Rare Species Ranking Definitions

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ORBIC participates in an international system for ranking rare, threatened and endangered species throughout the world. The system was developed by The Nature Conservancy and is now maintained by NatureServe in cooperation with Heritage Programs or Conservation Data Centers (CDCs) in all 50 states, in 4 Canadian provinces, and in 13 Latin American countries. The ranking is a 1-5 scale with 1 being critically imperilied and 5 being secure. When determining species rank many factors are taken into account, including the number of known occurrences, threats, inherent sensitivity, area occupied, and other biological and anthropogenic factors.

Global Ranks

Ranks are developed for different portions of a species range. The first and most critical rank describes the species status globally, and best describes the risk of extinction. This is called the Global Rank and begins with a "G". If the taxon has a trinomial (a subspecies, variety or recognized race), this is followed by a "T" rank indicator. A "Q" at the end of this line indicates the taxon has taxonomic questions.

State Ranks

Other status ranks are developed using the same criteria, but for smaller portions of the species ranges. The most widely used in the United States are the State Ranks. These State Ranks begin with the letter "S". For the Survey and Manage Species, Sranks were developed for each of the species in Washington, Oregon and California. In addition, the National Ranks describing the species status in the United States, was developed. These begin with the letter “N”. The ranks are summarized below:

1 = Critically imperiled because of extreme rarity or because it is somehow especially vulnerable to extinction or extirpation, typically with 5 or fewer occurrences.

2 = Imperiled because of rarity or because other factors demonstrably make it very vulnerable to extinction (extirpation), typically with 6-20 occurrences.

3 = Rare, uncommon or threatened, but not immediately imperiled, typically with 21-100 occurrences.

4 = Not rare and apparently secure, but with cause for long-term concern, usually with more than 100 occurrences.

5 = Demonstrably widespread, abundant, and secure.

H = Historical Occurrence, formerly part of the native biota with the implied expectation that it may be rediscovered.

X = Presumed extirpated or extinct.

U = Unknown rank.

? = Not yet ranked or assigned rank is uncertain.

More details on the Heritage Ranking system and more definitions can be found at the NatureServe Web site and on their Conservation Status page. A complete description of the ranking process is available in the Natureserve Methodology for Assigning Ranks publication (PDF, 335 kb).

Heritage Lists

ORBIC evaluates the NatureServe ranks and further refines the data for state application through placing species on their List 1 to 4. The fact that Oregon (and California) has such high rates of species endemism was the reason for the implementation of the1-4 Heritage Listing in Oregon and California. In particular, most heritage programs consider all G1, G2 or G3 taxa as needing protection. Based on a snapshot search of NatureServe explorer, 1138 taxa in Oregon are ranked G1-G3 (or T1-T3). Of these, 615 taxa ranked as a G3 (or T3, excluding taxa with G3 in range ranks). Many of these are rare endemics which have few documented threats. Because of this, ORBIC evaluates G3 taxa individually based the significance of the species Oregon distribution, and overall threats to the taxa. This evaluation is the primary analysis used to determine which ORBIC list they are placed on.

Brief Summary of ORBIC Lists:

List 1 contains taxa that are threatened with extinction or presumed to be extinct throughout their entire range. These are the taxa most at risk, and should be the highest priority for conservation action.

Any taxa with heritage Global ranks of G1 or G2 (or T1 or T2) would be included on List 1, as well as any G3 (or T3) taxa which are showing significant or dramatic declines rangewide.

List 2 contains taxa that are threatened with extirpation or presumed to be extirpated from the state of Oregon. These are often peripheral or disjunct species which are of concern when considering species diversity within Oregon's borders. They can be very significant when protecting the genetic diversity of a taxon. ORNHP regards extreme rarity as a significant threat and has included species which are very rare in Oregon on this list.

All taxa with Heritage State Ranks of S1 and most taxa with Heritage State Ranks of S2 are included on this list.

List 3 contains species for which more information is needed before status can be determined, but which may be threatened or endangered in Oregon or throughout their range. Many taxa on this list may eventually be determined to belong on List 1 or List 2, so it is important that they be looked for, and the few known occurrences be protected.

Taxa on this list often include those with range ranks (such as G2G4 or G2G3), indicating uncertainty about their status. In addition, these species can have a question mark in their status, or have an unknown status. Typical ranks might be G3?S3?, indicating that the species is probably at risk, but with significant status questions.

List 4 contains taxa which are of conservation concern but currently do not meet the criteria for begin considered threatened or endangered. This includes taxa which are very rare but are currently secure, as well as taxa which are declining in numbers or habitat but are still too abundant to be proposed as threatened or endangered. While these taxa currently may not need the same active management attention as threatened or endangered taxa, they do require continued monitoring.

Formerly considered a watch list, List 4 includes species that may be of conservation concern from a global or state perspective. The more abundant or less threatened S2 ranked taxa, along with taxa with state ranks of S3 which are showing significant or dramatic declines statewide would also be included on this list. Similarly, G3 taxa that are not significantly declining, or with less imminent threats would be included on this list.

Photo of big-flowered woolly meadow-foam (Limnanthes floccosa ssp. grandiflora)
© Alan St. John
 

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