Introduction
The IUCN Red List of Threatened Species (
https://www.iucnredlist.org), hereafter the “Red List”, is an extinction risk assessment tool that has informed and catalyzed species conservation for over five decades
\citep{BettsEtAl2020Conserv.Biol.}. The Red List has been instrumental in improving species conservation status and guiding their recovery
\citep{Hoffmann_2010,Grace_2021} while playing a major role in decision-making, policy implementation, planning, funding, and research
\citep{RodriguesEtAl2006TrendsinEcologyampEvolutiona,BettsEtAl2020Conserv.Biol.,Hoffmann_2008}. The Red List aims to compile the best possible information on the conservation and threat status of species, in a laudable effort to become the “barometer of life”
\citep{Stuart_2010}. However, Red List assessments cover a very small fraction of life on Earth. At present, only 150,000 species have been assessed
\citep{IUCN2023}, making less than 10% of about 2 million described species
\citep{BankiEtAl2023}. This percentage dwindles dramatically when considering recent estimates of global biodiversity, with the number of animal species exceeding 50 million alone
\citep*{Li_2022}.
The Red List has categorized 42,100 species as threatened \citep{IUCN2023}, yet over a million species are potentially facing extinction based on conservative estimates \citep{IPBES2019}. This number could be much larger given that undescribed species can have a higher extinction risk \citep{LiuEtAl2022Conserv.Lett.}. Furthermore, some assessed species could already be threatened, but reassessments have not been undertaken, which are recommended every 10 years \citep{IUCN2016a}. For instance, in the marine realm, over 40% of top-fished species have outdated assessments \citep{MiqueleizEtAl2022Fishes}. Overall, 28% of assessments have already been declared outdated \citep{IUCN2023}, potentially undermining the long-term use of the Red List \citep{RondininiEtAl2014Conserv.Lett.}.
Thus, while the Red List has undoubtedly been a major undertaking, we argue that as a standalone tool, it cannot provide an accurate, timely, and repeated assessment of the world’s threatened species. Furthermore, we assert that relying solely on the Red List is not sufficient for guiding conservation efforts, despite its influence on conservation planning and decision-making. Hence, in this review, we also expand on (a) the methodological drawbacks of the Red List (b) the limitations of the Red List categories and criteria (c) shortcomings of the assessment process, (d) recommendations for assessing extinction risk beyond the Red List and (e) guidelines to broaden species conservation and the setting of conservation priorities.
Rethinking the role of the IUCN Red List
The Red List underpins some of the most influential conservation mechanisms and initiatives at the global scale, such as the identification and designation of Key Biodiversity Areas
\citep{KBAStandardsandAppealsCommittee2022}. Notably, the Red List index is the headline indicator for the species component of Goal A in the Kunming-Montreal Global Biodiversity Framework (KM-GBF) of the Convention of Biological Diversity (
https://www.cbd.int/gbf/). This influence on the global conservation agenda can lead to downstream consequences such as the selection of conservation priorities, even though it was not explicitly designed to do so
\citep{IUCN2012,IUCNStandardsandPetitionsCommittee2022,MaceEtAl2008Conserv.Biol.}.
The conservation prioritizations driven by the Red List are often realized through funding allocation from governments, NGOs, and other global stakeholders \citep{WilsonEtAl2009Ann.N.Y.Acad.Sci.}, where funds are directed to species based on their threat status on the Red List \citep{MillerEtAl2013Conserv.Lett.,BettsEtAl2020Conserv.Biol.}. For instance, major organizations such as the Rainforest Trust or the Mohamed Bin Zayed Species Conservation Fund typically give priority to species listed as Critically Endangered (CR) or Endangered (EN). Furthermore, newer initiatives, such as the EDGE Protected and Conserved Area Fund and the Indianapolis Zoo Saving Species Challenge, also require target species to be listed in threatened categories of the Red List. Other funders, such as the North American Pangolin Consortium Grant, require endorsements from IUCN Specialist Groups.
Although well intended, such international funding can direct conservation actions that erase local context and conflict with those identified by regional experts \citep{MelickEtAl2012Conserv.Soc.,IsbellEtAl2023FrontiersinEcolampEnviron}, overlooking or undermining more pressing conservation efforts based on localized assessments \citep{McShane2003Conserv.Biol.,SmithEtAl2009Nature}. This is particularly concerning in the Global South, where financial resources are limited. In contrast, the Red List has less influence on prioritization in high-income economies such as North America and Europe \citep{LimEtAl2023BiologicalConservation}, where conservation strategies such as the US Endangered Species Act and the EU Birds and Habitats Directives are designed, implemented, and funded internally.
Given this context, threatened species on the Red List, particularly charismatic ones, attract more funding and scientific attention than non-threatened species \citep{TrimbleVanAarde2010Conserv.Biol.,SmithEtAl2012Conserv.Lett.,BrambillaEtAl2013BiologicalConservation}. Consequently, conservation efforts and research have mainly targeted a few selected species on the Red List \citep{Martin-LopezEtAl2011BiodiversConserv}. This creates a cycle of underfunding for many species \citep{HochkirchEtAl2021Conserv.Biol.} and prevents actions to support their persistence or recovery. We acknowledge that conserving threatened species on the Red List could benefit co-occurring organisms and their habitats \citep{CoxEtAl2022Nature}. However, it is important to carefully evaluate and justify their use as “umbrella" species rather than assuming it \citep{RobergeAngelstam2004Conserv.Biol.,SeddonLeech2008Oryx,WangEtAl2021BiologicalConservation}. In addition, less charismatic species or those perceived as uncharismatic, such as invertebrates or parasitic species, should also be considered for conservation efforts given their key roles in ecosystem functioning \citep{EisenhauerEtAl2019NatCommun,CarlsonEtAl2020BiologicalConservation,HughesEtAl2021BioScience}.
Overreliance on the Red List to select conservation priorities can also lead to misuse or exploitation \citep{Campbell2012Conserv.Soc.}. This can occur by elevating the threat status of a species \citep*{GodfreyGodley2008Endang.SpeciesRes.}, opposing downlistings to maintain funds \citep*{MallonJackson2017Oryx}, or elevating populations and subspecies to the species level, where they are placed in threatened categories \citep{ThomsonEtAl2021OrgDiversEvol}. Such situations can create a sense of urgency and fundraising opportunities due to the “high-profile” status of the Red List, which would be mitigated if species conservation was less dependent on a single system of threat categorization \citep*{SoberonMedellin2007Conserv.Biol.}.
In summary, the global influence of the Red List and its misuse in decision-making \citep{Lunney2017AustralianZoologist,PossinghamEtAl2002TrendsEcol.Evol.} can constitute a priority-setting ‘pitfall trap’ \citep{Martin-LopezEtAl2011BiodiversConserv} in which funds and other resources are disproportionately allocated to a small number of species that have been assessed, are threatened, and are charismatic. This is exacerbated by the globalization of conservation \citep{Chapin2004WorldWatchMagazine,RodriguezEtAl2007Science}, the concentration of power in centralized authorities, and the dominance of conservation priorities set by high-income economies and large conservation organizations \citep*{WybornEvans2021NatEcolEvola}.
Methodological drawbacks of the IUCN Red List
We are not the first to express concerns about the Red List \citep{Master1991ConservationBiology,Mrosovsky1997Nature,WebbCarrillo2000PopulationEcology,SoberonMedellin2007Conserv.Biol.,Keith2009Significance,Campbell2012Conserv.Soc.,Lovari2020} with controversies arising from inaccurate assessment of different species, including high-profile examples such as sea turtles \citep{Mrosovsky2003,GodfreyGodley2008Endang.SpeciesRes.,Campbell2012Conserv.Soc.} and primates \citep{Nijman2004TheRafflesbulletinofzoology,ThomsonEtAl2021OrgDiversEvol}.
The Red List was designed as a quantitative system to evaluate the relative risk of species extinction \citep{MaceEtAl2008Conserv.Biol.}. However, species assessments are often based on expert opinion, which may be biased or subjective \citep{Keith2009Significance,HaywardEtAl2015Front.Ecol.Evol.,deOliveiraCaetanoEtAl2022PLoSBiol}. For instance, a recent study revealed that the threat categories of stork species (family Ciconiidae) were based on unfounded assumptions regarding their ecology and natural history and not scientific evidence \citep{GulaEtAl2023BiologicalConservation}. The utility of the Red List has also been questioned for entire groups of organisms, such as invertebrates \citep{CardosoEtAl2011BiologicalConservation}, which constitute the majority of animal species on Earth \citep{BankiEtAl2023}. In fact, the Red List is currently biased towards terrestrial vertebrates, the first group of species that was evaluated (M). This group has over 90% of described species assessed, in contrast to only 2% of invertebrates, 0.5% of fungi and protists, and 15% of plants, although some groups, such as gymnosperms, have almost complete assessments (IUCN 2023).