Caves are almost entirely heterotrophic; they depend on the transfer of energy and nutrients from the surface environment. Food is brought in with plant roots, water (i.e., organic material brought in with percolating rainwater, flooding, streams), and animals, particularly those that feed in the outside environment but return to the cave for shelter during their inactive period (Howarth, 1983; Gnaspini and Trajano, 2000; Huppop, 2000). Although caves are generally considered food deficient, there is tremendous variation among and within caves. Food scarcity may be considered general, periodic (variation in time), or patchy (variation in space) (Huppop, 2000). The best examples of the latter are guano beds that can be several meters deep and support tremendous populations of invertebrates. These islands of life, however, "are surrounded by desert, as most of the underground space is severely oligotrophic and sparsely populated" (Gilbert and Deharveng, 2002).
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