Channel Islands Chapter
Rare Plants: Definitions
What is a rare plant?   That depends.  That depends on what?  Rarity can be looked at and defined in many different ways, depending on the context of the question.  The following is a detailed discussion on this topic, and some answers to this question. 

Several lists of rare plants have been developed over the years for a variety of purposes, such as plants listed under the California Endangered Species Act (CESA) or the federal Endangered Species Act (ESA), or the Native Plant Protection Act (NPPA).  California Native Plant Society {CNPS) has maintained and updated a list of plants it considers to be “rare” in California, with five separate lists, in its Inventory of Rare and Endangered Plants of California, now in its sixth edition.  Federal, state, and some local government agencies also maintain lists of rare plants found within their jurisdiction.

Special-Status Plant Species Definitions

Special-status species are plants (including nonvascular plants) that are either listed as endangered or threatened under the Federal ESA or CESA; or considered to be rare under the California NPPA; or considered to be rare (but not formally listed) by resource agencies, professional organizations (e.g. CNPS, California Lichen Society), and the scientific community.  Special-status plant species are further defined in Table 1 (below).

Table 1.  Definitions of Special-Status Plant Species


1. Plants and animals legally protected under the California ESA and Federal ESA or under other regulations.

2. Plants and animals considered sufficiently rare by the scientific community to qualify for such listing; or

3.   Plants and animals considered to be sensitive because they are unique, declining regionally or locally, or are at the extent of their natural range.

Special-Status Plant Species

¨       Plants listed or proposed for listing as threatened or endangered under the Federal Endangered Species Act (50 CFR 17.12 for listed plants and various notices in the Federal Register for proposed species).

¨       Plants that are Category 1 or 2 (species of special concern) candidates for possible future listing as threatened or endangered under the Federal Endangered Species Act (55 CFR 6184, February 21, 1990).

¨       Plants that meet the definitions of rare or endangered species under the CEQA (State CEQA Guidelines, Section 15380).

¨       Plants considered by the CNPS to be "rare, threatened, or endangered" in California (Lists 1B and 2 in CNPS [2001][1]).

¨       Plants listed by CNPS as plants about which we need more information and plants of limited distribution (Lists 3 and 4 in CNPS [2001]).

¨       Plants listed by the California Lichen Society as rare in California (Magney 1999[2]).

¨       Plants listed or proposed for listing by the State of California as threatened or endangered under the California Endangered Species Act (14 CCR 670.5).

¨       Plants listed under the California Native Plant Protection Act (California Fish and Game Code 1900 et seq.).

¨       Plants considered sensitive by other federal agencies (i.e. U.S. Forest Service, Bureau of Land Management) or state and local agencies or jurisdictions.

¨       Plants considered sensitive or unique by the scientific community or occurring at the limits of its natural range (State CEQA Guidelines, Appendix G).


Listed species are those taxa that are formally listed as endangered or threatened by the federal government (e.g. U.S. Fish and Wildlife Service [USFWS]) pursuant to the ESA or as endangered, threatened, or rare (for plants only) by the State of California (i.e. California Fish and Game Commission) pursuant to the CESA or the NPPA.

The California Natural Diversity Database (CNDDB) Element Ranking system provides a numeric global and state ranking system for all special-status plant and wildlife species and rare habitats tracked by the CNDDB.  The global rank (G-rank) is a reflection of the overall condition of an element (species or natural community) throughout its global range.  The state ranking (S-rank) is assigned much the same way as the global rank, except state ranks in California often also contain a threat designation attached to the S-rank.  This Element Ranking system is defined below in Table 2, Natural Diversity Database Element Ranking System.

Not all special-status species are tracked by the CNDDB, nor have global or state rarity ranking been given to all special-status species; therefore, the Channel Islands Chapter of CNPS has applied the rules described above to “rank” those special-status species lacking such ranking. 

Table 2.  Natural Diversity Database Element Ranking System

Global Ranking (G)


Less than 6 viable elements occurrences (populations for species) OR less than 1,000 individuals OR less than 809.4 hectares (ha) (2,000 acres [ac]).


6 to 20 element occurrences OR 809.4 to 4,047 ha (2,000 to 10,000 ac).


21 to 100 element occurrences OR 3,000 to 10,000 individuals OR 4,047 to 20,235 ha (10,000 to 50,000 ac).


Apparently secure; this rank is clearly lower than G3 but factors exist to cause some concern (i.e. there is some threat, or somewhat narrow habitat).


Population or stand demonstrably secure to ineradicable due to being commonly found in the world.


All sites are historic; the element has not been seen for at least 20 years, but suitable habitat still exists.


All sites are extirpated; this element is extinct in the wild.


Extinct in the wild; exists in cultivation.


The element is very rare, but there is a taxonomic question associated with it.

Subspecies Level
Subspecies receive a T-rank attached to the G-rank.  With the subspecies, the G-rank reflects the condition of the entire species, whereas the T-rank reflects the global situation of just the subspecies or variety.
For example:  Chorizanthe robusta var. hartwegii.  This plant is ranked G2T1.  The G-rank refers to the whole species range (i.e., Chorizanthe robusta, whereas the T-rank refers only to the global condition of var. hartwegii.

State Ranking (S)


Less than 6 element occurrences OR less than 1,000 individuals OR less than 809.4 ha (2,000 ac).

          S1.1 = very threatened
          S1.2 = threatened
          S1.3 = no current threats known


6 to 20 element occurrences OR 3,000 individuals OR 809.4 to 4,047 ha (2,000 to 10,000 ac).

          S2.1 = very threatened
          S2.2 = threatened
          S2.3 = no current threats known..


21 to 100 element occurrences OR 3,000 to 10,000 individuals OR 4,047 to 20,235 ha (10,000 to 50,000 ac).

          S3.1 = very threatened
          S3.2 = threatened
          S3.3 = no current threats known


Apparently secure within California; this rank is clearly lower than S3 but factors exist to cause some concern (i.e., there is some threat, or somewhat narrow habitat).  NO THREAT RANK.


Demonstrably secure to ineradicable in California.  NO THREAT RANK.


All California sites are  historic; the element has not been seen for at least 20 years, but suitable habitat still exists.


All California sites are extirpated; this element is extinct in the wild.

Notes:  1.  Other considerations used when ranking a species or natural community include the pattern of distribution of the element on the landscape, fragmentation of the population/stands, and historical extent as compared to its modern range.  It is important to take a bird’s eye or aerial view when ranking sensitive elements rather than simply counting element occurrences.

2.       Uncertainty about the rank of an element is expressed in two major ways: by expressing the rank as a range of values (e.g., S2S3 means the rank is somewhere between S2 and S3), and by adding a ? to the rank (e.g. S2?).  This represents more certainty than S2S3, but less than S2.  (CNDDB 2002.)

CNPS’s Inventory of Rare and Endangered Plants of California[3] categorizes rare California plants into one of five lists (1A, 1B, 2, 3, & 4) representing the five levels of species status, one of which is assigned to a sensitive species to indicate its status of rarity or endangerment and distribution.  A CNPS List is a more general designation than the three separate sets of information provided in a CNPS R-E-D Code (defined in Table 3, California Native Plant Society R-E-D Code).  However, the CNPS List is a significant designation in terms of a species’ overall status throughout all of California, and it works well in conjunction to the specifications of the R-E-D Code.  Table 3, California Native Plant Society List, provides a definition for each List code number.

Table 3.  California Native Plant Society List (CNPS List)




Presumed Extinct in California


Rare or Endangered in California and elsewhere


Rare and Endangered in California, more common elsewhere


Need more information


Plants of Limited Distribution

(CNPS 2001)

As described for the CNDDB ranking, not all special-status species considered rare in the Channel Islands Chapter region are tracked by the CNDDB or CNPS at the state level, nor have R-E-D codes been given to them; however, the Channel Islands Chapter has applied the rules described above to “rank” those special-status species lacking such ranking.  This applies to the rare moss, liverwort, and lichen taxa found in the study area, for which CNPS has not yet developed or incorporated into its Inventory of Rare and Endangered Plants of California (CNPS 2001).  CNPS incorporating nonvascular plants (i.e. mosses and liverworts) for the first time in its sixth edition of the Inventory (David Tibor, CNPS Rare Plant Botanist, pers. comm.).  Rare lichen taxa are published by the California Lichen Society.  Taxa for which CNPS R-E-D Codes have been devised for this report are followed by a “?” in parentheses, denoting tentative assignment.

Locally Rare Plants

Since California is such a very large and diverse state, it is not reasonable or scientifically valid to ignore the natural differences of plant distributions and abundance or rarity at a more local level.  For example, the Bigleaf Maple (Acer macrophyllum) has a widespread distribution over much of California and northward.  It is common in northern California, but becomes increasingly less common towards the southern part of the state.  It is known from only one population in San Diego County.  Bigleaf Maple is known from over 15 occurrences in Ventura County.  Looking at this species’ rarity at the county level, Bigleaf Maple is clearly rare in San Diego County, but is probably too abundant in Ventura County to be considered "locally" rare.

The Los Padres National Forest maintains a list of Sensitive Plants found on the forest (which includes portions of Ventura, Kern, Santa Barbara, San Luis Obispo, and Monterey Counties).  The Bureau of Land Management (BLM) maintains similar lists for the lands it administers in California.  The National Park Service also maintains such lists for each of its parks.

There has been no systematic, formal, assessment of the rarity of plants at the county level; that is not until recently.  Various lists have been made identifying rare and unique plants from different parts of the state; however, no one method has been employed.  To address this problem, CNPS started a project to develop local lists, focusing on counties.  The Channel Islands Chapter currently has two countywide lists of locally rare plants within the general bounds of the chapter: Ventura County, and Santa Barbara County, which are available at Channel Islands Chapter Rare Plants.

Ventura County Rare Plants

The Channel Islands Chapter, through David Magney, accepted the task and developed a list of plants rare in Ventura County.  As part of David Magney’s floristic research in Ventura County, the Channel Islands Chapter has developed a list of native plants that are rare in Ventura County.  Of the over 2,100 species, subspecies, and varieties of vascular plants known to occur in Ventura County, about 1,300 of them have 10 or fewer populations in the County, or are considered rare statewide.  Certainly, many of the species rare in Ventura County are widespread and may be common in adjoining counties; however, they are not in Ventura County.  A complete checklist of these plants can be downloaded from the CNPS website.  Below is the description of the project, and criteria used to develop the checklist.

This list represents a compilation of known records and observations (by the author) of all native vascular plant species with ten (10) or fewer populations within Ventura County.  Some taxa are considered rare or uncommon throughout California, as established by CNPS in it’s Inventory of Rare and Endangered Plants of California, sixth edition, even though more than ten populations are known from Ventura County, including Anacapa Islands and San Nicolas Island.  Currently there are 1,395 taxa included in this list, when the previous version contained only 1,050 taxa.  Why so many more?  A number of taxa were deleted when over ten occurrences were known to occur in Ventura County.  At the same time, many taxa (368) were added that are likely, or have been reported, to occur in the County but for which no voucher specimens or reliable locality data are available.  Many of the additions are known to occur in adjacent counties and expected to occur in Ventura County.

This list was compiled as the results of extensive research of the flora of Ventura County and is based on extensive field searches, examination of specimens collected by others housed at public herbaria, and published reports. 

Soon to be published separately, in Ventura County Rare Plant Species, each species listed includes additional data on them, including a brief description, habitat preferences, associated plant communities, elevation range, and blooming period.  That publication will include additional information for each taxon, including complete scientific name, known synonyms, common name(s), habit and size, rarity status, months it is in bloom or identifiable, and general and specific locality data, with supporting voucher collection information.  Each specimen cited for a specific location includes the name of the collector, the collection number (or collection date if no number was assigned), and the herbarium in which it is stored (some duplicate specimens have been deposited in more than one herbarium).  Location information is provided at two levels:  first a general geographic area (starting in the northwest corner of the county eastward, and southward to the southeast corner of the county) and specific locations.  Each geographic region is separated by semicolons and specific localities separated by commas. 

This list should be considered to be the best available information at the time of publication, as our knowledge of the Ventura County flora is dynamic, and growing with each botanical survey or foray.  This checklist will be updated periodically and revised according to any and all new verifiable information provided.  Occurrences of vascular plants found in Ventura County that add to or could change the status of any plant listed here should be submitted to the author via CNPS (attention: David Magney) or via email to david(at)magney(dot)org.  Any lists submitted must be supported by voucher specimens that are deposited in a public herbarium.

Plants not included in CNPS’s Inventory, or listed by federal and state agencies, that have more than ten populations in Ventura County will be dropped from this list, except for those plants representing range limits, such as the westernmost limit of the taxon’s range or the “Type Locality” sites (“Type Locality” sites are the sites from which the plant was originally collected and formally described from, and is represented by a “Type Specimen”, for which a note occurs after the herbarium at which it is deposited as “Holotype” or “Isotype”).  Only the populations that represent the limit of a plants range should be considered of concern, with the remaining population(s) noted to provide perspective.   Artemisia tridentata var. tridentata is an example of this, where it should be considered of local concern when it occurs in the western portion of the Santa Clara River Valley (the limit of its range) while the populations in the Upper Sespe Creek watershed and the northern part of the county are not of concern.

This list was compiled according to definitions under the California Environmental Quality Act (CEQA) under Initial Study Checklist section 6. subsections A and E.  Projects reviewed under CEQA should consider impacts to one or more of the species included in this checklist as potentially significant.  Generally, if a project would impact a population of one or more of the plants listed herein, that/those impact(s) should be considered significant unless substantial evidence is provided that may support an alternate conclusion. 

The large number of taxa on this list is the result of several factors:

1.      Development in Ventura County has eliminated a majority of the natural vegetation in the southern half of the County, eliminating many occurrences of native plant species;

2.      A number of taxa found nowhere else are present in Ventura County, local endemics, and several of these are listed as threatened or endangered by either the California Department of Fish and Game or the U.S. Fish and Wildlife Service;

3.      Ventura County is geographically situated such that floral influences from the north, south, and east enter the County at its edges (limits of some species ranges);

4.      Anacapa and San Nicolas Islands support many species found only on the California Channel Islands, including island endemics;

5.      Ventura County is topographically and climatically diverse, providing numerous habitats for uncommon and rare species; and

6.      Botanical surveys have not been conducted systematically everywhere in the county.  In other words, the flora of Ventura County is not fully understood or documented, and additional populations of a number of the plants listed below likely occur in the county, we just aren’t aware of them yet.

This checklist is updated regularly based on new information on the Ventura County flora.  Species are added and deleted from this list as the new data warrant; therefore, be sure to obtain the most up-to-date version of this checklist.  The most up-to-date version can be downloaded (as a PDF) from the CNPS website at Channel Islands Chapter Rare Plants.


* * *

CNPS welcomes your comments and suggestions about locally rare plants.  In efforts to always maintain current, up-to-date, and accurate data, please submit your observations on the native, and naturalized plants in the Channel Islands Chapter region.  Your input is greatly appreciated.  Contact CNPS about the “Rare Plants” page:  david(at)magney(dot)org.

[1] California Native Plant Society.  2001.   Inventory of Rare and Endangered Vascular Plants of California.  Sixth Edition.  Sacramento, CA.

[2] Magney, D.L.  1999.  Preliminary List of Rare California Lichens.  California Lichen Society Bulletin 6(2):22-27.

[3] California Native Plant Society.  2001.  Inventory of Rare and Endangered Vascular Plants of California.  Sixth Edition.   

Special thanks to Carlin Moyer for the beautiful illustrations on our site.

Last updated: 14 February 2007
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