Lichens of Conservation Concern

The California Lichen Society (CALS) provides information on the conservation concern for lichens in two formats. One is a series of lists that use the same criteria as those of the California Native Plant Society (CNPS). The other is rankings that are very similar to those used by the California Natural Diversity Database (CNDDB) and NatureServe. The primary difference in the CALS listing and ranking is that we cover additional states, as no similar organization exists in other southwestern states to provide lichen conservation information.


CALS works with CNPS to place lichens on appropriate lists for inclusion in the CNPS Inventory. A brief summary of the CNPS lists is given below; for further details, please refer directly to the CNPS list descriptions.

Summary of CNPS-equivalent lists:

  • List 1A – Lichens Presumed Extinct in State and Rare, Threatened, or Endangered Elsewhere. This list is restricted to species thought to be extinct or extirpated for the state in question.
  • List 1B – Lichens Rare, Threatened, or Endangered in State and Elsewhere. This list includes only species thought to be rare, threatened, or endangered throughout their global range.
  • List 2A – Lichens Presumed Extirpated from State, More Common Elsewhere.  These are thought to no longer survive in the state in question, but are known to be secure in some portion of their global range.
  • List 2B – Lichens Rare, Threatened, or Endangered in State, But More Common Elsewhere. This list includes lichens which are rare, threatened, or endangered within the state in question, but are thought to be secure in some other portion of their global range.
  • List 3 – Lichens About Which We Need More Information – A Review List. CALS currently does not intend to use List 3 for the large number of lichen taxa that are poorly understood and may only have a few reports for the state in question; many such lichens will prove to be quite common with more thorough surveys. Rather, this list will likely be used for lichens where there is a great deal of taxonomic uncertainty and significant reason for conservation concern. For example, a ‘species group’ expected to be divided into multiple species when adequate information becomes available, and which occurs infrequently or in limited areas, might be placed in List 3.
  • List 4 – Lichens of Limited Distribution – A Watch List. This list is used for lichens thought currently to be adequately secure in the state in question that they do not qualify for other lists, yet their distribution is sufficiently limited, or their populations are under sufficient threat, that their status should be monitored for future decline.
  • “CBR” – Lichens Considered But Rejected.  These have been researched by the CALS Conservation Committee to sufficient degree that the committee believes they are adequately secure in the state in question to not be placed on one of the lists above.  Such lichens may not be ‘common’; many infrequent taxa may be adequately secure.


The California Native Plant Society recognizes threat as a separate issue from rarity.  Threat ranks are frequently appended to a taxon’s List status (e.g. List 2B.2).

  • .1 – Seriously Endangered (threat rank)
  • .2 – Fairly Endangered (threat rank)
  • .3 – Not Very Endangered (threat rank)


CALS uses a ranking system very similar to those of the CNDDB and NatureServe. NatureServe and many state-level Natural Heritage Programs use a numeric system for individually ranking global (G-rank) and state (S-rank) conservation concern with 1 being of greatest concern and 5 being ‘demonstrably secure’. Ranks as given by CALS function as recommendations to the CNDDB, which retains authority to use different rank values as they see fit.

Rarity ranks in California are tied to population estimates in terms of the number of populations, number of individuals within populations, or area of population extents. Due to issues of practicality, most organisms ranked by the CNDDB are ranked by the number of populations, or “occurrences”. This will be particularly true for lichens due to the poor definitions of ‘individual’ and the difficulties of defining the physical area extent of populations. In general counts of occurrences are determined by the number of known sites for an organism, with multiple sites within 0.25 miles distance considered to be a single population. Other distance criteria may be scientifically justifiable. For example, more linear extents and network connections may be considered for aquatic taxa.
When some form of uncertainty exists, rarity ranks may reflect that uncertainty. A range rank incorporates uncertainty of what rank value is most appropriate (e.g. S2S3 – the proper rank could be either 2 or 3). If available data indicate a particular rank, but there is substantial reason to believe that the rank could change if thorough surveys are completed, then the uncertainty may be denoted by a question mark (e.g. S1? – current surveys indicate that it is an S1, but additional surveys could push it to an S2). Other rank values are available to denote special situations. The list below provides brief descriptions of common rank values. Please read the CNDDB Special Vascular Plants, Bryophytes, and Lichens List for further details.

Brief rank descriptions:

  • 1 – Critically Imperiled; generally 1-5 occurrences.
  • 2 – Imperiled; generally 6-20 occurrences.
  • 3 – Vulnerable; generally 21-80 occurrences.
  • 4 – Apparently Secure; uncommon but generally > 80 occurrences.
  • 5 – Secure; common, widespread, and abundant.
  • H – Occurrences known in historical data and potential habitat exists, but no sitings in the last 20 years.
  • X – All known sites extirpated (S rank) or extinct (G rank)

State of Knowledge of Lichens

CALS acknowledges that many lichen taxa are understudied and that the current number of known occurrences will often be less than the true number. For this reason, lichens are sometimes ranked as though they have a greater number of occurrences than is currently known, with consideration given to survey effort and rate of occurrence discovery.

Listing and Ranking Process

The CALS Conservation Committee requires all taxa proposed for listing and ranking to go through a sponsorship process that ensures a solid understanding of the current science and bieogeography.  See our page on Sponsoring Sensitive Lichens for details.

View the current Lichens of Conservation Concern in the CNPS Inventory.

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