mesocaverns, but is still often sluggish enough to accommodate small, slow-swimming cavernicoles (such as Niphargus fontanus), which avoid fast-moving vadose streamways.
In recent years cave divers have penetrated great distances into freshwater phreatic macrocaverns, and to considerable depths, but to date no detailed studies have been made of the biota of this remote and fascinating environment.
Water-filled mesocaverns
Most of our knowledge about the structure of mesocaverns comes from looking at exposed joints or bedding planes in limestone quarries, or occasionally caves, and from the work of karst hydrologists. We know that the very earliest stages of cave development occur under phreatic conditions, eventually creating inter-connected systems of conduits which may be in any plane, from horizontal to vertical. Phreatic mesocaverns may take the form of wiggly networks of small-diameter tubes (anastomoses), or thin, but laterally-extensive cracks, or narrow shafts – and are probably as common in other cavernous rocks, such as chalk or gypsum, as in limestone. Evidence that such spaces are inhabited comes from the animals found in well-water over the centuries. It is ironic that the earliest records of our cave fauna should be from a habitat about which little more is known today than a century-and-a-half ago, when Philip Henry Gosse wrote:
“recently, investigations in various parts of the world have revealed the curious circumstance of somewhat extensive series of animals inhabiting gloomy caves and deep wells, and perfectly deprived even of the vestiges of eyes … even in this country we possess at least four species of minute shrimps [all of which] have been obtained from pumps and wells in the southern counties of England, at a depth of thirty or forty feet from the surface of the earth.”
There is no way at present of collecting information directly about how aquatic cavernicoles use mesocavernous bedrock cracks, but it seems likely that great local variation exists within this habitat in terms of oxygen concentrations, pH and food supply. Such factors are likely to influence the distributions of the fauna, and will be discussed in Chapter 4. Fortunately there are other, more accessible types of water-filled mesocavernous habitat which are easier to study. They include the deeper interstices of stream-bed cobbles (phreatic nappes), and a peculiar sub-soil phreas of mesocavernous dimensions which occurs on the surface of impervious silt or clay deposits in mountainous areas of Europe (the hypotelminorheic medium). In both these habitats, the food supply comprises dissolved or finely particulate organic material, and the waters tend to be rather low in oxygen; and both contain faunas very similar to those of limestone mesocaverns.
Amphibious mesocaverns
As no detailed investigation of the biology of air-filled mesocavernous habitats has yet been attempted, we are forced to infer what we can about the conditions within them from studies of limestone caves and other similar habitats.
As soon as mesocaverns develop an airspace, they become available for colonization by terrestrial cavernicoles. However, cracks and anastomoses are extremely flood-prone, often filling up with water each time there is heavy rainfall at the surface. Vertical cracks probably flush more violently, but remain water-filled for shorter periods than horizontal cracks, and this may result in some differences in their faunas. Less immersion-tolerant organisms may tend to inhabit the wider, better-drained vertical cracks and humid terrestrial cave habitats, while the more aquatic organisms may prefer horizontal cracks or cave pools. It is likely that particulate organic material accumulates at specific points within the cracks (perhaps at the upstream ends of permanently flooded sections), so that some patches of habitat will be better supplied with food than others. Some areas may be too anoxic to support any life other than anaerobic micro-organisms, while some patches may harbour relatively large concentrations of detritivorous invertebrates. As previously discussed, there may be an almost complete overlap in distribution between the ‘terrestrial’ and ‘aquatic’ components of the fauna of such habitats.
The French biospeleologists Juberthie, Delay and Bouillon consider that mesocavernous spaces in fractured rock immediately below the soil constitute a habitat which is separate from caves, which they have termed the ‘Superficial Underground Compartment’ (SUC). Their claim rests on differences between the fauna found here and that found in deep caves. They consider the primary cause of such differences to be the greater temperature variation experienced in the ‘SUC’ compared with the ‘Deep Underground Compartment’ (DUC) represented by deep fissures and caves. While this may be so in SUC habitats beneath shallow soils of regions which experience a strongly seasonal temperate climate, I would doubt that the microclimate in deeply-buried SUC habitats or those of tropical karst differs a great deal from that of the ‘DUC’ – and there is evidence that cave faunas migrate up into the SUC periodically in order to exploit the resources they contain. For the purposes of this classification, I propose therefore to treat the ‘SUC’ as part and parcel of other intermittently-flooding mesocavernous spaces, whether they be immediately below the soil, within cave passages, or connecting one with the other.
There would seem to be little doubt that the SUC within calcareous rocks is by far the most extensive and important of all cave habitats in terms of the numbers and diversity of its biota. Since their ‘discovery’ of the ‘SUC’ (an environment previously well-known to karst hydrologists as the ‘subcutaneous zone’), Juberthie and Delay have gone on to show that this habitat and its biota not only occurs in limestone and other cavernous rocks, but also in ‘non-cavernous’ shales, granites, schist, gneiss, sandstones, etc. My first reaction on reading the paper announcing this discovery was to attack the bottom end of my garden with a pick and shovel. There, to my delight and amazement, I found tiny-eyed cave spiders (Porrhomma egeria) frolicking among the fractured chunks of Pennant Sandstone just one metre beneath the wreckage of the flower bed. As far as I know there has been no systematic investigation to date of the fauna of ‘SUC’ habitats in Britain and Ireland – an extraordinary gap in our knowledge which surely must be remedied before long.
A better-known mesocavernous habitat is contained in talus, or scree, whose surface can frequently become covered with vegetation and soil, turning it into a fair imitation of Juberthie and Delay’s SUC. When not sealed by soil, the upper levels of talus are unsuitable as a habitat for cavernicoles, being too cold in winter, too hot in summer and too dry for much of the time. However, if the scree is deep enough, the lower levels must surely provide exactly the conditions favoured by cavernicoles, though I know of no work on this deep-talus habitat in Britain.
I know of only two accessible ‘DUC’ mesocavernous habitats within caves. One is in the spaces within rock piles (underground talus), the other is in speleothem pockets. Rock piles may, or may not provide a suitable habitat for mesocavernicoles. If the pile is in an old, dry ‘fossil’ passage, as most rock piles tend to be, it is unlikely to contain enough food to support life (unless the cave contains bats, or other vertebrates, in which case the rocks may be over-run by guano-beasts). On the other hand, if the pile is sufficiently extensive, and is traversed by percolation water carrying organic material, it is likely to harbour a rich fauna of mesocavernicoles – although the depth within it at which a searching biologist can expect to ‘strike bugs’ will increase with the increasing dryness or breeziness of the surrounding cave atmosphere, precisely as would be the case with above-ground talus. Juberthie (1983) gives an interesting example of a schist-boulder pile in the great Salle de la Verna chamber in France’s Pierre Saint Martin cave. It is inhabited by a typical SUC fauna of Aphaenops beetles which appear to be quite oblivious of the fact that their schist scree habitat lies the best part of 1000 m underground.
Speleothem pockets are essentially just spaces of mesocavernous dimensions like all the others described in this section, but where they occur in the ‘deep cave’ or ‘stagnant air’ zone of caves (see the microclimate section, later in this chapter), they will often prove to be the very best places to look for mesocavernicoles. Speleothems
