The physical environment is rigidly constrained by the geological and environmental settings and can be defined with great precision because it is surrounded and buffered by thick layers ofrock. Caves can be water-filled or aerial.
Aquatic systems are best developed in limestone caves since water creates these caves. Debris-laden water in voids in nonsoluble rock will eventually fill caves.
A significant exception is found in young basaltic lava that has flowed into the sea. Here, subterranean ecosystems develop in the zone of mixing freshwater and salt water within caves and spaces in the lava. The system is fed by food carried by tides and groundwater flow. Frequent volcanism creates new habitat before the older voids fill or erode away. Aquatic cave environments are dark, three-dimensional (3D) mazes, in which food and mates may be difficult to find. In addition, the water can stagnate, locally becoming hypoxic with high concentrations of toxic gases including carbon dioxide and hydrogen sulfide.
The subterranean aerial environment is stressful for most organisms. It is a perpetually dark, 3D maze with a water-saturated atmosphere and occasional episodes of toxic gas concentrations. Many of the cues used by surface animals are absent or operate abnormally in caves (e.g., light/dark cycles, wind, sound). Passages can flood during rains, and crevices might drop into pools and water-filled traps. If the habitat is so inhospitable, why and how do surface animals forsake the lighted world and adapt to live there? It is the presence of abundant food resources that provides the impetus for colonization and adaptation.
The terrestrial environment in long caves is buffered from climatic events occurring outside. The temperature stays nearly constant, fluctuating around the mean annual surface temperature (MAST); except passages sloping down from an entrance tend to trap cold air and remain a few degrees cooler than MAST. Passages sloping up are often warmer than MAST. The environment is strongly zonal (Figure 1). Three zones are obvious: an entrance zone where the surface and underground habitats overlap; a twilight zone between the limit of photosynthesis and the zone of total darkness. The dark zone can be further subdivided into three distinct zones: a transition zone where climatic events on the surface still affect the atmosphere, especially relative humidity (RH); a deep zone where the RH remains constant at 100%; and an innermost stagnant air zone where air exchange is too slow to flush the buildup of carbon dioxide and other decomposition gasses. The boundary between each zone is often determined by shape or constrictions in the passage. In many caves, the boundaries are dynamic and change with the seasons.
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