Τα σπήλαια από βιολογική άποψη είναι κι αυτά ένα ξεχωριστό οικοσύστημα το οποίο διαφοροποιείται από τα επίγεια οικσυστήματα εξαιτίας των ιδιαίτερων χαρακτηριστικών και συνθηκών που επικρατούν σε αυτό. Όπως και όλα τα άλλα οικοσυστήματα αποτελείται τόσο από τα αβιοτικά (π.χ. πέτρωμα, νερό, αέρας, κτλ) όσο και από βιοτικά στοιχεία (βακτήρια, μύκητες, αρθρόποδα, νυχτερίδες, κτλ). Η βιοσπηλαιολογία είναι ο κλάδος της βιολογίας που ασχολείται με την μελέτη των έμβιων οργανισμών τα οποία περνάνε όλο ή κάποιο μέρος της ζωής τους σε σπήλαια. Με άλλα λόγια βιοσπηλαιολογία είναι η μελέτη των βιοτικών παραμέτρων του οικοσυστήματος των σπηλαίων. Η μελέτη αυτή έχει να κάνει με την συστηματική κατάταξη των σπηλαιόβιων ειδών, με τις ανατομικές ιδιαιτερότητές τους, τη σύγκριση μεταξύ σπηλαίων της ίδιας ή όχι περιοχής και πολλά άλλα. Όπως και με τα έμβια εκτός σπηλαίου, τα εντός χωρίζονται σε "βασίλεια", όπως τα βασίλεια των ζώων, των φυτών και των μυκητών.
Οι σπηλιές έχουν σταθερό περιβάλλον. Τις χαρακτηρίζει η σταθερή θερμοκρασία, η έλλειψη φωτός (κατ' επέκτασην και φωτοσυνθετικής δραστηριότητας φυτών), υψηλή σχετική υγρασία και συνεπώς χαμηλούς ρυθμούς εξάτμισης υγρών. Για τους οργανισμούς που αναπτύσσονται στα σπήλαια (π.χ. σπηλαιόβιοι οργανισμοί) η σταθερότητα αυτή είναι ιδανική.
Κατηγορίες οργανισμών που σχετίζονται με τα σπήλαια
Οι οργανισμοί που ζουν ή σχετίζονται με τα σπήλαια χωρίζονται σε τέσσερεις κατηγορίες. Τα φυτά και οι μύκητες αποτελούν την πρώτη και το ζωικό βασίλειο τις άλλες τρεις: Τα ζώα που περνούν τον όλο τον κύκλο της ζωής τους στην σπηλιά και αδυναμούν να ζήσουν εκτός αυτής ονομάζονται Τρογλοβίτες (ή τρογλόβια ή σπηλαιόβια). Ζώα που ζούν κάποια περίοδο (μικρή ή μεγάλη) της ζωής τους στο σπήλαιο και εξέρχονται από αυτό για οποιοδήποτε λόγο (π.χ. συλλογή τροφής, ζευγάρωμα) ονομάζονται Τρογλόφιλα. Τα ζώα που χρησιμοποιούνε περιστασιακά την προστασία του σπηλαίου (είτε από κυνηγούς, είτε από τα στοιχεία) ονομάζονται Τρογλόξενα.
Cave environments fall into three general categories:
- Endogean: the parts of caves that are in communication with surface soils through cracks and rock seams, groundwater seepage, and root protrusion.
- Parahypogean: the threshold regions regions near cave mouths that extend to the last penetration of sunlight.
- Hypogean: or "true" cave environments. These can be in regular contact with the surface via wind and underground rivers, or the migration of animals, or can be almost entirely isolated. Deep hypogean environments can host autonomous ecologies whose primary source of energy is not sunlight, but chemical energy liberated from limestone and other minerals by chemoautotrophic bacteria.
Cave organisms fall into three basic classes:
- Troglobites ("cave dwellers") are obligatory cavernicoles, specialized for cave life. Some can leave caves for short periods, and may complete parts of their life cycles above ground, but cannot live their entire lives outside of a cave environment. Examples include chemotrophic bacteria, some species of flatworms, collembola, and Blindfish.
- Troglophiles ("cave lovers") can live part or all of their lives in caves, but can also complete a life cycle in appropriate environments on the surface. Examples include cave crickets, millipedes, pseudoscorpions and spiders.
- Trogloxenes ("cave guests"): Frequents caves, and may require caves for a portion of its life cycle, but must return to the surface (or a parahypogean zone) for at least some portion of its life. Hibernating reptiles and mammals are the most widely recognized examples.
There are also so-called accidental trogloxenes which are surface organisms that enter caves for no survival reason. Some may even be troglophobes (“cave haters”), which cannot survive in caves for any extended period. Examples include deer which fell through a sinkhole, frogs swept into a cave by a flash flood, etc.
The two factors that limit cave ecologies are generally energy and nutrients. To some degree moisture is always available in actively-forming Karst caves. Cut off from the sunlight and steady deposition of plant detritus, caves are poor habitats in comparison with wet areas on the surface. The majority of energy in cave environments comes from the surplus of the ecosystems outside. One major source of energy and nutrients in caves is dung from trogloxenes, the majority of which is deposited by bats. Other sources are mentioned above. <ref> Dr. Paul Richter, Classifications of Cave Biota & Cave Environments (1996)</ref>
Cave ecosystems are very fragile. Because of their rarity and position in the ecosystem they are threatened by a large number of human activities. Dam construction, limestone quarrying, water pollution and logging are just some of the disasters that can devastate or destroy underground biological communities. <ref>National Speleological Society, The Fragile Underground</ref>
from other sources
Cave Inhabitants Trogloxene
Bats are Trogloxenes and spend the winter in caves to hibernate. Three types of bats, Eastern Pipistrelle, Little Brown and Northern Myotis, inhabit Crystal Cave between September and April for hibernation. Bats mate in the fall and then go into hibernation. While in hibernation, they will wake up about every twenty-five to thirty days. While in hibernation the bats live off their body fat. They can eat about three thousand bugs a night when feeding. If the bats are disturbed in the caves, they will lose body fat and leave early, often to their death. In the spring the eggs will be fertilized and the young will be born from late May to June. Eastern Pipistrelles are the first to enter hibernation each fall and the last to leave the caves in the spring (Figure 1). During hibernation they live off body fat. Eastern Pipistrelles prefer to hibernate alone in caves. They also may choose the exact same spot in a cave year after year (BCI, 2002). There are approximately sixty to eighty Eastern Pipistrelles hibernating in Crystal Cave each winter (Figure 1). The Northern Myotis is also a solitary bat in caves or is found in small groups (Figure 2). The Little Brown often forms colonies with hundreds of bats (BCI, 2002) (Figure 3).
These are species that have been confined to cave life and form adaptations in order to survive life only in caves. The adaptations are ones that will allow great struggle for species to live outside of the cave. Fish, salamanders, insects, spiders will have reduced eyes and in most cases no eye sight (Figure 4). In dark caves eye sight is not needed. Troglobites have great senses of smell and touch allowing a more efficient life for cave dwelling animals. Characteristics such as elongation of antennae and bristle covered legs give the species abilities unlike those of species similar to them above the surface (Hurd, 2001). An interesting aspect of Troglobite’s lives are the possibility that species of such could have survived in the caves as other species above ground became extinct. Changing conditions on the earth largely effects the surface environments, whereas protection of species living below the surface in cave environments will most likely remain relatively stable over long periods of time (Holsinger, 1998). These species may have occurred protected against many perturbations that effect surface environments. Evidence of Troglobite species that once lived in the cave and had been well preserved in the stable cave environment (Figure 5). Troglophile
Troglophile’s are animals that like to live in caves but are also widely seen above ground. They are attracted to the caves because of the moist and dark environment. These species are then commonly seen above ground in dark damp areas, such as under logs, rocks (Kramer, 1995). Differences between Troglophiles and Troglobites are the pigmentation and eye sight (Figure 4). Food For Cave Dwellers
Unlike the bats that live in the caves during the winter months which are using the cave as a hibernation location and have stored food, other cave dwellers need nutrients to survive. There are two main ways basic food resources become available in most ecosystems (Holsinger, 1988):
Stream Flow. Nutrients may be transported by streams that enter caves through sinkholes and other openings in the cave formation. Openings can be intense factor when flooding occurs and strengthens the velocity of the waterways. Percolation. Percolating groundwater enters the caves. Water passes through crevices and fissures holding nutrients in its liquid through the material above the cave. Decompositional material. Species in the cave will feed off of bat guano and dead species in the cave.
Figure 1. (Left) Distribution Range of Eastern Pipistrelle Bats in North America. (Right) A picture of an Eastern Pipestrelle . * Isolated or questionable record of bat inhabitants (BCI, 2002).
Figure 2. (Left) The distribution of Northern Myotis around North America. (Right) A picture of a Northern Myotis. * Isolated or questionable record of bat inhabitants (BCI, 2002).
Figure 3. (Left) The distribution of the Little Brown around North America. (Right) A picture of a Little Brown. * Isolated or questionable record of bat inhabitants (BCI, 2002).
Figure 4. Notice the coloration of the pictures in the left column compared to those in the right column. The pigmentation of the troglobites to the left is very dull. The coloration is an adaptation to cave life. Troglophiles (right) on the other hand are colorful and resemble species above surface. They are able to adjust from habitat to habitat (USGS, 2002).
Figure 5. Well preserved fossils of species that survived many years after surface species died off.
Characteristics of caves do not make life possible for trees, grass, or ferns. Photosynthesis is not a process that occurs in dark, damp places (Kramer, 1995). Although there is no sign of photosynthetic plants in the deep cave itself, flora largely exists in the entrance of the caves. Near the entrance of the cave there is a gradual decrease of light intensity with increased distance from the opening. As you move farther into the cave you find less and less flora (Figure 6). The lack of photosynthesis creates a lack of chlorophyll. The absence of chlorophyll may cause species to lose their pigmentation and have a whitish or pale yellow color (Ford, 1976). The flora found deep in caves is generally flora inhabited in other environments above the surface with low amounts of light. These species have adapted to a cave environment to allow inhabitance in caves. Most of these species are varieties of fungi and bacteria that have adapted to life in a cave (Figure 7). Fungi and bacteria receive the energy needed through animal droppings, dead animals, leaves, stems, and twigs carried through by stream water (Kramer, 1995). In cave inhabitants where humans venture through there are higher rates of exotic fungi that is accumulating (Ford, 1976). At Crystal Cave where we went on a field study there was few evidence of human induced life (figure 8). Fungi is critical in providing food sources to other species in the cave (Ford, 1976).
Figure 6. As the light intensity decreases moving away from the mouth of the cave, less evolved flora is found. Once you get to the point where is an absence of light penetrating into the cave you only find fungi and bacteria (Ford, 76).
Figure 7. Evidence at Crystal Cave of human induced flora growth.
Figure 8. Picture of fungi using bat guano as a sourceof energy to survive in a cave (Kramer, 1995).