Our current, empirical understanding of the relationship between biodiversity and ecosystem function is based on two information sources. First, controlled experiments which show generally positive relationships. Second, observational field data which show variable relationships. This latter source coupled with a lack of observed declines in local biodiversity has led to the argument that biodiversity‐ecosystem functioning relationships may be uninformative for conservation and management. We review ecological theory and re‐analyse several biodiversity datasets to argue that ecosystem function correlations with local diversity in observational field data are often difficult to interpret in the context of biodiversity‐ecosystem function research. This occurs because biotic interactions filter species during community assembly which means that there can be a high biodiversity effect on functioning even with low observed local diversity. Our review indicates that we should not necessarily expect any specific relationship between local biodiversity and ecosystem function in observational field data. Rather, linking predictions from biodiversity‐ecosystem function theory and experiments to observational field data requires considering the pool of species available during colonisation: the local species pool. We suggest that, even without local biodiversity declines, biodiversity loss at regional scales—which determines local species pools—may still negatively affect ecosystem functioning.

Marine biodiversity plays an important role in providing the ecosystem functions and services which humans derive from the oceans. Understanding how this provisioning will change in the Anthropocene requires knowledge of marine biodiversity patterns. Here, we review the status of marine species diversity in space and time. Knowledge of marine species diversity is incomplete, with only 11% of species described. Nonetheless, marine biodiversity is clearly under threat, and habitat destruction and overexploitation represent the greatest stressors to threatened marine species. Claims that global marine extinction rates are within historical backgrounds and lower than on land may be inaccurate, as fewer marine species have been assessed for extinction risk. Moreover, extinctions and declines in species richness at any spatial scale may inadequately reflect marine diversity trends. Marine local-scale species richness is seemingly not decreasing through time. There are, however, directional changes in species composition at local scales. These

The Living Planet Index (LPI) is a standardised indicator for tracking population trends through time. Due to its ability to aggregate many timeseries 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 non-linearly. We applied randomisation 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. Randomisation null models are useful for presenting uncertainty around indicators of progress towards international biodiversity targets. Nations of the world are in the process of negotiating the post-2020 Global Biodiversity Framework under the Convention on Biological Diversity. One of the ambitions of the post-2020 framework is "bending the curve of biodiversity loss" by first slowing down declines by 2030 and then improving the state of biodiversity by mid-century 1-3 . Reliable biodiversity indicators are essential for tracking progress towards global biodiversity targets 4 . A prominent indicator of species abundance over time is the Living Planet Index (LPI) 5-7 . The LPI aggregates population time-series for vertebrates from terrestrial, freshwater and marine systems into a relative index (where the baseline is

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