Climate change goes underground: temperature in caves and its effects on subterranean organisms

Abstract

Climate change is one of the most significant threats to all ecosystems. Yet, there are fundamental knowledge gaps in understanding its effects on caves, mainly due to the difficulty in accessing and studying these ecosystems. Caves provide incredibly stable conditions, such as constant temperature, often corresponding with the average annual temperature for the surface. This dissertation studies the temperature variation in caves and at their respective surface in several climatic regions, and tested the effect of temperature in cave-adapted species and compared it with previously published studies on cave-adapted species thermal tolerance. Temperature was measured in 12 different locations across different climatic zones during one year. Results showed that cave temperature corresponds to the average annual temperature of their correspondent surface, independently on the type of cave or location. Three types of thermal regimes were found with caves thermally similar to the surface, caves with a slight thermal delay, and caves with an extreme thermal delay from the surface. Daily thermal cycles were found in some caves, with implications for the fauna’s circadian rhythms. Thermal tolerance was assessed for six cave-adapted species from Western Portugal karst areas. Some species started dying at temperatures very close to the highest predicted temperature. It is crucial to consider sub-lethal effects that may be occurring prior to mortality. However, caves provide vital ecosystem services. Subterranean ecosystems store 97% of freshwater used in multiple human activities, in which groundwater species play the crucial role of maintaining its ecological equilibrium by filtering and cleaning the water. Furthermore, some terrestrial species are also key decomposers of organic matter, encouraging nutrient cycling. Considering the cave dependence on the surface to regulate its temperature, temperature increases at the surface will be reflected underground, impacting caves, their fauna and ecosystem services.