Our mission:
Saving species from extinction

The proof: Durrell Index

Durrell exists to save species from extinction. It is essential to know if we are achieving this mission, so we can constantly improve and demonstrate our impact to our supporters. This is where the Durrell Index comes in. This set of indicators, timelines and stories track the threats facing our species, the actions we take, and the difference we make to some the planet’s most threatened animals.

It provides proof that conservation works: a story of hope in the battle to save the world’s wildlife.

We are proud to be one of the first conservation organisations to have a ‘key performance indicator’ for its mission. The Durrell Red List Index of Species Survival demonstrates that, overall, we have improved the status of our species by over 100%, compared to a scenario of no conservation. It shows that at least 6 species are alive today that would have otherwise been lost forever.

Find out more about our programmes and our impacts - both globally and at the species level - by following the links below.

The Durrell Index measures the threats facing our species, the actions we take in response, the results we achieve, and ultimately the difference we make - our impact. Our highest-level indicator, the Durrell Red List Index shows how we are performing against our organisational mission of saving species from extinction.

Our impact


Our Results


Our actions




This part of the Durrell Index illustrates the impacts of our actions for our priority species, complemented by species timelines to provide a richer, story-driven measure of our conservation success. Dive in and find out more about our long-term efforts to save these species from extinction.

The science behind the Durrell Index

Thank you for your interest in the science behind the Durrell Index. Throughout the Index, we have drawn upon global biodiversity indicators and databases, as well as industry standard frameworks, to ensure we are being robust and transparent in our assessments. We have aimed to provide credible evidence of our results, using published research to support our findings wherever possible.


The global and species programme indicators are themed on the Pressure-State-Response framework, used by other conservation organisations such as BirdLife International.

To assess the type and severity of threats to our species we have developed a system based the Threats Classification Scheme (Version 3.2) of the International Union for Conservation of Nature (IUCN) and Conservation Measures Partnership (CMP). Our framework for assessing the conservation actions implemented for our species is based on the IUCN/CMP Conservation Actions Classification Scheme (Version 2.0).

IUCN Red List

One of the most important global programme indicators of impact within the Durrell Index is the ‘Red List Index of Species Survival’. We use the IUCN’s Red List Index to track the conservation status of a set of our target species over time, compared to a predicted counterfactual scenario of what would have happened if conservation actions had not taken place. This is the first use of this globally recognised indicator to evaluate the impact of a conservation organisation.

The methods and results are published in the peer reviewed scientific journal Biological Conservation. Our paper Accounting for conservation: Using the IUCN Red List Index to evaluate the impact of a conservation organization can be found here Young et al. 2014. As part of this analysis, we draw on a paper by Hoffman et al (2010) on The Impact of Conservation on the Status of the World’s Vertebrates published in the journal Science to determine which of our species have improved in Red List category due to conservation.

At the species indicator level, in order to track observed changes in the extinction risk of our species, we use - where available – conservation status categories published on the IUCN Red List, which are reviewed and verified by independent experts. If a species does not feature on the Red List, Durrell’s conservation scientists who are trained in the Red List methodology conducted the assessments following the 2001 IUCN Red List Categories and Criteria version 3.1.

Counterfactual Red List categories for our target species (the Red List category that we predict a species would have in the absence of conservation actions) are determined by Durrell’s Conservation Science team. Many of these counterfactual assessments have been externally reviewed by the scientific community during the peer review process during publication of the paper Accounting for conservation: Using the IUCN Red List Index to evaluate the impact of a conservation organization. For a number of the Mauritian bird species, we follow the counterfactual assessments produced by Butchart et al (2006) in the paper published in the journal Oryx entitled How many bird extinctions have we prevented?.