UNDERGROUND ECOSYSTEMS AND CLIMATE CHANGE

Climate change represents a complex global challenge, capable of triggering articulated ecological responses at different levels of biological organisation. To analyse its effects and reduce the influence of confounding factors, research often focused on isolated ecosystems, such as the subterranean one, considered an ideal model for in situ study of various environmental and biological dynamics, also in relation to the impact of anthropogenic activities. 

Thanks to their thermal stability and partial isolation from surface influences, hypogean environments offer, in fact, a particularly sensitive observation lens for detecting environmental alterations induced by climate changes. 

Only recently, however, has the scientific community begun to direct growing attention to underground ecosystems, although their potential was already recognised in 1969 by Poulson and White, in the pioneering contribution The Cave Environment, published in the prestigious journal Nature. 

This renewed attention has stimulated the development of various theoretical models, aimed at understanding whether and how climate warming could influence the underground microclimate. 

The formulated predictions have been subsequently confirmed by both modelling simulations and empirical observations, which have demonstrated how climate alterations of anthropogenic origin are already producing measurable effects even underground, determining an increase in internal temperature. However, such variations manifest themselves with a certain delay compared to the surface, due to the slow conduction of heat through rock. 

Besides temperature, relative humidity also represents a critical parameter undergoing alteration, particularly in Mediterranean karst areas. In these settings, the decrease in precipitation and the reduction of infiltration processes can lead to significant drops in the internal humidity of caves, with potential impacts on microclimate stability and on the organisms that populate such environments.

Underground life: extremely adapted, extremely vulnerable

Most affected by the progressive modifications of the underground microclimate are species strictly specialised for subterranean life, which show high levels of adaptation to stable environmental conditions. Among these are troglobionts, organisms that live exclusively in terrestrial underground environments, and stygobionts, their aquatic counterparts. 

The fruit of a long evolutionary process in conditions of darkness, isolation and limited energy availability, these species have developed highly specialised morphological, physiological and behaviour adaptations. Regression or loss of eyes, reduction of pigmentation, slowed metabolism, late maturation and remarkable longevity are among the most common evolutionary responses. 

However, in the context of climate changes and growing pressures exerted by anthropogenic activities, the capacity of hypogean species to respond to stresses is severely limited. These organisms share a series of traits that reduce their response, including low genetic variability, reduced metabolic flexibility and limited fecundity. To this, the often small size of populations has to be added: species with reduced abundances present, in fact, very limited response potential to environmental changes. 

Furthermore, spatial dispersal is strongly limited by geographical isolation, intrinsic to underground systems, and by the poor mobility of the species that inhabit them. Although some organisms can make seasonal vertical movements within caves, migration towards deeper sectors entails high energetic costs, in environments where resources tend to decrease with increasing distance from the surface. 

Being environments devoid of light, underground ecosystems depend on the outside for their energy sustenance. Although in some cases there is a contribution to primary production by chemo-autotrophic organisms – micro-organisms capable of deriving energy from inorganic chemical reactions rather than from photosynthesis – such input is generally limited. Energy flux is based mainly on the entry of organic matter transported passively by abiotic vectors (water and/or air currents) or by organisms (humans included). Consequently, the energy balance of hypogean habitats is subject to seasonal variations, and can be strongly influenced by land use changes. 

Beyond direct effects, climate change can cause significant indirect impacts, such as the introduction of alien or pathogenic species and the modification of underground hydrological regimes through aridification phenomena. 

Numerous recent studies have highlighted heterogeneous physiological responses to temperature increase amongst different underground organisms. Many hypogean species present narrow thermal tolerance (stenothermy, from the Greek stenós, "narrow", and thérmē, "heat"), an adaptive strategy that allows the optimisation of physiological functions within a very limited thermal range, but renders them vulnerable even to slight temperature variations (±2 °C). However, the possibility of drawing general conclusions is hindered by the lack of comparative analyses on a phylogenetic scale, which could help to better understand variations in thermal tolerance amongst evolutionarily distinct groups. 

In the darkness of conservation: when data are lacking

Despite the growing evidence of threats linked to climate change, systematic biological monitoring programmes, essential for evaluating the extent of ongoing transformations and defining effective conservation strategies, are still lacking. Moreover, the scarcity of long-term abiotic monitoring systems, with often fragmentary and non coordinated networks, limits the capacity to accurately detect and interpret the effects of climate change in underground environments. 

Filling current knowledge gaps is essential not only to protect underground biodiversity, but also to fully understand the crucial role of hypogean ecosystems in guaranteeing human wellbeing. The life forms hosted here, besides being highly specialised and vulnerable, perform fundamental ecological functions, such as natural water purification and conservation of underground aquifers, the main global reserve of drinking water. 

Therefore, it is necessary to promote integrated approaches and interdisciplinary collaborations that involve speleologists, ecologists, climatologists and other specialists, in order to build a shared and operational vision on the future of these unique environments. 

In this scenario, the Underground Climate Change (UCC) group has been established, which proposes to optimise the available resources, coordinate the present expertises and activate collaboration networks to make the study of climate change underground more effective, systematic and connected to global environmental challenges.