Incorporating coextinction in threat assessments and policy will rapidly improve the accuracy of threatened species lists

Threatened species lists are important policy tools for species conservation, and they are often used to help prioritize recovery actions. Current listing processes, such as the IUCN red list threat assessment, rely on accepted correlates of extinction risk, especially population size. Population size appears to be an effective indicator for threatened vertebrate species, but it is less relevant for invertebrate species, where population sizes are rarely known. Furthermore, many invertebrate species are dependent upon a host species for survival and reproduction, and thus host population sizes would be better indicators of (co-)extinction vulnerability. Improving listing criteria to be relevant to invertebrates is important, because faunal biodiversity consists predominantly of invertebrate species (>98%) and invertebrates likely represent a majority of the current global sixth mass extinction event. Here, we propose including the degree of host specificity of a dependent invertebrate species to threatened host species as an additional correlate of extinction proneness, where there is a continuum from generalist dependents to dependents that are completely specialized to use a single host species per life stage. For highly specialized dependent taxa, we recommend that that threat category assigned to the dependent species should match the threat category assigned to the host species. Incorporating this correlate of extinction risk into threatened species policy, such as the IUCN red list, could rapidly increase the rate of assessments for many invertebrates and justify their inclusion, where previously they may have been considered data deficient.