and a number of papers sought to explain this on theoretical grounds. For example, it was argued that Australia’s climate during the Pleistocene had not been harsh enough to exterminate the above-ground populations of its cavernicoles, and so any tendency on their part towards specialization for underground life would be continually cancelled out by gene flow from outside the cave. Having wickedly sub-titled his paper Why there are so many troglobites [= highly specialized cavernicoles] in Australia, Howarth makes the telling point that “One has to actually enter a cave and look for troglobites before proclaiming on theoretical grounds that none could exist.” I offer this creed to the reader in the context of the British cave fauna. Let us, as naturalists, devise ways to find out what lives in our underground world and get down and study it at first hand.
I have distinguished between ‘interstitial’ (microcavernous) and ‘cave’ (meso- and macrocavernous) habitats on the grounds that their biotas are substantially distinct. We might expect a similar distinction to exist between ‘aquatic’ and ‘terrestrial’ cave communities. Certainly, there are some cavernicoles which are essentially aquatic, and others which are essentially terrestrial. However, the atmospheres of most mesocavernous, and of some macrocavernous, gas-filled habitats are permanently saturated with water vapour. This poses physiological problems for many groups of terrestrial arthropods which, unless equipped to eliminate excess water from their tissues (as aquatic species do), would quickly die of ‘water poisoning’ through dilution of their body fluids. Not surprisingly, ‘terrestrial’ mesocavernicoles have been found to be physiologically specialized to cope with a hydrating atmosphere and seem able to withstand long periods of immersion in freshwater – an adaptation which is essential in habitats which are frequently flooded by downward-percolating rainwater or by fluctuations in the water-table. Some seem equally at home in air or water, and can frequently be seen feeding on the floor of cave pools among their aquatic counterparts. Conversely, many freshwater aquatic mesocavernicoles seem able to cope with ‘terrestrial’ life without undue physiological stress and have been recorded as living out of water for several weeks at a time. So we see that the distinction between terrestrial and freshwater aquatic cave habitats is not exactly cut-and-dried, although there is a clear distinction between the communities present in either zone and those found in marine cave habitats.
Not all terrestrial cave habitats are moist. Large caves with more than one entrance often experience drying airflows which can produce desert-like conditions which are lethal to the hygrophilic denizens of the mesocaverns. However, such caves are often easily accessible and attractive to vertebrates, and may (especially in the tropics) support vast populations of bats, birds and guano-associated invertebrates. Guanobious animals exhibit few or none of the morphological characteristics considered by European cave biologists to be the mark of a ‘true cavernicole’ or ‘troglobite’, yet they may be just as exclusively cave-dwelling as any mesocavern specialist. Above-ground human structures are usually designed to be as dry as possible and are seldom completely dark, and this makes them suitable as a habitat for only a very few cave-threshold specialists, such as the daddy long-legs spider Pholcus phalangioides which presumably originated somewhere in the Mediterranean region, but in the UK is found only in houses. In between the dry, draughty macrocaverns and the soggy, airless mesocaverns, there may be wide expanses of transitional cave habitats with a variable microclimate, posing a distinct set of problems for the communities which inhabit them. Terrestrial inhabitants of such places must cope with the physiological stress of desiccation some of the time and physiological drowning for the rest of the time. In the tropics, transitional cave habitats may be particularly extensive, with their own specialized faunas, often dominated by ‘bandits’ – marauding predators and scavengers which live off the scraps of the guano-based community. Climatically similar conditions are found in man-made culverts and other artificial tunnels, and these frequently attract transitional-zone cavernicoles such as the widely distributed cave spider Meta menardi. Later we shall distinguish a range of natural and artificial cave habitats principally on the basis of their microclimatic regimes.
In earlier discussing the criteria which may be important to cavernicoles in choosing their habitats, I included the apparently pedantic phrase ‘gas mix’, rather than ‘air’ in my list of the media which may fill mesocavernous voids. I did so because it seems that the atmosphere of mesocaverns may differ substantially from that found in open macrocaverns with a good air circulation (which generally have much the same atmosphere as the outside world). Bacterial decomposition of organic material in small spaces frequently results in unusually high atmospheric concentrations of carbon dioxide. Frank Howarth’s new Australian cavernicoles, mentioned earlier, were found in poorly ventilated lava caves which are thought to share the atmosphere of the mesocavernous spaces in the surrounding basalt. The air in these caves is saturated with water vapour and contains around 250 times more carbon dioxide than normal air. Bad air caves occur in Britain too, but have not yet been biologically investigated.
Finally, we may seek to distinguish cave habitats from non-cave habitats in terms of their food supply. Early cave biologists, whose experience of cave faunas was mainly confined to the larger, more easily explored ‘fossil’ macrocaverns (those no longer bearing the watercourses which formed them) of temperate European limestone areas, concluded that cave animals were perpetually starved. While food resources may be very thinly distributed in such cave habitats, in others (and particularly in the tropics) food may be superabundant. The biotas of food-poor caves are adapted to eke out what little energy is available, while those of food-rich caves are adapted to a life of plenty. Caves may contain a wide range of food sources, including living vegetation (tree roots, saprophytic plants and fungi which get their energy by digesting organic matter rather than by trapping sunlight, fruits carried in by vertebrates), living invertebrate or vertebrate animals, and all kinds of detritus. Cavernicoles may be plant-, fungus-, detritus- or bacteria-feeders, predators, parasites or a combination of these. In short, caves are more ecologically diverse than most biologists realize.
To summarize then, cave habitats may be defined as ‘perpetually-dark voids, more than one millimetre in diameter (and sometimes much larger), bounded by rock or similar inorganic materials, and filled with gas (‘fresh’ or ‘bad’ air) and fresh or salt water.’ Within such habitats, the microclimatic regime and the type and quantity of the available food-supply largely determines the species composition of the cave community. Only the largest (and often, in our islands, the least populated) cave habitats are accessible to human observers, so that we know a good deal less about the composition and functioning of cave communities in Britain and Ireland than we do about most other natural communities of our islands.
Of the voluminous literature dealing with the biota of caves, two works of this century stand out for sheer scope of vision. The first, B. Wolf’s Animalium Cavernarum Catalogus, published in three parts between 1934 and 1938, lists all animal species recorded from caves to that date. The second, by A. Vandel, published in French in 1964, discusses the biota and biology of caves worldwide. An English translation, published by Pergamon Press in 1965 as Biospeleology: The Biology of Cavernicolous Animals, is perhaps still the most useful general text despite its wacky view of evolution in caves. L. Botosaneanu’s book Stygofauna Mundi, published in 1986, gives a more up-to-date account of the fauna of subterranean waters, but there is need for a similar treatment of the terrestrial cave fauna to take into account the spate of biological discoveries in tropical caves during the decade and a half since Vandel’s book. The following brief summary illustrates the range of life forms presently known to inhabit caves.
Fig. 2.2 Leptodirus hohenwarti, a highly cave-evolved beetle discovered in 1832 by the Count von Hohenwart in the Slovenian cave of Postojna Jama.
Kingdom MONERA
Phylum Bacteria
Well
