that the cavernicole will distinguish between natural or man-made tunnels, or between subterranean and above-ground enclosed chambers, as long as the appropriate conditions of food supply and microclimate are present in the living space? Should we then include mines, adits, buried pipes, culverts, sewers, cellars, tombs, the London Underground System, or perhaps even houses and other enclosed buildings in our preliminary list of potential cave-habitats? To what extent should our definition specify the material bounding the cavity? Must a cave be rock-lined, or should we widen our brief to include animal burrows and other spaces present in soil, for surely it must be arbitrary to distinguish between an earthy burrow and a muddy hole of similar dimensions in rock?
Fig. 2.1 The main tunnel of Sleets Gill Cave in Wharfedale, Yorkshire Pennines – a classic phreatic tube, formed and enlarged by water filling the passage and so dissolving the limestone rock equally on all sides. (Chris Howes)
As it happens, soil faunas are very well documented, and while it seems that many animal groups are common to both soils and caves (and indeed to leaf-litter and the deep moss-carpets of tropical regions as well), the fauna of organically-rich topsoils is sufficiently distinct from that of most rock-space habitats to warrant a separate treatment. I shall therefore exclude soils forthwith from our definition of cave habitats (but see ‘Cave sediments’ under ‘Types of cave habitat’ later in this chapter). Similarly, the voids in other organic, living or once-living materials, such as wood or the guts or blood vessels of animals, have distinctive specialized faunas of their own, clearly distinguishable from those of habitats within inorganic materials – although some specialized xylophages, such as termites, and endoparasites, such as tapeworms or flukes, share certain morphological specializations (eyelessness, depigmentation) characteristic of the more specialized cavernicoles. When it comes to holes of human fabrication, most significant biological criteria must lead us to include them in our category of caves. That they have a very poor fauna in comparison with natural caves, is due less to their artificial nature than to their frequent isolation from sources of natural colonization and their often unfavourable microclimate.
Having narrowed our definition of the cave environment to ‘habitable voids bounded by walls of rock, or similar inorganic materials’, let us now consider the physical criteria which may determine their habitability: the presence or absence of light, physical space (the size of the hole), the medium filling the space (water or gas mix), the microclimate within the medium (the pattern of change in temperature, pH, etc. over time), and the nature and amount of available food.
Let us begin with the business of light, a variable of obvious biological significance. Beyond the limits of light penetration, the cavernicole will be obliged to rely on senses other than sight, and on foods other than green plants. Perpetual darkness is a characteristic of most rock void habitats anyway, so let us choose to define ‘the cave’ as a habitat entirely without natural illumination. This will substantially simplify our task, by excluding from the cave fauna a whole host of organisms which seek shelter in cave entrances, but also live in a wide range of other shady, sheltered habitats such as the woodland floor, or river gorges, or houses and other structures used by people. Later we will consider the illuminated portions of man-sized caves as a significant ‘cave-related habitat’ – the ‘cave threshold’ – simply because it is familiar and accessible to cavers, while ignoring all other lit, cave-related habitats.
In the world of dark holes, the physical dimensions of a potential habitat are of obvious importance in determining what creatures can colonize it. One has only to consider the relative body-diameters of a man (say 450 mm across the shoulders), a Greater Horseshoe Bat (60 mm), a cave spider such as Meta menardi (6 mm), a springtail (0.6 mm) or a nematode worm (0.06 mm) to appreciate that one creature’s spacious accommodation may be another’s unenterable squeeze, and that the cave biologist may be excluded physically from all but a tiny proportion of the very largest of cave habitats. Frank Howarth, an entomologist who works mainly on the fauna of Hawaiian lava caves, distinguishes three principal hole-size categories which appear to have biological significance for subterranean biotas. He terms these ‘macrocavernous’ (>200 mm diameter), ‘mesocavernous’ (1–200 mm diameter) and ‘microcavernous’ (<1mm diameter).
The characteristic inhabitants of Howarth’s microcaverns are sometimes termed ‘the interstitial fauna’. They include a distinctive suite of specialized, skinny-bodied crustacea (such as Bathynella and various harpacticoid copepods) and other tiny creatures (such as rotifers, nematodes and tardigrades) which mostly like to be in contact with a solid surface on all sides and typically inhabit the spaces in between unconsolidated, fine-grained sediments such as the sand and gravel of river beds and the seashore. I propose, on purely arbitrary grounds, to exclude this fauna from further discussion in this book (except for species which also frequently inhabit larger spaces), and to restrict the definition of the cave habitat to holes of 1 mm diameter upwards, that is, to mesocavernous and macrocavernous habitats.
Various vertebrates use macrocavernous caves (and the larger mesocaverns) for shelter and they, and the other species which depend on their presence, form characteristic communities which reach astonishing levels of diversity and abundance in tropical regions. I shall long remember my first visit to the spectacular Deer Cave in the Gunung Mulu National Park in Sarawak, where at dusk close on half a million bats stream out of the cave in a seemingly-endless cloud which winds its way across the sky with a rush of wings like the sound of Niagara Falls. British bat-watchers have to be content with the odd flap, but in spite of declining populations, cave-roosting bats are still widespread and bat caves do support their own suite of associated ‘batellite’ cavernicoles. Other cavernicoles may, for example, be specifically associated with the guano of cave-roosting crickets, or with cave sediments introduced by sinking streams.
Mesocavern-sized holes not only occur within karstic rocks, but also in screes, in the coarse gravels and rocky beds of upland rivers, between the pebbles and cobbles of exposed sea-shores, in the fractured zone of non-karstic rocks (especially shales) just beneath the soil, and as cooling cracks in lava flows and other igneous rocks. They represent a very much larger habitable subterranean space than do macrocaverns and so have developed a richer and often more specialized fauna, frequently dominated by species peculiar to this habitat and characterized by a reduction in the size of the eyes, loss of pigment and various other specializations. These ‘mesocavernicoles’ may also occur in soil spaces, or animal burrows, or even in large macrocaverns, provided there is an adequate food supply of down-washed organic material and a fairly stable humid microclimate. Not all species within the mesocavernous fauna will be found in all related habitats; some do not seem to occur in soil-spaces, others shun human-sized caves.
Simply as a consequence of our own species’ enormous body-size, we are physically excluded from the very habitats which are most likely to harbour a specialized fauna. In the absence of appropriate tools with which to peer inside mesocavernous habitats, cave biologists have so far been forced to infer what they can about them from the behaviour of their biotas where they pop up in the accessible portions of people-sized caves. These act as windows into the mesocavernous world, but it seems likely that they provide a distorted view, encouraging widely differing interpretations of the nature of what has been observed. The present situation in cave biology is a bit like that which prevailed among astronomers a century or so ago, when dependence on inadequate earth-based optical telescopes sustained the widely-held belief that Mars was criss-crossed by an elaborate network of irrigation canals built by Martians. Speculation and controversy abound no less in cave biology literature, while cavernicolous communities remain enigmatic and under-recorded. As a result, new species await discovery in most subterranean habitats in every part of the world including the British Isles. In short the whole subject of cave biology is very much still in its infancy. A nice illustration of this turned up on my desk in the form of a report from Frank Howarth, announcing his discovery of a brand-new diverse fauna of highly specialized cavernicoles in lava caves in tropical Australia. For
