Volume II, No. 3, Spring 1975
Though many people believe it to be true, a cave is not an isolated hole in the ground inhabited only by bats or hibernating animals. Many animals do hibernate in caves, and caves are bats' favorite roosting places, but life in a cave is extensive, varied and interrelated with life outside. The life forms range from bacteria and fungus through insects and up to larger mammals which come into the mouth of the cave for protection or predation.
Animals can be found in all parts of the cave. Some are found only in specific areas, while others range throughout.
The cave environment the animals live in is fairly uniform, not varying significantly from season to season, nor from the front of the cave to the back.
Some of the outstanding features of the cave environment which differ greatly from the surface are darkness, fairly constant humidity and constant temperature. Of these darkness and constant temperature most affect the cave life.
Since the only sunlight to reach a cave's interior is at the mouth, this automatically limits the number of life forms found in a cave. There are almost no green plants, for sunlight is essential to most green plants" food-making process of photosynthesis. The lack of sunlight limits the energy output in a cave, forcing life to use other sources.
Water is a necessity for almost all forms of life except perhaps bacteria and molds. Most animals can live several times longer without food than without water. Though caves are poor in food production, water is abundant in most caves, easily supporting the animals' needs. Humidity is high, the walls are quite damp, and water drips from the ceiling. Readings we took in a local cave showed the humidity at the mouth to be around 50%; at the center, 82%; and toward the back, 90%. Humidity can reach 100%. When a bat maternity colony roosts in the cave, fresh guano combined with bat respiration and urination can raise the humidity level from 60% to over 90%.
The level of humidity does not seem to directly affect the life in a cave. However, there are some so-called dry caves, which are not too common in our area, in which there is no moisture at all. It is almost impossible under these conditions for life to exist except for fungi and bacteria.
Besides high humidity, caves often have moving streams which provide a habitat for aquatic animals which in turn become food for larger animals.
Another factor affecting life in a cave is its even temperature. Sunlight cannot warm a cave, except at the mouth. Even there it is noticeably cooler in hot weather. However, the temperature throughout the cave may not vary more than a few degrees winter and summer.
Readings taken in a cave showed the temperature to range from 52°F to 58°F. The temperature at the mouth was 58°F, the temperature at the center, 52°F, but by the time we reached the back, it had warmed up to 56°F. These temperatures are fairly representative for caves in this area.
The ambient air temperature in a cave is nearly constant year round. Tom Aley, owner of the Ozark Underground Laboratory, Forsyth, Missouri, told us, "The reason it doesn't fluctuate is the rock surrounding the cave. The temperature of the rock is determined by incoming solar radiation. It will come to equilibrium with the average annual temperature of the area. So temperatures in a cave are determined by the rock. The temperature of the rock is determined by the location of the cave, so it is colder in caves in Wisconsin than in Missouri, and colder in caves in Missouri than in Texas."
Barometric pressure has little effect on cave life. Nevertheless, we checked it to see another of the differences between the cave environment and that in which we live. Standard barometric pressure is slightly over 30 inches, but readings we took were slightly lower than this, ranging between 29.5 and 29.54 inches.
The life existing in the cave can be divided into that which uses the entrance temporarily, that which makes the cave its home but feeds outside and that which is dependent entirely on the cave ecology for its existence.
Of the temporary dwellers of the past we can include Indians. From archeological research it seems evident that the Indians did not make permanent homes in the caves, but used them as temporary hunting camps, at least that is true of the Osage Indians who lived in villages in winter and went on hunting trips during the summer. The cave entrance provided shelter and protection and running spring water ,as well as a site for burial of the dead.
Man's contribution as an inhabitant to the cave life cycle was of minor importance. But many animals continue to use cave entrances as the Indians did for protection, a temporary home and some food.
Mammals like the pack rats, fox, skunk, opossum, groundhog and raccoon are often in caves. The fox may make his den in the protective blackness of a hole dug in the clay of the cave floor. The raccoon uses the cave for part of his food. The cave gives him a challenge to find his food in the darkness. He eats fish, crayfish, salamanders or anything he can catch hold of, going all over the cave to hunt for food.
Some birds use the natural openings and holes in the cave entrances and bluffs for their homes. Fly catchers and other cliff-dwelling birds like pigeons roost in cave entrances where the insect hunting is good.
The best known permanent cave inhabitant that feeds outside is the bat. The bat is probably the most read about cave animal in the world, and also the most misunderstood. Superstitions abound and because they operate only at night and because a very few species feed on blood, the whole order has been given a bad name. Most bats are very beneficial, eating thousands of insects and doing no harm other than scarring unenlightened people.
Because they like darkness, caves are a natural home for them. They are the main vertebrate animal that commonly lives in totally dark parts of caves. They live there by day, leaving at night to hunt food, which is not blood as some fear, but insects and fruit. Tom Aley claims that his colony of 150,000 bats eat a thousand pounds of insects a night--a far sight more than most birds, even martins.
Bats are usually divided into solitary or colony living types. In a ten by twenty foot section of a cave ceiling one might find as many as 155 bats per square foot. Or in another cave you may find a single bat. Their hind feet are modified for clinging. Toes bend inward with sharply curved claws so that the bat can hang upside down for hours from the rough surface of a rock with no muscular effort, even after death.
Bats Use the cave to have their young. The females separate themselves from the males and go to special nursery rooms in the cave where the young are born and nursed. Adult males do not enter. Young bats mature quickly learning to fly in one month. By the end of summer the nursery colonies are deserted.
Some bats hibernate, living on stored body fat. Other bats migrate, using caves as a sort of motel along the way. They stop off to sleep for a couple of days and are on their way.
Bats are beneficial to man because of the destruction of thousands of flying insects. Though they have eyes, they do not need them for they have a natural radar emitting high frequency sounds inaudible to humans. The echoes from these sounds bouncing back tell them what the object is. They can distinguish between echoes or insects, trees, rocks and rain.
Another benefit of bats to man and especially to the ecology of the cave is its guano or manure which is used as fertilizer. Tom estimates that his bats convert their nightly catch of a thousand pounds of insects into two hundred and twenty pounds of guano.
It is this guano which directly or indirectly supports the remaining animal life in the cave.
There are many kinds of insects, spiders, mites and other very small animals which make their home in caves. But cave insects, like flies, crickets, mosquitoes and gnats are not like the everyday kind found around the house. These insects are unique in that they have adapted to the darkness, being found not only at the entrance, but all over the cave system.
Spiders may feed on insects that come near the cave wall, or on mites living on the bat guano. Their web is unseen until hit by a light source of some kind. Upon that happening the web takes on a silver plated effect due to the moisture accumulated on it. The web itself is beautiful to the eye, but to the eyes of an insect, it means a quick death.
There are many different kinds of flies in the cave system, from the common house fly to fruit flies. They feed on other insects, bat guano and dead animals.
The crickets that inhabit a cave are well adapted to that sort of life. Over the years this small insect has also developed long antennae which give the cricket a high degree of sensitivity to any type of movement and air and temperature change. Their diet is about the same as the fly. The cricket is a migratory insect, migrating in and out of the cave every forty-eight hours. It feeds outside but leaves its guano in the cave.
Mosquitoes and gnats are different from the flies and crickets because they are found at the entrance near light. Female mosquitoes have to have the blood of animals in order to lay good eggs. Thus, they could not survive entirely in the darkness of the cave. But there are varieties of these insects that can be found anywhere in the cave that there is a food source, in the twilight zone and even in total darkness.
Several aquatic animals feeding off insects are part of cave life, too. Salamanders like other animals have adapted to cave life in and outside. Those at the entrances are colored and can see. These little amphibians are beautiful, coming in a variety of colors from the common cave salamander which is orange with black spots, to the blind white salamander.
Of these two salamanders the blind white cave salamander Js the most interesting. Over the many hundreds of years this little animal has adapted to the cave environment. It gets no pigment on its skin because it gets no sunlight. It has no eyes because it needs none to see in the darkness. This salamander mates and raises its young in total darkness. The Ozark blind salamander feeds on insects and most any animal that is small enough.
Salamanders have a unique way of protecting themselves when roughly handled. They may secrete a milky fluid through the skin which is poisonous to some animals.
The blind white crayfish is another animal dwelling in total blackness. This crayfish has no pigment nor sight, but has very long antennae which are very sensitive to movements in the water. It gets its food by preying on animals that fall in the water. It and the blind cave fish (only about two inches long) can detect any slight movement within or on the water for several feet away.
Molds and fungi in a cave are decomposers, growing on guano, dead animals, wood, string or anything they can cling to and obtain energy from. They are white since they produce no chlorophyll. They in turn become a grazing ground for some cave inhabitants.
In absence of sunlight the energy source above ground, a cave's basic food energy source is bat guano. This guano does not contain as much energy as sunlight but does have roughly the caloric value of beef steak. In some caves one can see guano piles several feet high underneath favorite roosting places.
If it were not for the guano deposited by a cave's bat population, the cave would be nearly devoid of energy input. Nearly all living things in a cave depend either directly or indirectly on the guano for life. Molds, bacteria and fungi live directly on the guano for life. Molds, bacteria and fungi live directly off the guano. Many cave insects such as mites, crickets and flies also receive nourishment in this fashion. Animals such as cave spiders feed on other cave inhabitants which derived their nourishment from the guano.
When guano falls into the water in a cave, the microscopic plants, animals and bacteria derive
nutrition. These in turn become food for larger aquatic animals such as isopods and amphoids.
These animals become food for salamanders that become food for raccoons.
Bats and other inhabitants or predators that die in the cave are treated no differently by the living inhabitants than is the guano. They simply become another source of energy, supporting the continuing life cycle.
The life in a cave does not lead a danger free existence, though. Besides the constant threat of predation, there are other threats to cave life.
One of these dangers is the disproportionate depletion of guano in many caves where the supply is being depleted faster than it can be replaced. With guano being the cave's main energy supply, the cave would soon be unable to sustain life if it were gone.
The main factor contributing to the guano depletion is the general decline in bat population, due to natural and human causes. Flash floods in caves wash out the guano. Collapses and increased molestation of the bats' roosting places by cavers, researchers and sightseers decrease the number of bats.
Another danger to cave life is related to the common practice in many parts of the Ozarks, and probably in many other parts of the country: that of dumping trash in sinkholes. Though this practice at first may not seem harmful to the ecology of caves, it can pose serious problems because few sinks hold water. Where does this water go? Most of it goes underground. The pollution from the dumps goes into underground streams.
Tom Aley estimates that he has done more ground water tracing than anyone else in the United States. Tom developed a process by which he stains Lycopodium spores. He then releases them into a stream system or into the ground either directly or by injection. He can pick them up later using calibrated nylon netting.
Lycopodium spores are approximately 33 microns in diameter. (About 850 laid side by side would equal an inch.) However small this may seem, they are about ten to fifteen times larger than most bacteria. A study made at the Underground Laboratory showed that 25 to 30% of a virus injected into the ground that came out in a cave stream was in virable (potent) condition. This shows that the Ozark sub-surface is not a good filter.
When it rains, bacteria and viruses are washed from trash in a sinkhole into the ground, traveling through the subsurface stream system. If the bacteria come out in a cave stream they pose a definite threat not only to the cave's aquatic life, but to ground water used for drinking. Aquatic life is killed, a vital link in the cave's food chain is broken.
Life in a cave is not in a separate unrelated world, but is intertwined with the surface ecology so that any change in water drainage or depletion of some species or change in a surface form can endanger the delicate balance. Most all the animals in the cave are adapted to life in darkness just as we are adapted to life above ground. Major disturbances such as the ceiling of a cave collapsing might destroy them, for many of the animals who had adapted to the dark, damp, consistent cave situation could not survive in the new environment.
Even such seemingly harmless activities as trampling through a cave threatens its life. As Tom said, "It changes the nature of the sub-strata. It squishes animals. To me it's one of the most damaging things you can do to the ecosystem in a cave."
The cave is not a world underground separated from the world up above. What happens above the cave is reflected in the cave, affecting the food supply, water supply or other environmental conditions within the cave. What happens inside the cave affects in many important and subtle ways the environment and life outside--including that of man. Careless and destructive use of caves which discourage bat colonies, for instance, can upset the whole food chain system which may cause over-populations of insects. Or wholesale using of chemicals on the surface seeping through the karat topography of the Ozarks, pollutes the underground, upsetting the balance there which in turn affects the outside (like contaminating ponds). Therefore, caves become show cases to give us better insights into what is happening to our environment and its possible effect on our future habitat.* * * *The Underground Laboratory is privately owned and operated by Tom Aley, "To encourage education, prudent resource use, and research in the cave and karst regions of the United States. It consists of 126 acres of Ozark hill-land which overlies 10,000 feet of known passage in Tumbling Creek Cave. Field trips for educational groups are by appointment." For more information write Thomas Aley, Director, Ozark Underground Laboratory, 1025 S. Roanoke, Springfield, Missouri 65807. Our appreciation to George Kastler, Naturalist at Bennett Spring State Park, for all his help on the cave series.
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