Freshwater ecosystems, despite covering only a small percentage of the Earth's surface, are exceptionally biodiverse. However, these ecosystems and their inhabitants are currently facing a dramatic decline in biodiversity, largely driven by human activities such as habitat destruction, pollution, and climate change. This biodiversity loss is often compared to past mass extinction events, particularly the K-Pg extinction 66 million years ago. While previous studies, primarily focused on vertebrates, suggested a relatively low impact of the K-Pg extinction on freshwater biota, this study uses the well-preserved fossil record of European freshwater gastropods to provide a more comprehensive assessment. Gastropods are ideal for this analysis due to their high diversity, widespread distribution, and extensive fossil record. The study aims to quantify the extinction rates of European freshwater gastropods during the K-Pg event and compare them to current extinction rates, inferred from IUCN Red List data, to gauge the severity of the current biodiversity crisis.

Previous research on the K-Pg extinction's impact on freshwater ecosystems primarily focused on vertebrates, leading to underestimations of the overall diversity loss. Estimates of freshwater species extinction during the K-Pg event ranged from 10-22%, considerably lower than the global average of 76%. This discrepancy motivated a reassessment using invertebrates, specifically freshwater gastropods, which possess a rich and well-documented fossil record in Europe. This study builds upon existing research on gastropod diversification and extinction, using improved methodologies and a larger dataset to provide more robust estimates. The use of the IUCN Red List to assess current extinction risk in freshwater gastropods adds a crucial contemporary perspective for comparison.

The study compiled a comprehensive dataset of European freshwater gastropod fossil occurrences from the Jurassic to the Pleistocene, encompassing approximately 20 million years. This involved extensive literature review and examination of museum collections. A rigorous taxonomic screening process was employed to ensure data quality, excluding uncertain records and applying the latest taxonomic revisions. The dataset, consisting of 24,759 fossil occurrences representing 3122 species, was then analyzed using a Bayesian approach to jointly infer speciation, extinction, and preservation rates. This approach accounted for the inherent uncertainties in the fossil record. The analysis utilized the open-source program 'ra' and an RJ-MCMC algorithm to estimate time-varying speciation and extinction rates. To assess current extinction rates, the study leveraged data from the IUCN Red List, focusing on 347 extant European freshwater gastropod species. The program 'iucnSim' was used to model future extinction based on current conservation statuses and transition rates, providing predictions for extinctions over 50, 80, and 100-year time horizons. The analysis considered the potential biases associated with underrepresentation of certain species in both fossil and modern datasets.

The analysis revealed that the K-Pg extinction event resulted in the extinction of 92.5% of European freshwater gastropod species, significantly higher than previous estimates based on vertebrates. The extinction phase lasted 5.4 million years, followed by a recovery period of 6.9 million years. The extinction rate during the K-Pg event was more than an order of magnitude higher than the background rate. However, the most striking finding is the current extinction rate of European freshwater gastropods, which is estimated to be three orders of magnitude higher than even the revised K-Pg extinction rate. Predictions based on IUCN Red List data suggest that extinction rates could range from 137.9 to 197.3 events per million species years (E/MSY) over the next 50-100 years. This alarming rate indicates that between 20.8% and 31.9% of extant species might go extinct within the next century. Moreover, the model projects that the 75% extinction threshold, typically defining a mass extinction event, could be reached by the year 2539. These results hold even when considering genera-level data instead of species-level data, demonstrating the robustness of the findings.

The study's findings highlight the critical severity of the ongoing biodiversity crisis in European freshwater ecosystems. The current extinction rate of freshwater gastropods far surpasses even the high extinction rates observed during the K-Pg event, suggesting an unprecedented crisis. The similarity between the K-Pg event and the current crisis, marked by environmental perturbations and increased extinction rates, reinforces the urgency of conservation efforts. The high extinction rates projected for the near future underscore the need for immediate and effective conservation strategies to mitigate the ongoing loss of freshwater biodiversity. The limitations of the study, such as focusing on a single geographic region and potential biases in the fossil and modern datasets, should be considered when interpreting the results. However, the dramatic difference in extinction rates between the K-Pg event and the current crisis strongly suggests a pressing need for conservation action.

This study demonstrates that the current extinction rate in European freshwater gastropods drastically exceeds that of the K-Pg mass extinction event. The projected high extinction rates for the coming centuries highlight the extreme urgency of implementing effective conservation measures to prevent a major biodiversity loss in freshwater ecosystems. Future research should expand to include global datasets and other invertebrate groups to provide a more complete understanding of the ongoing biodiversity crisis.

The study's focus on European freshwater gastropods limits the generalizability of the findings to other regions or taxonomic groups. Potential biases in the fossil record, such as incomplete sampling or taxonomic uncertainties, might influence the accuracy of past extinction rate estimations. Similarly, uncertainties in IUCN Red List data and potential variations in conservation status assessment across species could affect the accuracy of future extinction rate projections.