most notably in the families Phalangodidae, Travuniidae and Ischyropsalidae. The terrestrial mites (Acari) most frequently found in caves are tiny Gamasides within the families Parasitidae, Rhagidiidae and Eupodidae and there are many cavernicolous water mites in the families Hydrachnellae and Porohalacaridae. Spiders (Araneae) dominate the predator niches in most tropical caves and include hundreds of cavernicolous species worldwide. In temperate regions, the most specialized cavernicoles belong to the primitive families Dysderidae, Leptonetidae, Telemidae and Oonopidae, but there are also many specialized species of Linyphiidae, Erigonidae and Agelenidae.
Fig. 2.3 Neobisium spelaeum, a giant cavernicolous pseudoscorpion which preys on the cave beetle Leptodirus hohenwarti (shown in Fig. 2.2)
Phyla Tardigrada and Bryozoa
A few species inhabit groundwaters.
Phylum Echinodermata
Various detritivorous brittle stars (Ophiuroidea) and sea cucumbers (Holothuroidea) frequently occur in submarine caves, but no strictly cavernicolous species are known.
Phylum Chordata
Class Teleostomi: Freshwater cavernicolous fishes are found mainly in the more arid regions of the world. To date around 60 more or less blind and depigmented species representing 8 orders and 13 families have been collected in freshwater and submarine caves, springs and groundwaters. The most speciose families are Cyprinidae, Gobiidae and Bythitidae, followed by Pimelodidae, Characidae, Cobitidae and Amblyopsidae.
Class Amphibia: 14 species of cavernicolous Urodela are known from North American caves and one from Europe. The latter, Proteus anguinus was the first cavernicole to be recognized as such. In common with species of the American genera Gyrinophilus, Eurycea, Typhlomolge and Haideotriton, Proteus retains larval gills in the adult stage, and is thus able to live permanently beneath the water table.
Class Reptilia: The colubrid snake Elaphe taeniura is a common inhabitant of caves in south-east Asia, from China to Borneo, where it preys on bats and swiftlets.
Class Aves: While no birds live permanently in caves, swiftlets of the genus Aerodramus (Apodidae) and the oilbird Steatornis caripensis (Caprimulgidae) are dependent on caves as nesting sites and are capable of navigating long distances underground and in total darkness by echolocation using ultrasonic clicks.
Class Mammalia: It is possible that a South American mouse Heteromys anomalus and one or two species of tropical shrews in the genus Crocidura may establish permanent cave populations. Many bats (e.g. Tadarida spp.) are dependent on caves for shelter, but none are cavernicolous.
In summary, although green plants are largely absent from caves, cavernicolous species are found within most of the major classes of animals, and are particularly common among the Crustacea, insects, spiders and millipedes.
Of several cavernous rocks in Britain and Ireland, one above all others provides extensive integrated cave systems of a size which allows the biologist to study the life they contain with relative ease. It is the Carboniferous Limestone.
Fig. 2.4 The blind North American cave fish Speleoplatyrhinus poulsoni has an extraordinarily developed lateral line system, with correspondingly enlarged brain parts for processing tactile and positional information.
The cave-bearing limestones are concentrated in six major patches: in County Fermanagh (the largest, but least well prospected area), the Northern Pennines, Derbyshire’s Peak District, County Clare, Somerset’s Mendip Hills and South Wales. In the Northern Pennines alone, there are 1800 or more documented cave entrances and around 350 km of explored and mapped cave passages – while current estimates give the total length of mapped passages in Britain and Ireland at somewhere around 800 km. Cave exploration is still in a very active phase in these islands, and significant new discoveries are still being made. For example, recent explorations in Ogof Daren Cilau beneath Mynydd Llangattwg in South Wales have yielded over 20 km of new discoveries in less than two years, including the largest passages yet found in Britain.
By any standards, 800 kilometers of open cave passage represents a significant habitat, worthy of the attention of naturalists – yet passages of explorable size must form but a tiny proportion of the total cave habitat, the vast majority being of mesocavernous dimensions. In the absence of data, I would guess that the habitable surface area within the mesocaverns of limestone terrains must run to at least two or three orders of magnitude more than that within explored caves.
So what is it which makes limestones so spectacularly cavernous? To understand the process in which caves are formed, we must begin by examining the origins, nature and structure of the limestone rock itself. Limestone is a sedimentary rock, that is, it began life as suspended particles in an ancient sea, gradually settling to the ocean floor millimetre by millimetre over millions of years. During this inconceivably long period, there were intervals when conditions changed enough to interrupt the steady downward rain of lime, allowing some other type of deposit to intervene briefly in the sequence of otherwise pure calcium carbonate. Aeons later, and now hardened to rock, these geological glitches have become ‘fossilized’ as bedding planes – horizons of weakness between the solid layers of limestone.
The Carboniferous Limestones of the British Isles were laid down somewhere in the tropical seas of the southern hemisphere. Pushed along on a northward-drifting chunk of continental crust, they have had a bumpy ride. Some, like those of the Yorksire Dales, have survived their 340 million year journey the right way up, though somewhat fragmented by massive vertical faults. Others have fared less well. The Mendip limestones lie like a wrecked car, buckled and perched at a steep angle, so that the bedding planes dip downhill at an average gradient of 50° or so. In all cases, the rough ride has produced vertical stress cracks, called joints, which link with faults and with the original bedding weaknesses of the limestone to form a boxwork of crevices reaching from the highest hilltop to beneath the deepest valley.
Limestone is a strong rock and so frequently forms upland regions. Solid limestone is impervious to water, but water is able to flow through the cracks within it. It is these cracks which are the key to understanding the origins of caves. Limestone caves form principally by means of a simple chemical reaction in which hydrogen ions from groundwaters, acidified with dissolved carbon dioxide, act on the relatively insoluble carbonate ions in the limestone to produce soluble bicarbonate 10ns which are then flushed away. The reaction renders the limestone 25 times more soluble than it would be in pure water and the result is holes.
Some of the carbon dioxide in groundwaters is collected by raindrops falling through the atmosphere, and some from the breakdown of organic material picked up as the rain then trickles down through the soil. Immediately beneath the soil, the weathered surface layers of rock are more fractured than those at a greater depth and the acidified, aggressive soil waters have their maximum impact here – so that at any one time, up to 15% of the volume in the top three metres of limestone may be occupied by air-, or water-filled spaces (Stearns, 1977 – figures for Central Tennessee, USA). These mesocaverns act like the guttering beneath the roof of a house – collecting soil water and quickly conveying it to natural drainpipes, often developed on the intersections of major joint fractures, or steeply inclined bedding planes.
The
