A change in the status rank of a species may not always reflect a true change in the probability of extinction of that species, but rather reflect other reasons than a genuine change in status. These reasons include:
(1) Changes in taxonomy -an amalgamation of a species with other species or a split of a species into more newly identified taxa results in a change in status rank
(2) Changes in knowledge – a new account, new information about a species, or new information about threats indicates that a species is more or less endangered than formerly thought
(3) Criteria change –a change in the methods that are used to assess a species status, resulting in an altered status rank without a change in the underlying reasons for the species status.
In such cases, it is recommended to apply an approach termed ‘back-casting’, where by earlier Red List categorisations are retrospectively adjusted using current information and taxonomy. This allows the preconditions to be met by assuming that the current Red List categories for the taxa have applied since the set of species was first assessed, unless there is information to the contrary that genuine status changes have occurred (Butchart et al. 2007).
The Australian and British Columbia example of national RLIs illustrate the use of retrospective adjustments. Information and examples of how to carry out a retrospective assessment can also be found in Marco et al. (2014) and Teucher & Ramsay (2013).