The Habitat Dependency Index: A new tool for quantifying species-habitat relationships

Habitat loss is a key threat to ecosystems and species that rely on them, with direct consequences for human wellbeing. Quantifying the degree to which species depend on specific habitats is critical for many fields of knowledge, such as conservation biology or economic valuations, yet poses a complex challenge. We introduce a new method to objectively quantify how much species depend on a habitat by integrating their life-cycle habitat associations. Through a comprehensive literature review focusing on 36 species associated with kelp forests, we demonstrate how this 'Habitat Dependency Index' (HDI) effectively discriminates between closely related species and captures differences in their reliance on kelp forests. We suggest this new HDI constitutes a powerful tool for understanding the role of marine habitats in supporting fisheries, biodiversity, and other ecosystem services. Moreover, the approach is broadly applicable and can provide essential information for managing both aquatic and terrestrial ecosystems.