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Illinois Caves  

THE FRAGILE FAUNA OF ILLINOIS CAVES

by Steven J. Taylor and Donald W. Webb

Illinois has several hundred caves, many of them in nearly pristine condition. This unique and fragile environment is home to a diverse array of creatures, including organisms that are completely limited to the cave environment, species that may be found in similar habitats above ground, and the many animals that accidentally wander, fall, or are washed into caves. Many cave animals are highly adapted for the unique and harsh living conditions they encounter underground.

map1.gif (29283 bytes)In Illinois, caves can be found in four distinct karst regions: in the Mississippian limestone of the Shawnee Hills, in the Salem Plateau and in the Lincoln Hills, and in the Ordovician limestone of the Driftless Area. These caves have been forming though the interaction of geology vegetation, and rainfall for the past 300 million years. Shallow seas covered much of Illinois during the Mississippian Period. When the seas receded, forests grew over the exposed sedimentary rocks; and rainwater-which had become slightly acidic through interaction with carbon dioxide from both the atmosphere and the bacterial breakdown of organic material-then seeped into cracks and bedding planes. As the limestone dissolved, conduits formed. These conduits eventually developed the geologic features characteristic of karst terrain-caves, sinking streams, springs, and sinkholes.

INTO THE TWILIGHT ZONE

Caves can be divided into three ecological zones. The entrance zone is similar in light, temperature, and relative humidity to the surrounding surface habitat, and the creatures that live there resemble the animals that live in the moist shaded areas near the cave. Hear we find the eastern phoebe (Sayornis phoebe), a small gray bird whose nest is constructed on bare bedrock walls out of mosses and other debris. In the leaf litter, we find many animals of the forest floor: redbacked salamanders, harvestmen (or daddy-longlegs), snails, earthworms, millipedes, centipedes, beetles, ants, and springtails. Cave entrances are often funnel shaped or have sheer vertical walls, and organisms and organic debris tend to concentrate at the bottom. The entrance zone also provides a highly protected environment for overwintering organisms.

Deeper inside the cave, in the twilight zone, there is much less light, and photosynthesizing plants are no longer able to grow. The temperature and relative humidity fluctuate here, but the environment is usually damp and cool. Many animals from the entrance zone wander into the twilight zone, but most of these creatures must eventually return to the land above. Several species of cave crickets are common in this part of the cave, sometimes appearing in large numbers on walls or ceilings.

In larger caves, there is a dark zone characterized by constant temperature (about 54-58*F in Illinois) and the absence of light. Here, the relative humidity approaches the saturation point. Many animals in the dark zone are capable of completing their entire life cycles without leaving the cave although food is scarce in the absence of photosynthesis. In this zone, there are fewer species of organisms. Creatures who live here eat primarily organic debris-wood, leaves, and accidental animals. Dark-zone dwellers get some of their nutrients from the feces of bats and cave crickets, animals that leave the cave at night to feed on the surface. Raccoons, common cave explorers in Illinois, also leave their waste behind. A wide array of bacteria and fungi feast upon these nutrient-rich items. Other animals then feed upon the fungi and bacteria. Springtails, minute insects typically overlooked by the casual observes, are important fungus feeders, and a variety of beetles, flies, and millipedes get their nourishment this way as well. These organisms may then become the prey of cave-inhabiting spiders, harvestmen, predacious fly larvae known as webworms, and an occasional cave salamander. In the winter, pickerel frogs, mosquitoes, and some moths move into cave to wait for warmer weather.

ADAPTING AND SURVIVING

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Common Cave inhabitants include (left to right) the moth, Scoliopteryz libatrix, which does not have a comon name; the cave salamander (Eurycea lucifuga); and the monorail worm (Macrocera nonilis).

Animals that live in caves vary greatly in their degree of adaptation to the cave environment. Accidental animals live there only temporarily; they will either leave or die. Animals that frequent cave but must return to the surface at some point in there life cycles are know as trogloxenes. Bats and cave crickets are two examples. Troglophiles are animals that can complete their entire life cycles within a cave, but they may also be found in cool, moist habitats outside of caves. Tow troglophilic vertebrates found in or near Illinois caves are the cave salamander (Eurycea lucifuga) and the spring cavefish (Forbesicthys agassizi).

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Diane Tecic, district heritage biologist for the Illinois Department of Natural Resources, looks for cave-adapted organisms in organic debris with Illinois caver Tim Sickbert.

Most cave animals are trogloxenes and troglophiles; only 20 to 30% of the animals in North American caves are troglobites. Troglobites are animals that live exclusively in caves; they are especially interesting because of their unique morphological, physiological, behavioral, and life-history adaptations. Many troglobites, for example, lack body pigment. Because they live where there is no light, there is no evolutionary advantage for them in maintaining the colors that might be characteristic of their relatives and ancestors that live above ground. In cave-adapted species, the evolutionary pressure to maintain functional eyes is also greatly reduced, and these species have been under strong selective pressure to evolve other means of sensing their surroundings. Their legs and antennae usually have more sensory nerve endings than related above-ground species. These appendages serve important tactile functions and are often greatly elongated in cave-dwelling creatures.

Adaptations that allow species to exist in an environment with very low nutrient input are not as obvious. Many cave-adapted species produce fewer offspring than their surface-inhabiting relatives, but individual eggs may contain more nutrients. In some species, timing of reproduction may be synchronized with spring flooding and its new supply of nutrients. Other species, lacking the above-ground seasonal cues of temperature and photoperiod, may reproduce year-round. Cave adaptations may include a reduced metabolic rate, allowing animals to live on limited food resources for long periods of time. Illinois has many troglobitic invertebrates but no troglobitic vertebrates.

As cave-adapted species become specialized, they also tend to become geographically isolated. The geological and hydrological history of some areas may divide species into isolated populations, and these populations, over time, may evolve into distinct species. During glacial periods, caves, as serve as refugia for some aquatic, soil-, and litter-inhabiting animals. These species may become "stranded" in caves when glaciers retreat surface conditions are not suitable for recolonization.

VULNERABITLIY OF CAVE ENVIRONMENTS

Human disturbance affects cave ecosystems just as it affects other ecosystems. As a result of changes we make on the surface, we unknowingly alter cave environments, destroying unique and valuable organisms before we even know of their existence. The public knows very little about caves and the organisms that inhabit them. Small wonder then that the importance of protecting groundwater, caves, and cave life is not fully appreciated. It is not uncommon to find sinkholes filled with trash, serving as natural garbage cans for rural waste disposal. Visitors sometimes permanently damage caves with graffiti, break stalactites and stalagmites, and carelessly set fires.

The very adaptations that allow troglobites to survive in the harsh cave environment make these animals more vulnerable to changes made by humans. The reduced metabolic rates that allow these animals to survive in a nutrient-poor environment also make them less competitive when organic enrichment is introduced in the form of fertilizers, livestock and agricultural waste, and human sewage. In Illinois, this effect is commonly seen in stream-inhibiting amphipods (small shrimplike animals) and isopods (small crustaceans related to terrestrial pillbugs or sowbugs). These groups contain troglobites that are highly adapted to cave environments; they also contain more opportunistic troglophilic species, which have a competitive advantage in the presence of high levels or organic waste.

Amphipods and isopods feed on small particles of organic debris and on decomposers such as bacteria and fungi. Because they ingest large quantities of this material, they are exposed to contamination from a variety of pollutants. In Illinois, samples of these animals collected in 1992 were found to contain dieldrin and breakdown products of DDT. They were also found to contain moderate levels of mercury, although mercury was not detected in any water samples from the same sites.

Sedimentation also threatens aquatic species. Topsoil run-off from rural development and agricultural fields enters caves readily when vegetative buffers around sinkholes are too small or nonexistent. This sediment fills the spaces in gravel streambeds, eliminating the microhabitats that allow many cavedwelling species to exist. As a result, cave streams with high sediment loads ten to contain few species.

Sometimes, humans can't easily see the value of these subterranean systems, especially when their own interests conflict with the health of cave communities. Such a conflict is occurring now in our most biologically and hydrologically significant karst area, the Salem Plateau of Monroe and St. Clair counties. As part of the greater St. Louis metropolitan area, the Salem Plateau is experiencing rapid population growth. Scientists can estimate the level and types of threats that this growth brings to the biological integrity of the region, but it's much more difficult to develop protected areas, educational programs, and new regulatory mechanisms within the existing political, social, and geographic framework. Illinois caves are a high priority for conservation because cave organisms face serious threats from agriculture and increasing urbanization. Also, the unique and fragile cave and environment provides a home for organisms found nowhere else in the world.

It is not usually possible to include the entire drainage basin of significant caves within nature preserves or other conservation easements. To manage a cave effectively, scientists must understand the hydrology of a cave's subterranean conduits. This knowledge is gained by doing extensive dye tracing studies and cave mapping. Both of these activities are time- and labor-intensive. Already, the drainage basins of some of our largest cave systems are being compromised by agriculture and rural housing projects. Educating the public-particularly politicians, farmers, and children-about land use and the impact of human activities is key to the long-term health of cave communities. We must also enact appropriate regulations for rural residential development-especially wastewater treatment-and for agricultural activities in a karst landscape.

For more information on cave conservation and management, contact the National Speleological Society, 2813 Cave Avenue, Huntsville, AL 35810-4431, or Steven Taylor or Donald Webb at the Center for Biology, Illinois Natural History Survey, 607 East Peabody Drive, Champaign, IL 61820.

Steven J. Taylor is an aquatic entomologist in the Center for Biodiversity at the Illinois Natural History Survey in Champaign. Donald W. Webb is an insect systematist, also at the Center for Biodiversity.

A GOOD NEIGHBOR POLICY

In a few caves in Monroe and St. Clair counties, you can find a small shrimplike creature that exists nowhere else in the world. The Illinois cave amphipod has made our corner of the world its home, but it may not be here long unless humans take steps to protect its environment. This unassuming cave creature has been proposed for listing as a federally endangered species.

Cave amphipods inhabit the bottoms of pools and riffles in large cave streams, where they creep among cobbles and under stones, feeding on decaying leaf litter and organic debris. Food is scarce in this environment, and the amphipods have developed chemosensory structures that detect the odor of food sources, such as dead or injured animals.

Injured or dying amphipods are vulnerable to such predators as flatworms, cave salamanders, and even other amphipods. But the greatest threat these vulnerable creatures face is the deterioration of the environment. The Illinois cave amphipod lives near the greater St. Louis metropolitan area, a region that has been experiencing dramatic population growth for the past 10 years. Continued urbanization without appropriate sewage treatment and disposal is especially threatening to the amphipods existence. Other serious threats are siltation and the presence of agricultural chemicals in subterranean aquifers.

Fortunately for the amphipod, the quality of life for people on the land above depends on water quality in streams below. Because agricultural chemicals and bacteria associated with sewage have been found in well water, springs, and cave streams in this area, a concerted effort is being made to improve the water quality in this karst region. Efforts to provide communities with safe drinking water could also provide a healthy cave environment and help ensure the further existence of our underground neighbor, the Illinois cave amphipod.

 

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