fish and Wildlife Service reports 540 endangered critical habitat Habitat of a threatened or endangered species and 130 threatened species of flowering species that is itself threatened by destruction, dis- plants in the United States (U.S.) as of November 30, turbance, modification, or human activity, poten- 1998. tially resulting in a reduction in the numbers, distribution, or reasonable expansion or recovery of that species. I. INTRODUCTION endangered species Those species in danger of extinction throughout all or a significant portion of their Extinction has always played an important role in na- range. ture: 95 to 99% of all species that ever existed are now endemism Condition in which a species’ distribution extinct. It is the current rapid rate of extinction that is restricted to a given geographic region. has many plant ecologists worried. Worldwide, as many rare species Species with small world populations that as 654 species of plants have gone extinct since A.D. are not presently listed as endangered or vulnerable, 1600 (Heywood, 1995). In this time period, about 110 but are at risk because of their small population size. plant species may have gone extinct in Hawaii according threatened species Those species that are likely to be- to records of the U.S. Fish and Wildlife Service and come endangered in the foreseeable future through- The Nature Conservancy. Estimates of plant extinctions out all or a significant portion of their range. in the continental United States range from below 25 (IUCN, 1998) to above 90 (Davis et al., 1986; Flather Encyclopedia of Biodiversity, Volume 2 Copyright 2001 by Academic Press. All rights of reproduction in any form reserved. 465 466 ENDANGERED PLANTS et al., 1994). The discrepancy in estimates is due to populations in well-suited habitats and smaller popula- incomplete systematic surveys, limited monitoring ac- tions in marginal or sub-optimal habitats. Many threat- tivities, and poor information on viable seed banks. ened vascular plants in Finland were found to be rare Estimates of extinctions in other countries include 71 largely because the species were in marginal rather than plant species in Australia, 53 in South Africa, 47 in suitable habitat (Cropper, 1993). Along with the in- Mauritius, and 23 in Cuba. Three endemic species of creasing threats of rapid environmental change, habitat ebony (Diospyros spp.) went extinct in the Mascarene loss, contaminants, and invasive exotic species, we may Islands before they were described. Nuttall’s mudwort face an uphill battle in protecting our treasured botani- (Micranthemum micranthemoides) had been recorded in cal resources in these sub-optimal habitats. Delaware, Maryland, New Jersey, New York, Pennsylva- Maintaining biodiversity requires a considerable un- nia, Virginia, and the District of Columbia, but, despite derstanding of rarity and the processes and stresses that extensive searches it has not been seen since 1941. The promote the endangerment of certain plant species. This single individual of Dicliptera dodsonii, which clings to article reviews the kinds of rarity, patterns of endanger- life in western Ecuador (Gentry, 1986), may be the next ment, causes of endangerment, consequences of rarity, victim of accelerated extinction. and legal mandates to protect endangered plant species. Estimates of future plant species extinctions vary It then provides selected examples of endangered plants widely. Raven et al. (1986) estimated that about 40,000 and concludes with the management and social implica- tropical plant species may go extinct in the wild within tions of protecting endangered plants. the next several decades. The New York Botanical Gar- den suggests that about 700 of the more than 2000 species of threatened and endangered plant species in II. KINDS OF RARITY the United States may be extinct in the next 10 years. The Center for Plant Conservation reported that 680 There are many published definitions of rarity. In an U.S. plant species were critically endangered, with ap- extreme example, DuMond (1973) stated nine criteria proximately 253 species estimated to become extinct for rarity, including species that are: (1) found out of in 5 years and 427 species to become extinct in 10 years its expected context; (2) particularly subject to extinc- (based on unpublished data in 1988). Today, more than tion or severe reduction in total population size by 10 years later, there is little evidence of these mass human activities that have already caused a significant extinctions, but the concern may still be well founded population reduction; (3) found only in a very specific in several areas. habitat of limited occurrence; (4) thought to be a relict Hawaii provides a good example of endangered of a no-longer extant vegetation association; (5) an plants. Up to 47% of the Hawaiian flora may meet the indicator of a unique extant vegetation association; (6) criteria for official listing by the U.S. Fish and Wildlife recognized as an example of a wide, disjunction pattern; Service, with as many as 16% immediately threatened (7) at its natural distribution limits within the area in with extinction (see the Missouri Botanical Garden web- question; (8) known to be introduced and has become site listed in the Bibliography). The genus Hibiscadel- naturalized only on a very small scale; or (9) does not phus in Hawaii includes only six species (with two ex- consistently occur as a member of any particular natural tinct species) with a total of 14 live individuals, and plant community. Gaston (1997) described rarity in one species with only one individual (Cody, 1986). The another way, noting that rare species can be delimited Hawaii Endangered Plant Task Force, which includes on the basis of one, two, or at most a few of the following many federal, state, and non-government partners, now variables: abundance, range size, habitat specificity tracks 597 threatened, endangered, and rare plant (habitat occupancy), temporal persistence (e.g., taxon species. age), threat (probability of, or time to, extinction), gene There is a growing backlog of candidates for listing flow, genetic diversity, endemism, and taxonomic dis- as endangered species. Between July, 1976, and August, tinctness. Rare species in the IUCN Red Book (1998) 1992, about 21 species per year were added to the U.S. are described in terms of population size rather than endangered species list. In the second half of that 16- an assessment of extinction risk, which is reserved for year period, 33 species per year were added to the list threatened and endangered status. The Nature Conser- (Flather et al., 1994). More than 200 plant species are vancy and associated network of Natural Heritage Pro- being petitioned for future listing in the United States. grams rate species endangerment based on five levels It has been said that ‘‘all species are rare somewhere’’ of global rarity (i.e., G1, G2, G3, etc.), national rarity (Orians, 1997), because most plant species have larger (N1, N2, etc.), and state rarity (S1, S2, etc.). ENDANGERED PLANTS 467 The kinds of rarity that concern conservation biolo- vaded broadly since the last Ice Age, and they are gists relate specifically to the potential vulnerability buffered from extinction with widely scattered pop- to extinction. Commonly recognized kinds of rarity ulations in several countries (Davis et al., 1986). (Rabinowitz, 1981; Cropper, 1993) include species • Small populations of new species. For example, a with: newly discovered species of Calyptranthes from El Yunque, Puerto Rico, has a population of four • Small populations, large geographic range, and oc- trees, and Auerodendron pauciflorum from Quebradil- currences in several habitats. For example, Ameri- las and Calyptranthes luquillensis from the Luquillo can chaffseed (Schwalbea americana) had a histori- Mountains have only five individuals each (Cody, cal distribution from Mississippi to Massachusetts. 1986). It resides now in 20 relatively small populations in five states, with the greatest populations in South Generally, species may be ranked for protection Carolina. It can survive in several vegetation types based on overall rarity, magnitude of potential threats as long as there is enough light in the understory. to populations, and the immediacy of the threats. Thus, Fire suppression, which allows for succession and a monotypic genus with high-magnitude and imminent canopy closure, threatens several Schwalbea popula- threats might receive a higher priority ranking than a tions. Another example is Hypochoeris maculata, subspecies with moderate or non-imminent potential which has small populations over a wide range in threats. Great Britain. • Large populations, large geographic range, but oc- currences in specific habitats. For example, several III. ENDANGERMENT PATTERNS grassland species of Calochortus (lilies) in Califor- nia have large populations following wildfires. They Slightly over 90% of threatened plants are single-coun- were thought to be far more widespread prior to a try endemics. Species with restricted ranges face the century of fire suppression and encroachment of greatest threat of extinction. It follows that endemic shrubs into grassland habitats.

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