Moving towards better risk assessment for invertebrate conservation

Global change threatens a vast number of species with severe population declines or even extinction. The threat status of an organism is often designated based on geographic range, population size, or declines in either. However, invertebrates, which comprise the bulk of animal diversity, are conspicuously absent from global frameworks that assess extinction risk. Many invertebrates are hard to study, and it has been questioned whether current risk assessments are appropriate for the majority of these organisms. As the majority of invertebrates are rare, we contend that the lack of data for these organisms makes current criteria hard to apply. Using empirical evidence from one of the largest terrestrial arthropod surveys to date, consisting of over 33 000 species collected from over a million hours of survey effort, we demonstrate that estimates of trends based on low sample sizes are associated with major uncertainty and a risk of misclassification under criteria defined by the IUCN. We argue that even the most ambitious monitoring efforts are unlikely to produce enough observations to reliably estimate population sizes and ranges for more than a fraction of species, and there is likely to be substantial uncertainty in assessing risk for the majority of global biodiversity using species-level trends. In response, we discuss the need to focus on metrics we can currently measure when conducting risk assessments for these organisms. We highlight modern statistical methods that allow quantification of metrics that could incorporate observations of rare invertebrates into global conservation frameworks, and suggest how current criteria might be adapted to meet the needs of the majority of global biodiversity.