Nature's untold stories: an overview on the availability and type of on‑line data on long‑term biodiversity monitoring

Long-term field-based monitoring is time and resource demanding. Consequently, there are few robust biodiversity databases that contain both a baseline and repeat measurements. On-line repositories represent a potential goldmine of conservation-relevant data, and are increasingly incentivized by funding agencies. However, there remains scarce information on their distribution and availability, limiting the possibility to exploit them to their full potential. Here we comprehensively searched and assessed open-access datasets where biodiversity has been monitored in the same site for at least 4 years, and where species and site locations were clearly reported. We located data on 75,669 field sites (9436 of which are in biodiversity hotspots), for a total of 28,723,226 records, monitoring a total of 15,046 different taxa. We found strong geographic and taxonomic biases. Monitoring sites were predominantly located in the Palearctic and Nearctic biogeographic realms and in the forest biome. Where fauna was monitored, the focus was mostly on amphibians and birds. Supporting open-access policies and developing strategies to fill the identified gaps will be crucial for improving our understanding of global biodiversity trends. Our results suggest, however, that we are on the right trajectory, with a vast storehouse of readily available (and often high quality) yet largely under-analysed biodiversity data now available online from a range of sources. We argue that such data can provide the required biodiversity baselines for national- or local-scale studies.