Calculating population reductions of invertebrate species for IUCN Red List assessments

Population reductions are often used to assess extinction risk of species in the IUCN Red List. Guidelines for Red List assessments describe specific methods for calculating the amount of reduction for species with strongly fluctuating populations. Recently, an alternative approach that involves expert opinion has been suggested for calculating population reduction in insect species. We argue that, while populations with high temporal variability do present challenges, the alternative suggestion is unnecessary, and inconsistent with the IUCN Red List Categories and Criteria. Consistent application of standardized methods for calculating population reductions allows robust and objective assessment of extinction risk faced by invertebrate species.