Random population fluctuations bias the Living Planet Index

The Living Planet Index (LPI) is a standardized indicator for tracking population trends through time. Due to its ability to aggregate many time series in a single metric, the LPI has been proposed as an indicator for the Convention on Biological Diversity’s post-2020 Global Biodiversity Strategy. However, here we show that random population fluctuations introduce biases when calculating the LPI. By combining simulated and empirical data, we show how random fluctuations lead to a declining LPI even when overall population trends are stable and imprecise estimates of the LPI when populations increase or decrease nonlinearly. We applied randomization null models that demonstrate how random fluctuations exaggerate declines in the global LPI by 9.6%. Our results confirm substantial declines in the LPI but highlight sources of uncertainty in quantitative estimates. Randomization null models are useful for presenting uncertainty around indicators of progress towards international biodiversity targets.DATA AVAILABILITY: Empirical data of population time series in the Living Planet database are available from the dedicated website maintained by the Zoological Society of London (ZSL) (http://stats.livingplanetindex.org/) and are subject to the Data Use Policy by the Indicators & Assessments Unit at the ZSL and WWF International. Simulated data to replicate the results are available from https://doi.org/10.5281/zenodo.4744533.CODE AVAILABILITY : All simulation outputs and code (R scripts) to reproduce the results in this manuscript are available from https://doi.org/10.5281/zenodo.4744533.EXTENDED DATA FIG. 1: The nine steps to calculating the Living Planet Index (LPI).EXTENDED DATA FIG. 2: The Living Planet Index (LPI) for randomly fluctuating populations that are stable on average.EXTENDED DATA FIG. 3: Starting population sizes of time series added to the Living Planet Index have declined between 1950 and 2015.EXTENDED DATA FIG. 4: Larger population fluctuations cause less precise estimates of the Living Planet Index (LPI) in nonlinear population trajectories.EXTENDED DATA FIG 5: Population fluctuations cause generalised additive models (GAM) to misestimate starting and ending populations when populations decrease from 100 to 40 individuals.EXTENDED DATA FIG 6: Population fluctuations cause generalised additive models (GAM) to misestimate starting and ending populations when populations increase from 100 to 160 individuals.EXTENDED DATA FIG 7: The reshuffling null model used to account for random population fluctuations.EXTENDED DATA FIG. 8: Cumulative population declines can occur in the Living Planet Index even when average population declines are zero.EXTENDED DATA FIG. 9: Cumulative population changes represent empirical trajectories more accurately than average changes as time series lengths increase.The National Research Foundation of South Africa and the Jennifer Ward Oppenheimer Research Grant.https://www.nature.com/natecolevolhj2022Zoology and Entomolog