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Have you seen any of Colorado's rare plants? If you came across a plant you hadn't seen before, might it be one that is protected under Federal or State law? How many plants in Colorado are considered "rare", and how is that determined? Answers to these questions come from the biological study of rarity and the social and political interpretation of those study results.

The study of rarity is a fascinating analysis of complex ecological factors that determine the distribution and abundance of an organism. Why is it that some Colorado plants are found in a wide range of habitats, and others are restricted to particular soil types? Why are populations of some species abundant, whereas others have been reduced to several thousand individuals in the state? How does human intervention (through habitat alteration) impact the long-term viability of populations of Colorado plants?

Answers to these complex questions have characteristically been uncovered after detailed study of a particular rare plant species. Studies of single species are an important source of information to guide land managers in their management efforts to increase populations of that species. Another common study approach is to derive comparisons between rare and common species in the same genus. This type of study helps separate intrinsic (e.g. reproductive, physiological) and extrinsic (e.g. interspecific competition, habitat destruction) factors which may be contributing to the rare species' decline. Because organisms may be rare for a variety of reasons (and perhaps for multiple reasons), knowledge of particular causes of rarity is critical for effective species management.

Once botanical research has identified species that might be in peril, political, social, and economic decisions are made to determine which species will receive conservation attention. The biologist's list of "rare" Colorado plants is likely to be considerably more extensive than the politician's (and no two lists will agree!).

Of the approximately 3200 flowering plant species in the state, 173 have been categorized as rare by a consortium of Federal, state, and private botanists in the Colorado Rare Plant Field Guide. This excellent guide is available without charge from any of the contributing agencies. Although 173 plants are considered "rare" in the Guide, only fourteen are protected under the Federal Endangered Species Act (ESA). An additional 52 plants listed were formerly recognized by the Federal government as potential candidates for ESA listing (so-called C2 species); the remaining are "sensitive species" found on Bureau of Land Management and U.S. Forest Service lands. An additional 227 plant species in the state are "of special concern" to the Colorado Natural Heritage Program, but were not included in the Guide.

Endangered plants in Colorado (see "Do You Know?" for complete list) include two Astragalus species, a buckwheat, a cactus, a Penstemon sp., and a Phacelia species. Plants categorized as threatened under the ESA include two cacti, three mustards, and an orchid. Two additional species, an Astragalus sp. and a Penstemon sp., are the only plants now categorized as Candidate (C1) Species. Specimens of all of these protected plants can be found in the CSU Herbarium.

Among Colorado rare plants listed in the Rare Plant Field Guide, the greatest number (24) are in the genus Astragalus (Fabaceae), including nine species that are endemic to the state. Other families with numerous rare Colorado representatives include Asteraceae (15 species); Brassicaceae (14 species); and Scrophulariaceae (14 species, of which 12 are in the genus Penstemon, including six Colorado endemics).

The families above are also well-represented in the common state flora, but such is not always the case. Some families have a disproportionate number of rare members. All Colorado members of the fern-like family Ophioglossaceae, for instance, are rare and endangered. (In North America as a whole, the family Cactaceae is in the most endangered condition; 44% of cacti require protection (Belousova and Denisova 1992)).

Generalizations about rarity at levels below family are also difficult to make. Some genera have both rare and common relatives; Colorado has a rare species of ragweed, Ambrosia linearis (Asteraceae). Other genera (e.g. Bolophyta, in the same family) contain only rare members in Colorado (three species).

Some rare plants are members of genera with many other species (e.g. Erigeron, with three rare Colorado species and many other relatives). In other instances, a rare plant may be the only member of its genus (e.g. Askellia nana, formerly Crepis nana; this plant has a wide distribution outside of Colorado).

How does Colorado compare with other states in terms of the number of rare plant species contained within its borders? In 1985, Colorado (ranked 14^th) had 2.3% of the total U.S. flora protected by Federal law (endangered, threatened, and candidate C1 and C2 species) (Belousova and Denisova 1992)..

A rare plant in the United States is most likely to be found in Hawaii; 50.6% of that state's isolated island flora requires urgent measures for conservation, and 75% of the island's species are endemics. Similar statistics for the continental U.S. vary depending on the thresholds of rarity used, but as a conservative estimate, 10.7% of continental U.S. plant species require protection in the near future (Belousova and Denisova 1992). California, with its diverse ecosystems, has by far the greatest number of rare plant species outside of Hawaii, with Florida next in line.

Two important resources for tracking rare plants on an international scale are the IUCN (International Union for the Conservation of Nature) Red Data Book. and Appendix I of CITES (Convention of International Trade in Endangered Species of Wild Flora and Fauna in Commerce), which lists species banned in trade without special permission. Although scientists compiling the IUCN Red List previously estimated that 20,000 species of plants were rare worldwide (Belousova and Denisova 1992), they now realize that over 33,000 species of plants (or 1 in 8 species worldwide) are in peril (Nature Conservancy 1998). Of the Colorado rare flora, Pediocactus knowltonii is on the Red List, and both species of Sclerocactus are listed in Appendix 1 of CITES (Belousova and Denisova 1992).

If knowing which family a plant is in doesn't necessarily tell you if it's likely to be rare, are there other distinguishing characteristics that might? Probably not! Scientists are discovering that rare plants fit many different descriptions. Rare and common members of the same genus may be quite similar in appearance; in fact, 25 of the 173 species in the Rare Plant Field Guide are so similar to other taxa as to have been afforded varietal or subspecific status.

Although many rare plants are restricted in their distribution to specialized habitats, others (e.g. Astragalus microcymbus ) can be found on a variety of soil types and in plant communities that are otherwise common throughout the state. Even local population sizes may not provide a clue to a species' rarity; a plant which has a globally restricted distribution may be quite abundant in a particular region of the state (e.g. Cleome multicaulis; see accompanying article). Colorado's rare flora affords a fascinating opportunity to explore a wide diversity of plant families. Although many rare plants of the state are restricted to specialized ecological habitats, others are not so easily categorized by habitat or other attribute. As our knowledge of the phenomenon of rarity increases, efforts to preserve Colorado's diverse rare flora will become more effective, helping to ensure that future generations will have the opportunity to discover the fascinating variety of rare plants to be found in the state.

References:

Belousova, L.S. and Denisova, L.V. 1992. Rare Plants of the World. Russian Translations Series 93. A.A. Balkema, Rotterdam. 348 pp.

Nature Conservancy, July/August 1998.

Spackman, S. et al. 1997. Colorado Rare Plant Field Guide. Prepared for the Bureau of Land Management, the U.S. Forest Service and the U.S. Fish and Wildlife Service by the Colorado Natural Heritage Program.

Do You Know?

Colorado plants protected under the Federal Endangered Species Act are the following:

ENDANGERED SPECIES:

Astragalus humillimus

Mancos milkvetch

Astragalus ousterhoutii

Osterhout milkvetch

Eriogonum pelinophilum

Clay-loving buckwheat

Pediocactus knowltonii

Knowlton's cactus

Penstemon penlandii

Penland beardtongue

Phacelia formosula

North Park phacelia

THREATENED SPECIES:

Eutrema edwardsii ssp. penlandii

Penland alpine fen mustard

Lesquerella congesta

Dudley Bluffs bladderpod

Physaria obcordata

Piceance twinpod

Sclerocactus glaucus

Uinta Basin hookless cactus

Sclerocactus mesae-verdae

Mesa Verde cactus

Spiranthes diluvialis

Ute ladies'-tresses

CANDIDATE SPECIES:

Astragalus tortipes

Sleeping Ute milkvetch

Penstemon debilis

Parachute penstemon