NatureServe Conservation Status: A Framework for Measuring Biodiversity Imperilment
In the effort to protect the planet's biological diversity, scientists require a standardized way to communicate how much danger a species or ecosystem is in. The NatureServe conservation status system provides this essential language. Developed in the United States during the 1980s by The Nature Conservancy (TNC), this system is now maintained by NatureServe in cooperation with the Natural Heritage Network.
These designations, often called NatureServe ranks or Natural Heritage ranks, categorize the relative imperilment—the degree of threat or risk of extinction—of plants, animals, ecological communities, and other organisms. By applying these ranks at global, national, and subnational levels, conservationists can prioritize resources to protect the most vulnerable parts of our natural world.
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Key Facts
- Purpose: To rank the relative imperilment of species and ecological communities.
- Scope: Methodology is global, with primary data for the United States and Canada.
- Scale: Uses a one-to-five numerical scale for vulnerability and letter notations for special cases.
- Exclusions: Ranks focus on natural occurrences and ignore individuals in captivity or non-native invasive species.
- Geographical Levels: Ranks are applied at Global (G), National (N), and Subnational (S) levels.
How the Ranking System Works
The NatureServe system evaluates the status of an entity based on its natural presence in the wild. It specifically ignores populations maintained in captivity or cultivation (such as zoos or botanical gardens) and excludes non-native occurrences established through human intervention, such as invasive species.
Geographical Levels of Assessment
Conservation status is not a single number; it is a multi-layered assessment that can be applied at three distinct levels:
- G-rank: The status at the global or range-wide level.
- N-rank: The status within a specific nation.
- S-rank: The status within a subnational unit, such as a state or province.
These ranks can be concatenated to provide a complete picture. For example, a G5N3S1 designation tells us a species is globally secure (G5), nationally vulnerable (N3), but critically imperiled within a specific state (S1).
Understanding the Numerical Ranks
Most assessments use a five-point scale to describe how secure or vulnerable a species is. The lower the number, the higher the risk of extinction.
| Rank | Status | Typical Criteria |
|---|---|---|
| 1 | Critically Imperiled | 5 or fewer occurrences, or ≤ 1,000 individuals |
| 2 | Imperiled | 6 to 20 occurrences, or 1,001 to 3,000 individuals |
| 3 | extVulnerable | 21 to 100 occurrences, or 3,001 to 10,000 individuals |
| 4 | Apparently Secure | 101+ occurrences, or 10,001+ individuals |
| 5 | Secure | Common, widespread, and abundant |
Special Designations and Letter Notations
Because nature is complex, numerical scales cannot cover every scenario. NatureServe uses specific letters to denote special biological circumstances.
Extinction and Presence Uncertainty
- X: Presumed extinct (globally) or extirpated (lost from a specific area, but exists elsewhere).
- H: Possibly extinct or extirpated; there is a reasonable hope of rediscovery.
- R or ?: Recorded in the area, but the local status is unknown or indeterminate.
- C: When added to X or H, it indicates the taxon exists only in captivity or cultivation.
Taxonomic and Biological Variations
The system also accounts for specific biological nuances:
- T-rank: Used for infraspecific taxa, such as subspecies or plant varieties.
- E: Indicates non-native (exotic) taxa introduced by human intervention.
- HYB: A placeholder for modern interspecific hybrids that are not primary conservation targets.
- U: Unrankable, used when information is too conflicting or absent to assign a status.
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Frequently Asked Questions
What is the difference between extinction and extirpation?
Extinction refers to a global phenomenon where a species no longer exists anywhere on Earth. Extirpation refers to a species that has been lost from a specific national or subnational area but still exists in other parts of the world.
Does a NatureServe rank include invasive species?
No. NatureServe ranks focus on natural occurrences. Non-native species established through human intervention are marked with an "E" (exotic) designation and are not ranked using the standard native methodology.
How are subspecies ranked differently than species?
Subspecies and plant varieties are assigned T-ranks. These are appended to the global G-rank of the parent species to provide specific data for that particular taxonomic group.
What does a rank like G2G3 mean?
This notation indicates a range of uncertainty. It means the global status is estimated to be somewhere between rank 2 (Imperiled) and rank 3 (Vulnerable). If the uncertainty is too large (e.g., a gap of more than two ranks), the status is listed as "U" for Unrankable.
Are species with a rank of 4 or 5 a priority for conservation?
Generally, species with G, N, or S rankings of 4 (Apparently Secure) or 5 (Secure) are not the primary focus of major conservation actions, as they are considered stable or abundant.