Aim: Groundwater ecosystems harbour a unique biodiversity, but remain poorly studied, mainly due to difficulties in accessibility and imperfect species detection. Consequently, knowledge on the state and change of groundwater biodiversity remains highly deficient. In the context of global warming and excessive groundwater extraction, understanding groundwater from an ecosystem-perspective, including organism diversity and distribution, is essential. This study presents the largest ever systematic assessment of groundwater amphipods, which are a key component of European groundwater biodiversity. Location: Switzerland (41,285 km2), including data from 906 sampling sites. Taxon: Groundwater amphipods, genera Niphargus and Crangonyx (Crustacea, Amphipoda). Methods: We applied a highly standardized citizen science approach to collect repeated groundwater fauna samples in collaboration with municipal drinking water providers. Using detection–nondetection data of the genetically identified groundwater amphipod species, we assessed the overall species diversity of both rare and common species. The distribution of commonly found species was predicted using multispecies occupancy modelling. Results: We retrieved 3882 samples from 906 sites, yielding 2350 groundwater amphipod individuals. We identified a remarkable species diversity, comprising few commonly and many rarely found species. Considering commonly found species, we identified distinct distribution ranges, low local species richness and a predominance of negative co-occurrences. In contrast, a major portion of species were found rarely (generally at just one or two sites each), distributed uniformly throughout the study area and unrelated to common species' recognized hotspots. Many of these rarely found species are not yet formally described. Main Conclusions: Our results give robust emphasis on the rare occurrence and narrow distribution of many groundwater dwellers. Our systematic and standardized sampling data of groundwater amphipods suggest that rarity is particularly prominent and inherent to groundwater organisms. We emphasize the need of systematic datato integrate rare groundwater species in biodiversity assessments, especially in times of global change.

Funding was provided by the Swiss Federal Office for the Environment FOEN/BAFU (project ‘AmphiWell’ to FA and RA) and the Swiss National Science Foundation (grants nr. PP00P3_150698 and 31BD30_209583/Biodiverse DarCo to FA) and the University of Zurich Research Priority Programme on Global Change and Biodiversity (URPP GCB to FA). CF was funded by the Slovenian Agency for Research and Innovation through the core program P1-0184 and project J1-2464.

Original research article: Knüsel, M., Alther, R., Locher, N., Ozgul, A., Fišer, C. & Altermatt, F. (2024). Systematic and highly resolved modelling of biodiversity in inherently rare groundwater amphipods. Journal of Biogeography, https://doi.org/10.1111/jbi.14975.