Spatial and environmental drivers behind the composition of groundwater copepod assemblages: insights from Italian caves

Subterranean ecosystems, with their unique assemblages of microorganisms, invertebrates, and vertebrates, represent a largely untapped frontier in biodiversity studies. Traditionally, these ecosystems have been sidelined in biomonitoring and conservation initiatives due to the challenges inherent in their exploration and study. The concept of the “Racovitzan impediment” succinctly captures these challenges, including limited accessibility, extreme abiotic conditions like total darkness and low oxygen levels, and biotic constraints such as nutrient scarcity. Although recent changes in scientific perspectives have emphasised the critical need to overcome these challenges to better understand and protect groundwater biodiversity, the spatial and environmental factors that drive the composition of groundwater communities are still poorly understood. To counter these knowledge gaps, we conducted an analysis on copepod assemblages (Crustacea Copepoda) sampled in 12 caves across various Italian karst regions between 2019 and 2022. We collected 71 copepod species (of which 45 are obligate groundwater-dwellers, i.e. stygobites), and the resulting presence-absence matrix was analysed to estimate: (i) the number of species (alpha diversity) potentially present but not observed; (ii) the taxonomic beta diversity and its two components of turnover and nestedness between individual pairs of caves; (iii) the relative weight of geographical distance and climate differences on observed beta diversity patterns between the location areas of the sampled caves. From the results of our analyses, we observed that the alpha diversity estimators indicate that further sampling would increase the number of species recorded across the Italian peninsula. Total beta diversity was high for most cave pairs, with turnover being dominant over nestedness. We hypothesized that both spatial isolation and regional climate influence the composition of copepod assemblages in cave waters. We used Generalized distance-decay models for assessing the influence of spatial isolation, which only partly explained total beta diversity and turnover patterns. In order to assess the contribution of regional climate in the analysis, we used Generalized Dissimilarity Models (GDM) which include surface climate conditions. Based on these models, we inferred that the contribution of seasonal temperature averages is generally more significant than that of geographical distance. Furthermore, by expanding the spatial extent from which the climate variable data were collected, the explanatory power of GDM increased significantly, along with the contribution of temperature. The results obtained further consolidate the exceptional diversity of copepod assemblages and strengthen the correlation between regional climate conditions and the composition of the copepod assemblages in karstic groundwater.