Goal A: outcomes for species
The first Goal of the KMGBF seeks to conserve biodiversity at the level of ecosystem, species and genetic diversity (see CBD [1992] Article 2 for CBD’s definition of biological diversity). It mandates that:
The integrity, connectivity and resilience of all ecosystems are maintained, enhanced, or restored, substantially increasing the area of natural ecosystems by 2050; Human induced extinction of known threatened species is halted, and, by 2050, the extinction rate and risk of all species are reduced tenfold and the abundance of native wild species is increased to healthy and resilient levels; The genetic diversity within populations of wild and domesticated species, is maintained, safeguarding their adaptive potential.
Here, we consider the second set of three clauses (in italics above) relating to species, which deal with three complementary aspects of species’ conservation status: extinctions, extinction risk, and abundance. The 37 words in these clauses contain some complex concepts and a considerable degree of nuance that have important implications for implementation. We address each in turn:
“Human induced”. These words establish that this clause refers directly to extinctions driven by human activities. Species evolve and go extinct naturally. The intent of Goal A is to focus attention on extinctions that are driven by human activities, rather than ‘natural’ extinctions such as those caused by, for example, volcanic eruptions (e.g. San Benedicto Rock Wren Salpinctes obsoletus exsul in 1952: Brattstrom, 1990), which would be extremely difficult to avoid. Some natural phenomena, such as hurricanes, floods and fires are occurring at far higher intensity and frequency because of climate change (e.g. Habibullah et al. 2022, Tripathy et al. 2023: see also Preston-Allen et al. 2024), and these in turn are driving extinctions (Murali et al. 2023). As recent climate change is human-driven (IPCC, 2023), it is appropriate that such extinctions should be considered human-driven, although in practice it may be difficult to confirm that climate change has led to particular extinctions. Hence, actions need to tackle the full range of threats that are driving species towards extinction, both directly and indirectly, and including climate change.
“Known threatened species”. The Goal focuses on extinction of species that have been documented as threatened. There are an estimated 8.1 million species on Earth (Mora et al. 2011), of which 1.5 million have been described (e.g. Costello et al. 2013). More than 150,000 species have been assessed for their risk of extinction using the categories and criteria of the IUCN Red List of Threatened Species (IUCN 2023a), which is widely considered to be the most comprehensive and objective system for classifying species in terms of extinction risk (Rodrigues et al. 2006; Stuart et al. 2010; Roberts et al. 2023). Of these, over 42,100 are assessed as threatened with extinction. Numerous additional species have been assessed for their extinction risk at the national scale for national red lists, often using the guidelines for regional and national application of the IUCN Red List categories and criteria (IUCN 2012). The number of species being assessed at various scales is continually increasing (e.g. Hochkirch et al. 2023). There are, therefore, certain to be many more species that are threatened, but are not yet formally documented as such, including large numbers of species that have not even been described to science. However, these undocumented species that may be threatened are excluded from the focus in this clause, as a practical approach to guide the baseline for measurement. It is therefore critical to identify the factors impacting known threatened species, and to address these to prevent their extinction and help recover their populations.
“Halted” makes clear the intention to put in place the action necessary to stop extinctions from a starting point of December 2022, when the Framework was adopted. Hence in the first clause Parties have committed to preventing human activities from driving any further extinctions of those species that have been identified as threatened. This is ambitious, but arguably feasible, as evidenced by conservation impact in preventing extinctions over the last three decades (Bolam et al. 2021). Halting further extinctions will require urgent, targeted, interventions, as called for in Target 4 (see below).
“By 2050, the extinction rate…of all species [is] reduced tenfold”. This element of the second clause focuses on all species, complementing the focus in the first clause on those that are known to be threatened. Hence it implicitly recognises that while we should aim to halt further extinctions of all species, it is practically challenging to halt extinctions of species that have not yet been described to science (“Centinelan extinctions”; Wilson 1992), or to document having done so. A tenfold reduction in the rate of all extinctions means that the rate must fall to 10% of the ‘current’ rate of extinctions by 2050. The CoP15 Decision on the Monitoring Framework indicates that the reference period for determining the current rate, unless otherwise mentioned, is 2011-2020 (see paragraph 2 of CBD 2022b). The challenges of detecting extinctions (see Akcakaya et al. 2017; Roberts et al. 2023) mean that there are often time-lags before a species can be declared extinct, so extinction rates are most accurately estimated retrospectively. Moreover, absolute extinction rates are also difficult to estimate given considerable uncertainty over the total number of species on the planet. It is therefore useful to compare relative extinction rates, expressed as extinctions per million species per year (E/MSY; Pimm et al. 1995). Mean fossil species’ lifetimes produce a background extinction rate of 0.1–1 E/MSY, while human activities have driven species extinct at a rate perhaps 1,000 times higher (Pimm et al. 2014). Difficulties in assessing extinction rate mean that it is not possible to make more precise estimates (Purvis et al. 2019).
For current extinction rates to be assessed meaningfully, Rounsevell et al. (2020: see also Diaz et al. 2020, Supplementary Information, pp 11 and 18) suggested an approach that is based on known extinctions, proposing to model extinction rates based on extinctions that have been documented in the IUCN Red List. This has been considered problematic for several reasons, including failing to capture the diversity of biodiversity and variation in knowledge across realms and spatial scales, and the practical limitations in detecting extinctions (Raven et al. 2020). An alternative for assessing the extinction rate of all species, including those not yet described by science, is to apply modelling approaches using proxies, for example relating to the severity of threats such as habitat loss (Pimm et al. 1995, Visconti et al. 2016). Further work is therefore needed to explore the best approaches for capturing progress towards this element of the goal, and for understanding associated uncertainties.
“By 2050, the extinction risk…of all species [is] reduced tenfold”. This element of the second clause calls for extinction risk to be reduced tenfold, i.e. to 10% of its current level. This reduction should apply to all species, but as discussed above the extinction risk of most species is not known, and many species remain undescribed. Reduction in extinction risk can therefore only be measured directly by quantifying trends in extinction risk for those species for which it has been assessed (i.e., through the IUCN Red List of Threatened Species). Simplistically, it could be assumed that this could be assessed by taking the proportion of species assessed for the IUCN Red List (or equivalent national lists) that are classified as threatened (i.e. in the categories of Critically Endangered, Endangered or Vulnerable) in 2050, and comparing it with the proportion in 2022 (Smith et al. 1993). However, this would fail to account for two important factors. Firstly, it is highly likely that the pool of species assessed for their risk of extinction will change between 2022 and 2050: for example, the number assessed for the global IUCN Red List has increased from 150,388 in December 2022 to 157,190 in December 2023 (IUCN 2023a). Species added to the pool may differ in their overall risk of extinction compared with those assessed prior to 2022. Secondly, even for the same pool of species, most reclassifications on the Red List result from improved knowledge or revised taxonomy (e.g. improved information on the population size, trends and distribution of species) rather than genuine improvement or deterioration in status (Butchart et al. 2004, 2005). Hence an overall change in the proportion of threatened species may reflect changes in knowledge more than the effectiveness of conservation efforts in addressing the drivers of extinction (Cuarón 1993).
It is therefore essential to consider only genuine changes in IUCN Red List category when assessing trends in species extinction risk. The Red List Index is designed to do exactly this, and can therefore be used to assess progress towards achieving this clause in Goal A. To avoid bias, the index is calculated only for groups that have been comprehensively assessed (i.e. those in which all species have been assessed) or those representing a random sample. The 2022 value of the global aggregated index (which is a measure of survival probability ranging from 0, where all species are extinct, to 1, where all species are Least Concern) was 0.724. Its inverse value (i.e. a measure of extinction risk) was therefore 0.276. A ten-fold reduction is this value would be 0.028 (equating to a Red List index value of 0.972). Reducing extinction risk could be achieved through efforts to improve the status of the most threatened species (e.g. leading to all Critically Endangered species being ‘downlisted’ to Endangered), or the least threatened (e.g. leading to all Near Threatened species being downlisted to Least Concern), or by reducing the extinction risk of a combination of species (e.g. leading to all species moving to the next lowest category of extinction risk) (Figure 1).