Groundwater is a vital ecosystem of the global water cycle, hosting unique biodiversity and providing essential services to societies. Despite being the largest unfrozen freshwater resource, in a period of depletion by extraction and pollution, groundwater environments have been repeatedly overlooked in global biodiversity conservation agendas. Disregarding the importance of groundwater as an ecosystem ignores its critical role in preserving surface biomes. To foster timely global conservation of groundwater, we propose elevating the concept of keystone species into the realm of ecosystems, claiming groundwater as a keystone ecosystem that influences the integrity of many dependent ecosystems. Our global analysis shows that over half of land surface areas (52.6%) has a medium‐to‐high interaction with groundwater, reaching up to 74.9% when deserts and high mountains are excluded. We postulate that the intrinsic transboundary features of groundwater are critical for shifting perspectives towards more holistic approaches in aquatic ecology and beyond. Furthermore, we propose eight key themes to develop a science‐policy integrated groundwater conservation agenda. Given ecosystems above and below the ground intersect at many levels, considering groundwater as an essential component of planetary health is pivotal to reduce biodiversity loss and buffer against climate change.

The stability of temperature and humidity in caves is well known. However, little is known if higher or lower cave environmental stability (temperature, humidity, light and others) implies changes in the structure of the biological communities. Number, position and size of entrances, then size, depth, host rock and extent of the cave, the amount and type of food resources are all factors that can have strong influence on the cave biological communities. The objective of the present study was to evaluate the correlation between the presence of water bodies, size of entrances and the linear development of caves with the terrestrial invertebrate richness and species composition in 55 limestone caves located in the Brazilian Savannah, sampled from 2000 to 2011. Invertebrates were sampled by active search throughout the caves, prioritizing micro-habitats (sites under rocks) and organic resources (litter, twigs, feces and bat guano). We recorded 1,451 invertebrate species. Species richness was positively correlated with presence of cave streams, width of entrances and linear development of the caves. The richness of troglomorphic species was positively correlated to the presence of perennial pools and linear development of the caves. The presence of cave streams was a decisive factor for determining the community structure, increasing the number and the similarity of troglophile species among the caves. Flood pulses can cause disturbances that eventually select the same species besides importing resources. However, for the terrestrial troglomorphic species the disturbance caused by cave streams may decrease the number of species.

Environmental stability and oligotrophy are considered the main drivers of species distribution within caves due to physiological and nutritional requirements presented by many cave dwellers. However, such patterns are poorly evaluated in tropical caves, especially with regard to habitat selection and interspecific competition between invertebrate groups. Considering that troglobitic species are usually highly specialized, presenting specific requirements for environmental conditions, we hypothesize that troglobitic species will be preferentially associated with deeper areas inside the cave. These areas are stable and present trophic and physical constraints, which may favors the troglobites in competitive interactions with non-troglobitic species. The study carried out in the Águas Claras Cave System revealed a new hotspot of subterranean biodiversity, represented by 30 cave-restricted species (29 invertebrates and 1 fish species), being 73.3% terrestrial, 16.7% amphibian, and 10% aquatic. The richness of troglobitic species did not respond to physical attributes or resources availability as postulated, but increased with temperature, humidity content and with non-troglobitic species richness. The similarity of the troglobitic species along the cave was determined by the moisture content. Furthermore, the richness of troglobites was higher in those areas with greatest taxonomic distinctness of non-troglobitic species and higher values of the TB/nTB species richness ratio. The habitats requirements of the troglobitic species were not coincident, thus indicating that such species avoid niche overlapping. We highlighted the studied cave system as a singular subterranean habitat that contributes to both local and regional biodiversity. Additionally, the condition of high temperature and humidity seems to be key factors that are favoring the existence of a high number of endemic species. Unfortunately, this cave system is devoid of any official protection, thus deserving urgent actions to ensure its conservation.

The Água Clara Cave System (ACCS) in Brazil is the richest hotspot of subterranean biodiversity in South America. In this study, we present an updated list of cave-restricted species in the ACCS and compare it with previously published hotspots in Brazil. Our list of cave-obligate fauna comprises 31 species, including 23 troglobionts and 8 stygobionts. The exceptional diversity of the ACCS can be attributed to factors related to the high dispersal potential of cave fauna within the system, high surface productivity, and the large size of the cave system size. Notably, we observed highly troglomorphic species in the ACCS, some of which are the most troglomorphic species in their respective groups in Brazil. The huge volume of galleries, high humidity, and trophic conditions prevailing in the ACCS may have played a role in shaping the strong troglomorphic traits observed in these species. However, all the obligate cave species in the ACCS require conservation attention and are at an elevated risk of extinction due to their limited ranges, few occurrences, and many potential threats. This study sheds light on the biodiversity and conservation status of cave-restricted fauna in the ACCS and highlights the importance of protecting these unique ecosystems.

The present study evaluated the seasonal variation of a population of Hypena sp. in the Gruta Taboa (Sete Lagoas, Minas Gerais, Brazil), in relation to changes in temperature and humidity during the dry (July 1999 and July 2000) and rainy (January 2000 and January 2001) seasons. The Hypena sp. population responded to external seasonality, being distributed closer to the cave entrance during the rainy season, in which temperature and humidity fluctuated around 21 °C and 85%, respectively. During the dry season abundance was higher in sections farther from the entrance (deeper sections) (19.2 °C temperature and 80% humidity). The results showed that this species is influenced by external environmental factors, even in a tropical region where the external climate fluctuations are lower compared to temperate regions.