The Ecology of Virginia Caves

scientists can access some caves on Cedar Creek (Frederick County) via kayak, where the surface/groundwater connection is obvious
scientists can access some caves on Cedar Creek (Frederick County) via kayak, where the surface/groundwater connection is obvious
Source: US Geological Survey (USGS), Valley and Ridge, Piedmont, and Blue Ridge Aquifers

Caves are a unique habitat, and very simplified ecosystems due to the absence of light. Animals are far more common than plants in caves. Beyond the cave openings, there is no light for photosynthesis.

Without life, there are no plants. There are bacteria that rely upon chemosynthesis, most notably Movile Cave in Romania and hydrothermal vents at the Mid-Atlantic Ridge. Such bacteria also exist in Virginia, but they have not been found in caves near the surface. Virginia caves rely upon a food web based on photosynthesis, even though there is no light in the cave, as food from the surface is washed in or brought into the cave by animals.

In 1992, companies exploring for oil and natural gas in the Taylorsville basin northeast of Fredericksburg drilled into a pocket nearly two miles underground. They found microbes that relied upon iron or manganese instead of photosynthesis to oxidizize carbon. Those microbes had been living without access to the sun since the dinosaurs had left footprints in those sediments, 200-250 million years ago in the Triassic Period.1

There are also chemosynthetic microbes at methane vents on the Outer Continental Shelf. The methane has been generated for at least 15,000 years, from organic matter decomposing in the seafloor sediments rather than formation fluids from deep aquifers.

microbes use chemosynthetic processes to generate food from methane at seeps on the Outer Continental Shelf, but such microbes have not been discovered in Virgina caves - yet
microbes use chemosynthetic processes to generate food from methane at seeps on the Outer Continental Shelf, but such microbes have not been discovered in Virgina caves - yet
Source: US Geological Survey (USGS), Atlantic Methane Seeps Surprise Scientists

The food chain for animals that live exclusively in a Virginia cave is based on the little bit of detritus that washes in during storms, or is brought in by the animals that use caves for shelter but leave the cave for food. Bats feed outside the cave, and return with full bellies. The bats then create great mounds of guano underneath their roosting sites, far from where sunlight can reach. Various forms of life from bacteria to cave crickets can live on the energy provided by that food source.

Artificial lights installed for commercial cave tours are altering cave ecology. Visitors rely upon those lights to walk on the trail, and to see stalactites and other rock formations. If the lights are left on long enough each day, then algae spores can germinate and grow on the cave formations in the moist environment. The chlorophyll used by the algae to photosynthesize food from the artificial light adds an unnatural green color and texture to the cave formations.

In Virginia, caves are too dark for animals to enter very far except for bats, who rely upon a form of sonar rather than eyes to navigate without light. Salamanders, fish, and insects are protected against predators in a cave. Moist, dark, protected caves offer an attractive habitat.

Virginia caves are typically muddy, moist, and cool - and dark
Virginia caves are typically muddy, moist, and cool - and dark

"Facultative" cave dwellers, such as a raccoon or human hunters in colonial days looking for shelter from a rainstorm, use caves when convenient. "Obligate" cave dwellers (also known as troglobites) are those animals that have evolved to the point where they are no longer able to survive outside the cave. Some obligate cave dwellers have lost the use of eyes, which provide no benefit in a cave and require substantial energy to maintain.

A reduction in metabolic requirements will enhance survival in a pitch black cave, where food resources are very limited. The Lee County cave isopod has evolved so it does waste energy growing and maintaining eyes.

the Lee County isopod is not charismatic megafauna like a grizzly bear, but the first rule of environmental management is to save all the pieces
the Lee County isopod is not charismatic megafauna like a grizzly bear, but the first rule of environmental management is to "save all the pieces"
Source: Virginia Tech Extension Service, Endangered Species and Pesticide Regulation

Caves also provide a steady temperature, reflecting the average temperature of the area. Only near the entrance of a cave will the temperature change between day and night, and with the seasons. Virginia caves are about 54-56 degrees, every day, every hour. The one commercial cave in North Carolina, Linville Caverns, is always 52 degrees. It is located south of Virginia but at a higher elevation in the mountains, so its average temperature is cooler.1

Caves are not always a static environment; they can change dramatically at times. A summer thunderstorm can result in a rush of water into a cave, importing leaf debris, raising the temperature, and even drowning bats and breaking cave formations if the cave floods completely.

Humans exploring a cave can also be caught and drowned. Caving clubs known as "grottoes" warn new members to be cautious before entering a wild cave if rainstorms are likely in the area. While underground, people inside a cave will not hear thunder and know to head back to the entrance befoe water levels might rise and cut off the escape route.

Caves are vulnerable to pollution from the surface. In the karst region of Lee County known as The Cedars, roughly 75% of the water reaching the Powell River travels as groundwater for at least a portion of that journey. Whatever was on the surface - manure from wildlife and cattle, hydrocarbons from highways, pesticides from agricultural operations, etc. - can be carried down into cave systems.2

the Powell River drains the eastern side of Cumberland Mountain on the Virginia-Kentucky border
the Powell River drains the eastern side of Cumberland Mountain on the Virginia-Kentucky border
Source: US Geological Survey (USGS), Streamer

Contaminants and nutrients introduced into the groundwater from surface application of fertilizer, from septic system leach fields, or from stormwater ditches. Whatever flows into sinkholes can reach the cave undernath and affect the sensitive biological balance.

Caves can suffer if the perception is "out of sight, out of mind" regarding waste disposal. Until 2001, the highway maintenance facility for the Virginia Department of Transportation in The Cedars karst region (Jonesville, Lee County) channeled its stormwater runoff directly into a sinkhole.3

In the 1980's, water seeping through piles of sawdust (leachate) from a local sawmill and flowing into Thompson Cedar Cave had very low levels of dissolved oxygen. The pollution appeared to eliminate the cave's local population of the Lee County Isopod (Lirceus usdagalun), which was listed as an endangered species in 1992. After most of the sawdust was removed, water quality improved. Biologists rediscovered the species within the cave in 2002, presumably after survivors in upstream refugia expanded their range into the restored habitat.4

rain seeps underground through sinkholes and porous limestone rock in karst landscapes, carry pollution into caves underground
rain seeps underground through sinkholes and porous limestone rock in karst landscapes, carry pollution into caves underground
Source: Virginia Department of Mines, Minerals and Energy (DMME), Sinkholes and Karst Terrain

Preservation of the Madison Cave isopod (Antrolana lira) in the Shenandoah Valley faces a similar requirement to conserve the quality of groundwater. The Madison Cave isopod has been fund only where fissures link the surface to the groundwater table near Harrisonburg. As described in the Final Recovery Plan:5

Urban and agricultural development threatens the quality of its groundwater habitat and thus its survival... The Madison Cave isopod appears to have low reproductive potential, and the small population size at most of its sites indicates that it is highly sensitive to disturbance.

fertilizer from cropland and manure from livestock can get into the groundwater of caves from small openings in the porous limestone bedrock and via rainwater flowing into larger openings
fertilizer from cropland and manure from livestock can get into the groundwater of caves from small openings in the porous limestone bedrock and via rainwater flowing into larger openings
Source: US Fish and Wildlife Service, Madison Cave Isopod (Antrolana lira) Recovery Plan (Figure 2)

References

1. Richard A. Kerr, "Life Goes to Extremes in the Deep Earth and Elsewhere?," Science, Volume 276, Number 5313 (May 2, 1997), p.703, http://www.jstor.org/stable/2892318 (last checked September 17, 2016)
2. "Atlantic Methane Seeps Surprise Scientists," US Geological Survey, June 16, 2016, https://www.usgs.gov/news/atlantic-methane-seeps-surprise-scientists; N.G.Prouty, D.Sahy, C.D.Ruppel, E.B.Roark, D.Condon, S.Brooke, S.W.Ross, A.W.J.Demopoulos, "Insights into methane dynamics from analysis of authigenic carbonates and chemosynthetic mussels at newly-discovered Atlantic Margin seeps," Earth and Planetary Science Letters, Volume 449, September 1, 2016, p.332 (last checked September 17, 2016)
3. "Tips for Visitors," Linville Caverns, http://www.linvillecaverns.com/planyourvisit/tipsforvisitors.html (last checked August 3, 2014)
4. "Lee County Cave Isopod (Lirceus usdagalun) Recovery Plan," US Fish and Wildlife Service, September 1997, p.8, http://www.fws.gov/ecos/ajax/docs/recovery_plan/970930c.pdf (last checked August 3, 2014)
5. "Lee County Cave Isopod (Lirceus usdagalun) 5 Year Review: Summary and Evaluation," US Fish and Wildlife Service, Summer 2008, p.6, http://www.fws.gov/northeast/EcologicalServices/pdf/endangered/LeeCountyCaveIsopod.pdf (last checked August 3, 2014)
6. "Sometimes It's the Little Things that Matter," Endangered Species Bulletin, US Fish and Wildlife Service, Fall 2007, p.14, http://www.fws.gov/endangered/news/pdf/ES_Bulletin_09-2007.pdf (last checked August 3, 2014)
7. "Madison Cave Isopod (Antrolana lira) Recovery Plan, US Fish and Wildlife Service, September 30, 1996, http://ecos.fws.gov/docs/recovery_plan/960930d.pdf (last checked August 3, 2014)


Caves and Springs in Virginia
Virginia Geology
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