Dark extinctions in a rapidly changing world

We are currently witnessing a mass extinction crisis due to increasingly overwhelming human impacts on the biosphere. Extinctions from the last few hundred years have been well-known for some taxonomic groups, but represent only the tip of the iceberg. While there is accumulating evidence that the number of historic human-induced extinctions is far higher than documented in the ‘gold standard’ for assessing extinction risks - the IUCN Red List - the extent and characteristics of poorly documented or ‘dark extinctions’ remain insufficiently known. Such dark extinctions, which are preceded by undocumented local declines and extinctions, have profound implications for assessing long-term trajectories of biodiversity change of local assemblages.Dark extinctions and undocumented local species declines span a gradient from events that left no direct trace, to documented losses supported by substantial evidence (e.g., subfossil remains, ancient DNA, historic descriptions, specimens in herbaria or museums, observation records) waiting to be formally accepted as actual local decline or extinction by rigorous scientific standards. Here, we synthesize the state of knowledge on the discrepancy between documented and dark extinctions, and assess the implications of undocumented local species losses on biodiversity assessments. We identify six essential dimensions for Red List assessments, and we provide an assessment of their coverage in the IUCN Red List. To this end we review the evidence on how extinction drivers have changed in importance over time, and how well they are actually covered by Red List assessments. We identify ten sources of gaps and biases in documenting extinctions and local species losses and evaluate the role of temporal lags in these processes. We discuss the implications of these findings for studying species richness and the temporal dynamics of local assemblages, and more broadly, for assessing the true scale of the unfolding extinction crisis and assess the implications for conservation policy. Finally, we identify key steps to better account for undocumented species declines from local to global scales.