Recent discoveries in the extremely low energy caves of the Southwestern USA have extended these ideas to the world of bacteria. A large number of new species, some with potential medical benefits, have been discovered living in extremely nutrient-poor cave environments. Most cavers don't notice the microorganisms, or even the small insects and springtails in the cave pools and the mud banks. They do notice, however, the larger visitors that use the cave as a temporary dwelling place or refuge from the surface world. Many a caver has unexpectedly encountered a snake hiding in an entrance passage, a pack rat who has chewed partway through his favorite rope, or a porcupine den with a pile of scat obscuring a passage that needs surveying.
Some caves are host to hundreds or thousands of individuals.
In fact, the largest concentration of mammals known in the entire world is a bat colony in a Texas cave. Unfortunately, careless behavior of cavers in past decades, and even deliberate mass slaughter, had brought the population of these interesting, beneficial animals to dangerously low levels. Strong conservation efforts are now taken to protect bats in their most vulnerable environments, such as seasonal prohibitions on visiting caves known to house hibernating bats or maternity colonies. You might also want to check out the Biospeleology web site at UTexas, which has quite a few cute pictures of cave life. These fellows are just sleeping during the day, in preparation for an evening meal of several hundred flying insects each.