Using decision science to evaluate global biodiversity indices.

Global biodiversity indices are used to measure environmental change and progress towards conservation goals, yet their fitness for purpose is poorly understood. Few indices have been evaluated comprehensively for their capacity to detect trends of interest, such as declines in threatened species or ecosystem function. Using a structured approach based on decision science, we evaluated nine indices commonly used to track biodiversity at global and/or regional scales against five criteria relating to objectives, design, behaviour, incorporation of uncertainty, and constraints (e.g. costs and data availability). We identified four key gaps in indices assessed: i) pathways to achieving goals (means objectives) are not always clear or relevant to outcomes decision makers wish to achieve (fundamental objectives); ii) index testing and understanding of expected behaviour is often lacking; iii) uncertainty is seldom acknowledged or accounted for; and iv) costs of implementation seldom considered. These gaps may render indices inadequate in certain decision-making contexts and are problematic for indices linked with biodiversity targets and sustainability goals. Ensuring index objectives are clear and their design is underpinned by a model of relevant processes are crucial in addressing the gaps identified by our assessment. Uptake and productive use of indices will be improved if index performance is rigorously tested, and assumptions and uncertainties are clearly communicated to end-users. This will increase the value of indices in accurately tracking biodiversity change and supporting national and global policy decisions, such as the post-2020 global biodiversity framework of the Convention on Biological Diversity. Article impact statement: Evaluating biodiversity indices against criteria testing fitness for use means more effective and robust application in decision-making. This article is protected by copyright. All rights reserved.