What do functional diversity, redundancy, rarity, and originality actually measure? A theoretical guide for ecologists and conservationists

• Functional diversity, redundancy, rarity, and originality (or distinctiveness) are fundamental concepts in ecology and conservation biology. • However, the precise meaning and relationships between these measures are often unclear. • This paper aims to provide a comprehensive theoretical framework to elucidate what each of these measures captures and how they interrelate. Functional diversity, redundancy, rarity, and originality (or distinctiveness) are fundamental concepts in ecology and conservation biology. Despite their frequent use, the precise meaning and relationships between these measures are often unclear. This paper aims to provide a comprehensive theoretical framework to elucidate what each of these measures captures and how they interrelate. By integrating traditional community-level diversity metrics with species-level specificity measures derived from fuzzy set theory, we bridge the gap between these concepts. Our framework reveals that while all four measures address distinct aspects of community-level and species-level functional resemblance, they can all be traced back to a common conceptual and formal background. This guide is intended to help ecologists and conservationists understand the meaning of these measures and apply them more effectively in their research and conservation strategies.