Testing the Living Planet Index’s ability to capture complex biodiversity change

Background/Question/Methods

The Living Planet Index is a crucial tool to track biodiversity change at the global scale, but necessarily sacrifices information to summarize thousands of populations’ changes into a single communicable trend. Evaluating when and how this information loss affects the LPI’s performance is essential to to ensure interpretations of the index are consistent and reflect the truth as reliably as possible. Here, we evaluated the ability of the Living Planet Index to accurately and precisely capture known, true trends of population change under various scenarios of biodiversity change. We used mechanistic simulations of population dynamics through time to generate scenarios of declining, stable, or growing populations fluctuating synchronously, asynchronously, or independently, to test the LPI’s performance when applied to covarying populations. We then calculated the Living Planet Index for each set of populations, and compared it to the known true trend, and assessed the consistency between the LPI’s confidence intervals and the true uncertainty inherited from error in the data and and accumulated through the methodology.

Results/Conclusions

We find that the Living Planet Index systematically underestimates population change trends, particularly when applied to growing populations and asynchronous populations. Although this bias appears generally small (< 0.05), the true trend often lies outside or at the upper bound of the LPI’s 95% confidence interval. The LPI’s confidence limits also tend to underestimate the error inherited from the raw data and the smoothing process involved in the calculation of the index, regardless of the direction of population changes or synchronized population dynamics. These findings echo suggestions that a more complete assessment of the variability in population change trends, with particular attention to covarying populations, would enrich the LPI’s already key influence on conservation communication and decisions.