Patterns and determinants of richness and composition of the groundwater fauna

Understanding patterns of groundwater biodiversity is a challenging task because of the inherent difficulty in drawing links between environmental factors, eco-evolutionary processes, and patterns. Using studies in Europe, the Americas, and Australia, we focus on patterns that are common enough among taxonomic groups and continents in species richness and taxonomic composition of the groundwater fauna to be potentially recognized as “rules”. The obligate groundwater fauna is dominated by crustaceans, which represent 65%–78% of species at continental to global scales. Local species richness of groundwater species communities is low, at least compared to surface aquatic communities; however, regional to continental species richness is high due to strong spatial turnover characteristic of groundwater species composition. Macroecological studies have challenged the long-held view that historical factors primarily drive continental-scale patterns of regional species richness by reviving the importance of energy resources and spatial heterogeneity in controlling speciation and extinction. Aquifer properties, such as the size of voids, their interconnectedness, and hydrological connection to the surface environment, drive differences in species composition among localities of a region. Continental-scale patterns in community turnover may essentially be controlled by historical climates: species replacement is higher in European regions of stable climates where ecological and/or nonecological speciation events can accumulate over time, whereas community nestedness is higher in regions where variable climates can promote extinction and recolonization. However, we still lack a quantitative understanding of the contribution of environmental filtering and dispersal limitations to patterns of species richness and community turnover at regional to continental scales. Groundwater biologists are increasingly incorporating the phylogenetic and functional facets of biodiversity for understanding the mechanisms contributing to the assembly of communities. This multifaceted approach is revealing the importance of niche specialization and ecological specialization–dispersal trade-off in shaping geographic patterns of subterranean biodiversity.