The numbers of species appearing in each category of threat in The IUCN Red List change each time the Red List is updated. In order to monitor the status of biodiversity, it is important to reassess species periodically.
The reassessment may result in species moving into a different Red List Category for non-genuine or genuine reasons:
- New information has become available since the last assessment (e.g., more recent data are available on population sizes, threatening processes, rates of decline or recovery, etc.).
- There has been a taxonomic revision resulting in the species no longer being the same concept as it was before (e.g., it is now split into several species each with smaller ranges, population sizes, etc.; or it has been merged with other species so the range, population size, etc. are now larger than they were previously).
- An error has been discovered in the previous assessment (e.g., the wrong information was used; the IUCN Red List Categories and Criteria were applied incorrectly; etc.).
- The previous assessment used an older version of the IUCN Red List Categories and Criteria and the reassessment uses the current criteria which have slightly different thresholds.
- The main threats are no longer present, or conservation measures (e.g., reintroduction, habitat protection or restoration, legal protection, harvest management, etc.) have successfully improved the status of the species enough to downlist it to a lower category of threat.
- The main threats have continued unabated, have increased, or new threats have developed causing the status of the species to deteriorate enough to move it into a higher category of threat.
IUCN relies on valuable research from around the world to provide new and better information for species. Each category change on The IUCN Red List has the reasons for the change recorded, which allows us to quickly identify species that are genuinely improving or deteriorating in status. Each time The IUCN Red List is updated, a list of species that have changed category is provided along with the reasons for these changes (Summary Statistics).