Cave Zones: Entrance, Twilight and Dark Zones

Caves are often regarded as dark recesses of the earth. However, most are not aware that there are zones in these subterranean environments. This zoning system is basically dictated by the presence of light, or absence of it, and temperature dynamics. Cave organisms are also commonly attributed with each specific zones.

---

The entrance zone is the closest part to the surface of the earth. This zone includes the entrance itself and the areas touched by sunlight. The temperature is highly variable and is mostly affected by the outside climate [2].

Many animals live and/or visit the entrance zone, but never staying permanently, as they are not dependent on it for survival. These animals are called trogloxenes and can include, cave bears, frogs, etc. The entrance zone is being used by these trogloxenes for feeding, sleeping, or nesting [3].

Want to know more about trogloxenes? Check this article:
TROGLOXENES: THE PART-TIME CAVE RESIDENTS

• 

• 

• 

---

Moving deeper in to the cave, the twilight zone connects the sunlight-touched areas and the totally dark subterranean chambers [4]. When you are inside the cave, the indirect lighting in this zone often appear as soft bluish glow at some point [5]. It is alternately wet and dry, with a high and variable potential evaporation rate [6].

In the twilight zone, both the trogloxenes and some troglobites share a part of the cave as habitat. Animals in this zone need moisture and coolness to survive [3], apparently, just what the twilight zone offer. Predators, scavengers and above ground animals looking for shelter are the most common animals in this zone.

Want to know more about trogloxenes? Check this article:
TROGLOBITES: THE CAVE’S PERMANENT DWELLERS

• 

• 

• 

---

Explore even deeper into the subterranean chambers and you’ll find yourself in the dark zone. The presence of total darkness in this area is one of the criteria for it to be called a dark zone.

Troglobites live in the dark zone. These permanent cave-dwellers have undeveloped eyes, poor pigment and long antennae because they’ve adapted to live in this environment [3].

In cave archaeology, the dark zone is considered as the most powerful part of the cave in terms of its effects to humans [4]. This is primarily based on the concept that it “is an ancient, undisturbed darkness, a darkness that has been here since the dawn of the world” [7].

Traditionally, this zone’s temperature remains constant. However, recent researchers further divided it with some added criteria. For example, a study proposing a new category within trogloxenes in Korean limestone caves divided the dark zone into dark zone with varying temperature (DZVT) and dark zone with constant temperature (DZCT) [8]. Another group of authors cited five zones, with the additional three coming from subdivisions of the dark zone. The three additional are transition dark zone, deep dark zone, and stagnant air dark zone. Their description are as follows [6]:

---

In summary, cave zones are mainly divided into entrance, twilight and dark zones. The presence or absence of light, temperature, and fauna are the main defining criteria. Moreover, practicing the leave-no-trace principles would highly ensure the preservation of these zones and everything withing.

References:

[1] Bairagya, H. 2014. “Environmental conditions of Borra Cave, Visakhapattanam, India”, International Journal of Environment, 3(2), pp. 150-166. doi: 10.3126/ije.v3i2.10526.

[2] Nadeau, I., 2006. Caves. 1st ed. New York: Rosen Pub. Group’s PowerKids Press.

[3] Ronca, D., 2009. How Cave Biology Works. [online] HowStuffWorks.com. Available at: <https://science.howstuffworks.com/life/biology-fields/cave-biology2.htm> [Accessed 1 July 2020].

[4] Dowd, M., 2015. The Archaeology Of Caves In Ireland. Oxford, UK: Oxbow Books.

[5] Hajenga, J., 2005. The twilight zone. West Virginia Wildlife Magazine, [online] pp.8-10. Available at: <https://www.wvdnr.gov/wildlife/magazine/Archive/05Winter/twilightzone.pdf> [Accessed 1 July 2020].

[6] Howarth, F. G., Moldovan, O. T. 2018. Cave Ecology. Ecological Studies, pp.28-29.

[7] Jenkins, M., 2009. Deep South. [online] Nationalgeographic.com. Available at: <https://www.nationalgeographic.com/magazine/2009/06/tag-caves/> [Accessed 1 July 2020].