Not evaluated
Not evaluated (NE) is a designation in the IUCN Red List Categories and Criteria for taxa that have not yet been systematically assessed for their risk of extinction using the IUCN's quantitative criteria.[1] This category signifies a lack of sufficient evaluation rather than a judgment on threat level, and such taxa are not included in the published IUCN Red List database.[2] As of April 2025, the IUCN has assessed approximately 169,420 species, leaving the vast majority of the estimated 1.5 to 2 million described species in the not evaluated status due to resource constraints and prioritization of certain taxonomic groups.[3] The designation underscores gaps in biodiversity knowledge, emphasizing the need for expanded assessments to inform conservation priorities, particularly for understudied invertebrates and microorganisms.[4] While not implying safety or peril, the not evaluated status can delay targeted interventions until data collection enables classification into categories such as data deficient, least concern, or threatened.[5]Definition and Criteria
Core Definition
The Not Evaluated (NE) category designates taxa that have not yet undergone assessment against the IUCN Red List criteria for evaluating extinction risk.[6] This status indicates a lack of formal evaluation rather than any inference about the taxon's conservation condition, distinguishing it from categories like Data Deficient (DD), where assessment has occurred but data insufficiency prevents categorization.[6] NE applies to the vast majority of Earth's described species, estimated at over 99% as of recent assessments, due to limited resources for comprehensive global biodiversity evaluation.[7] Taxa assigned NE are excluded from the published IUCN Red List, which focuses on assessed species to inform conservation priorities.[1] The category underscores gaps in taxonomic and ecological knowledge, as only select groups—such as mammals, birds, and certain plants—receive routine scrutiny, leaving insects, fungi, and many invertebrates unevaluated.[2] Reassessment may elevate a taxon from NE once sufficient data becomes available, potentially revealing previously unrecognized threats.[8]Distinction from Other IUCN Categories
The IUCN Not Evaluated (NE) category applies exclusively to taxa that have not undergone any formal assessment against the Red List criteria, distinguishing it from all other categories which require an evaluation process to determine extinction risk.[8] Unlike the eight published categories—Extinct (EX), Extinct in the Wild (EW), Critically Endangered (CR), Endangered (EN), Vulnerable (VU), Near Threatened (NT), Least Concern (LC), and Data Deficient (DD)—NE taxa are not included on the IUCN Red List itself, as no data compilation or risk analysis has occurred.[1] This procedural status reflects a lack of systematic review rather than any inference about population status or threats.[9] A key distinction lies between NE and Data Deficient (DD), where DD denotes taxa that have been evaluated but for which inadequate information prevents a reliable assignment to threatened, Near Threatened, or Least Concern categories, often due to genuine knowledge gaps on distribution, abundance, or threats.[9] In contrast, NE indicates no evaluation attempt has been made, potentially encompassing undescribed species, newly discovered taxa, or those overlooked in assessment priorities; DD assessments, however, involve documented efforts yielding insufficient quantitative data for criteria application, such as population size reduction thresholds or geographic range metrics.[8] The threatened categories (CR, EN, VU) and conservation-dependent ones (NT, LC) further diverge from NE by quantifying extinction probability over defined time frames—e.g., CR requires a 50% decline in three generations or a 10% probability of extinction in 10 years—based on empirical evidence like habitat loss or exploitation rates, whereas NE provides no such risk appraisal.[9] EX and EW, meanwhile, confirm absence in the wild or entirely, verified through exhaustive searches and absence records spanning 50 years post-last sighting.[8] Thus, NE serves as a placeholder for future scrutiny, not a risk indicator, underscoring the Red List's incomplete coverage of global biodiversity, with millions of species remaining unevaluated as of 2023.[4]Historical Development
Origins in IUCN Red List
The Not Evaluated (NE) category was formally introduced in the IUCN Red List Categories and Criteria version 2.3, adopted in 1994, to designate taxa that had not yet been assessed for extinction risk using the established quantitative criteria.[10] This marked a shift from earlier IUCN Red Data Books, which began publication in 1966 with volumes on mammals and later birds, focusing primarily on listing species considered rare, endangered, or vulnerable without explicit categories for unevaluated taxa.[5] In pre-1994 assessments, species absent from these lists were implicitly unevaluated, but lacked a standardized designation, reflecting the qualitative and selective nature of initial Red List compilations starting from 1963.[5] The 1994 framework defined NE alongside other categories such as Extinct, Critically Endangered, Endangered, Vulnerable, Lower Risk (with subcategories), and Data Deficient, emphasizing that NE applied to taxa not subjected to the criteria-based evaluation process.[10] This introduction addressed the need for a comprehensive system applicable across diverse taxa, acknowledging that only a fraction of described species—estimated at around 1.8 million at the time—could be practically assessed given resource constraints. The category underscored the preliminary stage of global biodiversity evaluation, where NE status highlighted gaps in knowledge rather than implying low risk.[10] Version 3.1 of the criteria, adopted by the IUCN Council on February 9, 2000, and published in 2001, retained the NE category without substantive changes to its definition, maintaining it as a marker for unevaluated taxa while refining overall assessment methodologies through extensive testing and consultation since 1989.[11] Unlike assessed categories, NE taxa are not published on the IUCN Red List website, distinguishing them from Data Deficient species, which have undergone evaluation but lack sufficient information for categorization.[1] This persistence of NE origins in the 1994 system established a foundational principle of transparency about assessment coverage, informing subsequent expansions where, by 2024, over 157,000 species had been evaluated, leaving the majority in NE status.[1]Evolution of the NE Category
The Not Evaluated (NE) category emerged as part of the formalized IUCN Red List Categories and Criteria system introduced in 1994, following six years of development to establish quantitative standards for extinction risk assessment.[8] Prior to 1994, the IUCN Red List, initiated in 1964 through early Red Data Books, focused primarily on compiling lists of threatened species using qualitative judgments without standardized categories for unevaluated taxa.[5] The 1994 framework (version 2.3) explicitly defined NE as applying to taxa that had not been assessed against the new criteria, thereby highlighting the distinction between unassessed species and those evaluated but data-deficient.[10] Subsequent revisions, such as version 3.1 adopted in 2001, preserved the NE definition without substantive changes, maintaining its role as a marker for species outside the formal evaluation process.[9] This stability allowed for consistent tracking of assessment coverage, revealing that NE encompassed the vast majority of described species—estimated at over 2 million eukaryotic taxa—while only 157,000 had been assessed by 2024.[12] The category's design ensures that the Red List publishes only evaluated species, excluding NE to emphasize empirical evaluations over assumptions of safety.[1] Over time, the NE category has underscored systemic gaps in biodiversity data, prompting initiatives like specialist group collaborations and data mobilization efforts to transition species from NE to assessed statuses.[13] Despite these advances, resource constraints and the sheer scale of undescribed or unstudied taxa have sustained NE as a critical indicator of incomplete global conservation knowledge, with no alterations to its criteria reflecting a commitment to unaltered baselines for longitudinal comparisons.[8] This evolution from an implicit unassessed state to a deliberate category has enhanced transparency in reporting assessment progress without implying lower risk for NE species.[2]Assessment Processes and Challenges
IUCN Evaluation Methodology
The IUCN evaluates species extinction risk using a standardized quantitative framework outlined in the Red List Categories and Criteria, version 3.1, adopted in 2001 and retained in the second edition published in 2012.[9][14] This methodology classifies taxa into one of nine categories—Extinct (EX), Extinct in the Wild (EW), Critically Endangered (CR), Endangered (EN), Vulnerable (VU), Near Threatened (NT), Least Concern (LC), Data Deficient (DD), and Not Evaluated (NE)—based on the probability of extinction in the wild.[15] A taxon qualifies as threatened (CR, EN, or VU) if it meets quantitative thresholds under any of five criteria (A–E), prioritizing empirical data on population trends, range, and threats over qualitative judgments.[15][14] The five criteria assess different facets of extinction risk:- Criterion A: Reduction in population size, measured over specified time frames (e.g., 10 years or three generations), with CR requiring ≥90% decline, EN ≥70%, and VU ≥50%.[15]
- Criterion B: Restricted geographic range, combining extent of occurrence (EOO) or area of occupancy (AOO) with fragmentation or decline; CR applies to EOO <100 km² or AOO <10 km², EN to EOO <5,000 km² or AOO <500 km², and VU to EOO <20,000 km² or AOO <2,000 km².[15]
- Criterion C: Small population size with observed or projected decline; CR for <250 mature individuals with ≥25% decline in three years or one generation, EN for <2,500 with ≥20% in five years or two generations, and VU for <10,000 with ≥10% in 10 years or three generations.[15]
- Criterion D: Very small or restricted population, with CR for <50 mature individuals, EN for <250, and VU for <1,000.[15]
- Criterion E: Quantitative analysis, such as population viability analysis, estimating extinction probability; CR for ≥50% within 10 years or three generations, EN for ≥20% within 20 years or five generations, and VU for ≥10% within 100 years.[15]