The risk of underestimating generation length for extinction risk assessments

Generation length is a key parameter for extinction risk assessment in the IUCN Red List, defining the biological horizon over which population decline is measured. Generation length data are largely unavailable, and a naive time frame of 10 years is often used to represent the three-generation horizon required for assessing decline in short-lived species. This assumption risks introducing biases in the assessment. Here, we quantified how using a 10-year time frame, instead of one based on predicted generation lengths, affects the extinction risk assessments of amphibians: one of the most imperiled groups worldwide. We measured past deforestation trends for 1,278 forest-dependent amphibian species and predicted future climate-driven habitat change for 1,559 species. Using predicted generation lengths systematically increased the percentage of declining species from deforestation (3-29%) and climate change (1-20%). This was also reflected in a higher proportion of species potentially assessed as threatened. Relying on a 10-year period to assess extinction risk can lead to substantial underestimations across both past and future time frames, compared to using predicted generation length. Predictions of generation length can allow applying IUCN Red List criteria in the absence of empirical data and improve consistency of extinction risk assessments.