Cryptic function loss in animal populations

• Cryptic function loss can occur in species with persistent populations.
• Function loss occurs at different scales, from partial to complete loss.
• Changes to populations, communities, and habitats drives cryptic function loss.
• Cryptic function loss must be included in assessments of functional diversity.

The essential functional roles performed by animal species are lost when they become locally extinct, and ecosystems are critically threatened by this decline in functional diversity. Theory that links function, diversity, and ecosystem stability exists but fails to assess function loss that occurs in species with persistent populations. The entire functional role of a species, or a critical component of it, can be lost following large population declines (functional extinction), following population increase, or after behavioural adaptations to changes in the population, community, habitat, or climate. Here, we provide a framework that identifies the scenarios under which ‘cryptic’ function loss can occur in persistent populations. Cryptic function loss is potentially widespread and critically threatens ecosystem stability across the globe.