Testing the performance of ecosystem indices for biodiversity monitoring

Understanding how biodiversity is changing is a crucial step in mitigating our environmental impacts. Biodiversity indicators synthesise and communicate status and trends in biodiversity, including progress towards global targets. Despite their importance in decision making, few indicators have been tested for their performance and behaviour. Here, we tested the sensitivity, responsiveness and complementarity of three indices synthesising status and trends in collapse risk, spatial distribution and health of ecosystems (i.e., the IUCN Red List Index of Ecosystems, the Ecosystem Area index, and the Ecosystem Health Index). We used a stochastic coral reef ecosystem model to evaluate the indices’ ability to detect changes in biodiversity under various future scenarios. We aimed to: i) test the indices’ sensitivity to detecting change under low and high threat; ii) examine their responsiveness in detecting changes at the onset of and changes in threat levels; iii) assess complementarity among the indices’ trends and values; and iv) explore the impact on the indices’ sensitivity and responsiveness to sampling decisions, including the scale of ecosystem classification, ecological variables used in their calculation, and frequency of reporting. Our study revealed that the ecosystem indices are sensitive, responsive and offer complementary information on changes in ecosystems, although the Red List Index of Ecosystems sometimes behaved counter-intuitively. The indices’ sensitivity and responsiveness depended on the nature of the threats and could be adversely affected by decisions made in the assessment process; careful variable selection in the Red List of Ecosystems assessment process is critical to reliability detecting ecosystem change. Our results suggest that that 5-year intervals in ecosystem assessment can reveal informative trends, suggesting that current guidelines for reassessment are sufficient. Our study is one of the few examples of a systematic evaluation of biodiversity indicator performance. Our findings will help improve our understanding of the reliability of these ecosystem indices as decision-support tools in local to global contexts.