Skip common site navigation and headers
United States Environmental Protection Agency
Great Lakes Ecosystems
Begin Hierarchical Links EPA Home > Great Lakes Ecosystems > Great Lakes Biological Diversity >  Issues and Opportunities > Appendix 2
GL Basin Ecosystem
GL Biological Diversity
Green Landscaping
Aquatic Ecosystems
Upland Ecosystem
Ecosystem Funding


Conservation of Biological Diversity in the Great Lakes Basin Ecosystem:  Issues and Opportunities

Appendix 2
The Heritage Ranking System

Element Ranking

An element's Global Rank is assigned on the basis of the following definitions:

Basic Ranks

G# = Numeric rank: A numeric rank (G1 through G5) of relative endangerment based primarily on the number of occurrences of the element globally.

G1 = Critically imperiled globally due to extreme rarity or due to factor(s) making it especially vulnerable to extinction. (Typically 5 or fewer occurrences or very few remaining individuals or acres)

G2 = Imperiled globally due to rarity or due to some factor(s) making it very vulnerable to extinction throughout its range. (6 to 20 occurrences or few remaining individuals or acres)

G3 = Either very rare and local throughout its range or found locally (even abundantly at some of its locations) in a restricted range (e.g., a single western state, a physiographic region in the East) or due to other factors making it vulnerable to extinction throughout its range. (21 to 100 occurrences)

G4 = Widespread, abundant, and apparently secure globally, though it may be quite rare in parts of its range, especially at the periphery. Thus, the element is of long-term concern. (Usually more than 100 occurrences)

G5 = Demonstrably widespread, abundant, and secure globally, though it may be quite rare in parts of its range, especially at the periphery.

Note: Other factors in addition to the number of occurrences are considered when assigning a rank, so the numbers of occurrences suggested for each numeric rank above are not absolute guidelines.

G#-G# = Numeric range rank: A range between two of the numeric ranks. Denotes a range of uncertainty about the exact rarity of the element (this range may encompass no more than three ranks). For the purposes of this analysis, elements with uncertain ranks were ascribed the higher (most imperilled) status.

G? = Unranked: Element is not yet ranked globally.

GU = Unrankable: Possibly in peril range-wide but status uncertain; need more information. This rank is rarely used. Whenever possible, the most likely rank is assigned with a question mark (e.g., G2?) to express uncertainty, or a numeric range (e.g., G2G3) is used to delineate the limits (range) of uncertainty.

GH = Historical: Of historical occurrence throughout its range, i.e., formerly part of the established biota, with the expectation that it may be rediscovered

GX = Extinct: Believed to be extinct throughout range (e.g., Passenger Pigeon) with virtually no likelihood that it will be rediscovered.

HYB = Hybrid: Element represents a hybrid of species.


T = Taxonomic subdivision: rank applies to a subspecies or variety.

? = Inexact: denotes inexact numeric rank.

Q = Questionable taxonomy: taxonomic status is questionable; numeric rank may change with taxonomy.

C = Captive or clutivated: element is present only in captivity or through cultivation.

A = 0-5
B = 6-20
C = 21-100
D = 101+

Element Ranking Considerations

Global Estimated Element Occurrences
Estimated number of occurrences believed extant for the element on a global basis.

Estimated Element Occurrence is the single most important factor influencing the Global Rank when occurrences are few. If the letter code for this field is A or B, the element will inevitably qualify for a 1 or 2 rank (ie. G1 or G2).

Global Abundance
The present estimated global abundance of the element.


A = Fewer than 1,000 individuals
Fewer than 2,000 acres
Fewer than 10 miles of stream length
B = 1,000-3,000 individuals
2,000-10,000 acres
10-50 miles
C =

3,000-10,000 individuals
10,000-50,000 acres
50-250 miles

D = over 10,000 individuals
over 50,000 acres
over 250 miles

Global Trend
The description which best characterizes the trend in the element's distribution over its global range:

A = Declining rapidly
B = Declining
C = Stable
D = Increasing

Global Protected Element Occurrences
Estimated number of occurrences of the element that have been adequately protected globally.

A = Believed to be none protected.
B = At least one protected EO.
C = Several protected EO's.
D = Many protected EO's.
U = Unknown whether any protected.

Global Threat
Degree to which the element is directly or indirectly threatened globally.

A = Very threatened range-wide; species or community directly exploited or threatened by natural or man-made forces.
B = Moderately threatened range-wide; habitat or community lends itself to alternate use.
C = Not very threatened range-wide; self-protecting by unsuitability for other uses.
D = Unthreatened on a range-wide basis, although it may be threatened in minor portions of the range.

Global Fragility
Fragility indicates how susceptible an element is to non-destructive intrusion.

A = Extremely fragile (eg. some cave communities, some freshwater mussels).
B = Fragile (eg. bald eagle, fens).
C = Fairly resistant (eg. northern raven).
D = Tough (eg. redwoods).

Global Other Considerations
Other considerations used to determine rank that are not apparent from the letter codes representing the other categories. Generally, these considerations should raise rather than lower the rank.


  • "Goes through unexplained population fluctuations."
  • "Never sexually reproduces."

Element Occurrence Ranking Guidelines

Rank Description

A Excellent Occurrence: Protection of A-ranked occurrences is essential for conservation of maximum diversity and viability of an element in a jurisdiction. The occurrence should be among the best in existence throughout the element's range. A-ranked communities are essentially undisturbed by humans, or have nearly recovered from early human disturbance. Species composition shows little departure from original structure and composition (except in seral or disturbance-dependant communities). A-ranked communities are among the largest of their type, and large enough to provide reasonable assurance for long-term viability of component ecological processes and species diversity. A-ranked populations of sensitive species have large numbers of individuals, are stable or growing, show good reproduction, and exist in a natural, sustainable habitat.

B Good Occurrence: Protection of these occurrences is important to the survival of an element in a jurisdiction and for the conservation of regional diversity, especially if A-ranked occurrences are scarce or absent. A B-ranked community is still recovering from early disturbance or recent light disturbance (recoverable departure from original structure and composition of the site is required, except in seral or disturbance-dependant communities). B-rank communities may include exotic species (localized and/or a minor component of the community). While not necessarily of an ideal size, they are among the larger examples of their type, or at least large enough to minimize edge effects and sustain component species and/or ecological processes. B-ranked populations of sensitive species are at least stable, in a minimally disturbed habitat, and of moderate population size.

C Fair Occurrence: Protection of these occurrences helps conserve the biotic diversity on a regional or local level and is important to jurisdiction-wide conservation only if no higher ranked occurrences exist. A C-ranked community is in an early stage of recovery from disturbance, or its structure and composition have been altered such that the original vegetation of the site will never rejuvenate, yet with management and time partial restoration of the community is possible. They may also be of relatively high quality, but too small to sustain ecological processes and/or species composition in the long-term. C-ranked populations of sensitive species are in clearly disturbed habitats, small in size and/or number, and possibly declining.

D Poor Occurrence: Protection of these occurrences is seldom worthwhile except for historical reasons for if no better occurrences exist. D-ranked communities are severely disturbed, their structure and composition have been greatly altered, and recovery to original conditions, despite management and time, essentially will not take place. In addition to their degraded quality, they may be too small to recover to a viable state. D-ranked populations of sensitive species are very small with a high likelihood of dying out or being destroyed, and/or exist in highly disturbed and vulnerable habitats.


back to index

Begin Site Footer

EPA Home | Privacy and Security Notice | Contact Us