Deep, dark, wet and eerily silent they may seem; but caves are one of the most striking natural formations of the Earth. From the world’s longest cave in sandstone situated in Meghalaya to the marvellously sculptured Ajanta and Ellora caves; humans have always been fascinated – more of being dependent on caves. Prehistorically, we needed caves for shelter which than became a medium of communication through rock paintings such as the ones seen at Bhimbetka in Madhya Pradesh. Then historically, caves became places of worship. India abounds with such caves of spiritual value. Caves also have some of the most unique ecosystems.
Taking millions of years to form, caves not only hold historical and spiritual significance but are unique ecosystems in themselves, harbouring life forms that are found nowhere else. The unique environment inside a cave favours species that can sustain in low-oxygen levels, under complete darkness with little connection to the ‘outer’ world. Thus, caves are fascinating areas to explore and the study of caves and cave systems is known as speleology. Biospeleology is the study of the cave organisms or the ‘troglofauna’ (troglos is Greek for caves).
How are caves formed?
Rainwater becomes acidic when it takes carbon dioxide from the atmosphere and groundwater takes acids from the soil. Caves are formed when this acidic water etches its way through rock. This water causes weathering of the rocks (generally limestone) creating incredible cave formations. The acidic water widens the already existing cracks in the rock to create passages which widens even further to form channels and ultimately caverns are formed. This process takes millions of years to occur.
The acidity of the water, the rock type, the external environmental conditions etc. are the factors that determine cave formation. The dripping of mineral- saturated water from the cave roof created pointed deposits hanging from the roof (stalactites) and rising from the floor (stalagmites). Overtime, these can join together to form columns. Thus, caves are a matrix of passages, tunnels, streams, pools and columns harbouring a rich variety of unique biota.
Cave Organisms
From a cave adapted specimen of the Golden Mahsheer measuring 40 cm and more to the tiniest of critters, caves are home to various species of bats, fishes, insects, crustaceans, arachnids and microbes that utilize a cave in different ways; spatially and temporally.
What are the different zones of a cave?
Caves are divided mainly into three zones and various organisms are adapted to each of these:
The Entrance Zone
This area receives sunlight and hence, there is variability in the environmental conditions. Green plants can be found here. The animals that are found here are known as Troglophiles or cave lovers. These organisms can survive both inside and outside the cave. Troglophiles include some beetles, worms, salamanders, amphibians etc.
The Twilight Zone
This area receives comparatively less sunlight and no green plants grow here. The temperature is relatively constant and the organisms that live here are known as Trogloxenes. They require moisture and coolness to survive and usually leave and enter the cave at their will. These organisms use caves for specific part of their life cycle like hibernation, birth etc. Trogloxenes include bats, bears, racoons etc.
The Dark Zone
This forms the deepest part of the cave and sunlight does not enter here. The temperature remains constant throughout the year and the organisms that live here need to adapt to dark, damp and hypoxic conditions. The animals that live in this dark zone are known as Troglobites. They have underdeveloped eyes, little or no pigmentation and long antennae for communication. These organisms spend their entire lives inside the cave. Examples include cave fishes, cave shrimps, cave arthropods etc.
Caves harbour unique food chains where the main source of nutrients is the water that flows from outside and brings plant and animal material inside a cave. Various organisms inside the cave like bacteria and fungi then feed on this organic matter. These bacteria and fungi are then consumed by millipedes, crustaceans, and insects; which then are eaten by larger organisms like fishes, centipedes, spiders etc. Bat poop containing guano is also an important source of nutrients for various cave- dwelling species. Thus, a cave- food pyramid is formed with the decomposers at the base and large carnivores at the top.
How do organisms ‘enter’ caves and survive?
Organisms generally enter a cave accidently due to rainwater influx and get trapped inside the cave. The populations that survive slowly adapt to the cave conditions and over generations can survive in the dark environment. They generally loose eyesight as the dark environment favours senses like hearing and smell. Maximum of the troglobites have underdeveloped eyes, but instead have long antennae for olfactory purposes. They lose skin pigmentation and become white (dull) coloured. These organisms can also survive over long periods of time without food.
An amazing example is that of a ray-finned fish Indoreonectes everzadi . Most of the populations of this fish are found in streams of the Western Ghats. While two cave adapted forms survive in the Kutumsar caves in the Kanger Valley National Park in Chhattisgarh. The cave populations of this fish are generally albinistic, with poorly developed eyes than their Western Ghats counterparts.
Other examples of cave-dwelling organisms include:
- Schistura larketensis – A species of blind fish found in the Krem Khung caves in Meghalaya
- the Olms – salamander species inhabiting caves of Slovenia and Croatia
- Blind salamanders– found only in underground cave systems in the U.S.A.
- Wroughton’s free-tailed bat (Otomops wroughtoni) – an extremely rare and elusive bat found in the Jaintia hills of Meghalaya; and many species of cave spiders, cave pseudoscorpions, cave crabs etc.
Mammoth Caves-huge cave systems as long as 563 km have been discovered in the U.S.A. They are the longest cave system in the world. Exploring caves and their life forms is as fascinating as diving into the space as what lies ahead is completely ‘dark’ to us.
With long tunnels, passages, narrow channels, deep pools, and towering pillars, caves are a difficult terrain to study and explore. With proper equipment like positioning systems, ropes, helmets, harnesses, carabiners, torches etc., caves can be explored to study geology, water systems, biology, archaeology; along with cave exploration for leisure purposes. Sustainable tourism and recreational activities in caves are an important source of knowledge about caves and cave ecosystems. Studying and protecting cave systems will go a long way in preserving these unique and fascinating ecosystems. They can certainly be used as a form of wildlife tourism.
Written by: Vaishnavi Apte
References
- Debra Ronca, How Cave Biology Works, howstuffworks.com
- Divya Candade, Life Without Light: Creatures of Meghalaya’s Caves, Roundglass Sustain , sustain.round.glass
- Bryan Nelson, 10 Creatures that Thrive in Caves, treehugger.com
- Knowledge Encyclopaedia, Dorling Kindersley Limited (2017)