The main energy source in limestone caves is sinking rivers, which carry-in abundant food not only for aquatic communities but also via flood deposits for terrestrial communities. Rivers are less important in nonsoluble rock, such as lava, but percolating runoff washes surface debris into caves through crevices. Other major energy sources are brought in by animals that habitually visit or roost in caves, plants that send their roots deep underground, chemoautotrophic microorganisms that use minerals in the rock and accidentals that fall or wander into caves and become lost.
Generally in surface habitats, accumulating soil filters water and nutrients and holds these resources near the surface where they are accessible to plant roots and surface-inhabiting organisms. However, in most areas with underlying caves, the soil is thin with areas of exposed bare rock because developing soil is washed or carried into underground voids by water or gravity. Soil formation is limited, and much of the organic matter sinks out of the reach of most surface animals.
Except for guano deposits, flood deposits, scattered root patches, and other point-source food inputs, the defining feature of cave habitats is the appearance of barren wet rock. Visible food resources in the deep cave are often negligible, and what food deposits there are would be difficult for animals to find in the 3D maze. Food resources in the system of smaller spaces is difficult to sample and quantify, but in theory, some foods may be locally concentrated by water transport, plant roots, or micro point source inputs such as through cracks extending to the surface. These deposits would be more easily exploited than would widely scattered deposits.
In each biogeographic region, a few members of the surface and soil fauna have invaded cave habitats and adapted to exploit this deep food resource. The colonists usually were pre-adapted; that is, they already possessed useful characteristics resulting from living in damp, dark habitats on the surface.
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