IUCN Green Status of Species

The IUCN Green Status of Species is a conservation assessment system published by the International Union for Conservation of Nature (IUCN) that grades the impact of recovery and conservation efforts for individual species. The first version of the Green Status assessment guidelines was published in 2018, and integration of Green statuses into Red List assessments was formalized as an optional component in 2020. The second version of the framework was published in 2021.

History
The creation of the Green Status system began with the formal call of the World Conservation Congress (WCC) in 2012 for the creation of a "Green List" of ecosystems, nature preserves and species based on a set of measurement systems for conservation success. In Resolution 41, the WCC noted that merely preventing extinction of species or loss of ecosystems, the goal of the Red Lists, was insufficient to retain biodiversity, preserve the valuable ecological services provided by ecosystems and species and maintain their resilience in the face of threats like those posed by climate change.

Ultimately, the Green List of Species was developed separately from what became the IUCN Green List of Protected and Conserved Areas. In 2020, the IUCN decided to rename the Green List of Species the IUCN Green Status of Species due to methodological differences between it and the Green List of Protected and Conserved Areas and concern that having a species receive a Green "listing" might be perceived as implying that it is not at risk of extinction. The Green Status complements the Red List assessment but does not replace it: both assessments are performed by the IUCN for a given species and, with the exception of species extinct in the wild that would require reintroduction as a conservation measure and whose current Green Score is by definition 0%, one status does not determine the other.

Pilot program
As of April 2020, preliminary IUCN Green Status assessments had been performed for 179 species. Among the IUCN Species Survival Commission Specialist Groups and IUCN Red List Authorities in existence in 2018, 52 out of the 135 working groups chose to contribute to the Green Status pilot. In interviews of stakeholders performed by the IUCN, it was suggested that Green List assessments may be most effective if performed at multiple spatial scales, such as in a regional assessment. Interviewees expressed concerns over the difficulty of establishing baseline Green Scores, especially for species that live in places difficult to survey, like the ocean, and in places, such as Europe, where human change has been occurring for a long time. They were also concerned about the cost of producing the new, complex assessments. The pilot was judged successful by the IUCN, leading to the launch of the program in mid-2021 and publication of Green Status assessments in the IUCN Red List using the updated Green Status of Species standard.

Assessment
The score (Green Score) is an average of spatial units currently occupied or occupied in the past by a species weighted by their integrity. Representative values of assigned weights are 0, if the species is not present in the area, 3, if the species is present, 6, if the population is viable and 9, if the population is assessed as functional, although depending on the exact criteria used by the assessor, the functional weight can be assigned to 8 or 10 and decimal weights may be used. The exact meaning of these terms varies by assessor and species, but the IUCN suggests conducting the assessment as would be done when assigning a regional Red List status, with the exception of assessing functionality, which is based on the ability of the population within the spatial unit to carry out natural processes, such as migration, the integrity of its interactions within its habitat, such as predator-prey relationships with other species, and its contributions to ecosystem processes within the unit, such as seed dispersal. Spatial units can represent reproductively isolated populations or subspecies, areas where the species faces a unique threat, division by ecosystem types the species inhabits or may be based on geographical features with some barrier to dispersal. National borders may also be considered when delineating spatial units. The definition and number of spatial units chosen by the assessor directly influences the Green Score and conservation metrics that are obtained. The Green Score is expressed as a percentage equal to:

$$\frac{\sum_{S=1}^N W_S}{W_F*N} * 100$$

Where WS is the weight (integrity) of the spatial unit, N is the total number of spatial units and WF is the weight of a functional unit (highest weight possible). A Green Score of 100% is defined for a fully recovered or non-depleted species that is present in all parts of its historic range (prior to any major human disturbance), each with viable populations that are ecologically functional, a score that may not be realistically attainable for many species even if they achieve their Recovery Potential.

Conservation metrics
A Green Status assessment also includes four conservation metrics that represent changes in Green Score in different conservation scenarios over periods of time. In the first version of the assessment, the first metric is Conservation Legacy, which measures the difference in the estimated change in Green Score from 1950 to present if no conservation actions had been undertaken (counterfactual scenario) to the actual change to present. If no conservation actions had occurred in this time, the Conservation Legacy would be 0%. Conservation Dependence, the second metric, assesses the change in Green Scores between the present and the short-term future, defined as three generations of the species or 10 years, whichever is longer, in the first version of the assessment, or 10 years alone in the second version, if no conservation actions are undertaken. Conservation Gain, the third metric, is the change in Green Scores between the present and short-term future with conservation action. Finally, Recovery Potential is the change in Green Score between the present and the long-term future, defined as 100 years after present, in an optimal conservation scenario.

In the case of assessing Conservation Legacy, the spatial units used for calculating Green Score reflect indigenous range, such as the range the species occupied before 1500 (estimate of the beginning of European expansion) or 1750 (approximate beginning of the Industrial Revolution). Expected additional range, such as habitats that a species may begin to occupy under anticipated climate warming scenarios, is used in calculating long-term future Green scores.

While the Green Status and the Red List statuses showed a moderate negative correlation among species assessed in a Green Status pilot, with progressively more depleted species being more likely to be threatened with extinction, among conservation metrics, only Recovery Potential showed differences between IUCN Red List categories, with currently imperiled species generally possessing a higher Recovery Potential than the species of Least Concern.

Green Status
The Green Status or Species Recovery Category is expressed in words in the second version of the Green Status Assessment. It is based on the Green Score, also known as the Species Recovery Score, which is a point estimate (SRSbest), with a corresponding confidence interval (bounded by SRSmax and SRSmin). The present-day Green Status or Species Recovery Category is defined as follows:

The conservation metrics are also expressed as point estimates with their own confidence intervals and verbal descriptors. They assess the effectiveness of conservation measures as measured by predicted changes in species' Green Scores over time. The verbal descriptors have criteria based on absolute change in Green Score (magnitude of the conservation metric), change relative to the baseline present-day Green Score or any benefit that prevents extinction, in cases of species with high conservation needs. The metrics can have zero and negative values.

In the case of the Conservation Dependence and Conservation Gain metrics, "false negative" values have been attributed to use of a static Green Score baseline that does not indicate whether the species is projected to decline or recover if threats to a species change in the short term, and future versions of the Green Score assessments are expected to instead use a dynamic baseline to compensate.