Mines, tunnels, cellars and tombs
As artificial caves, such spaces may be inhabited or visited by any cavernicoles which have both the motive and the opportunity to do so. The motive will be a suitable medium/microclimate and food supply (see following sections). The opportunity will fall to any cavernicole whose habitat intersects the artificial cave.
Chlorophyll and sunlight are the elements of life on earth and the source of that unique green glow which identifies our living planet when seen from space. Photosynthesis lies at the base of the food chains in which all life is meshed: fish, fowl and fungus; tree, turtle and tiger – or almost all.
In 1977, an American ship on an oceanographic survey south of the Galapagos Islands, located some active volcanic vents on the sea floor at a depth of 3 kilometres. An instrument pod was sent down, armed with video cameras to record the scene. As the probe moved towards the vents, the watching scientists were amazed to see their monitor screens fill with a writhing mass of enormous worms, 10 centimetres thick and up to three metres long. Close by were beds of 30 cm-long clams. Among them swam shoals of fish, and white crabs scuttled across the black basalt rocks. At these depths, far beyond the reach of sunlight, life is generally thin on the ground. A few starfish, crinoids and crustaceans subsist on the steady drizzle of detritus, and are in turn eaten by predatory fish. To find such an abundant local concentration of large organisms clearly pointed to an unusually rich food source. The mystery was soon solved. The volcanic vents, it seems, were spouting superheated, sulphurladen water. As this cooled, clouds of black sulphides formed and were immediately consumed by great concentrations of bacteria. The worms and the clams were feeding on these bacteria and they in turn supported the scavenging fish and crabs. The bacteria concerned are chemo-autotrophs – that is, they can harness the chemical energy in the volcanic sulphides to power their own vital processes. What is more, this whole process and the mini-ecosystem which revolves around the ‘volcanic bacteria’ is quite independent of sunlight.
Sulphur bacteria, and relatives which derive energy by reducing ferric iron compounds, are common in caves – in sediment banks as remote from solar rays as are the depths of the deepest ocean trench. The muds where they live are home to nematode worms, known bacteria-feeders, which are hunted by tiny, scurrying beetles. How many visitors, catching sight of a cave beetle, appreciate that they may be watching one of the rarest of all living phenomena – a predator sustained, at least in part, by a food chain independent of sunlight.
The energy source for cave-based chemosynthesis generally originates outside the cave, as Carboniferous Limestone itself contains very little in the way of iron and sulphur minerals. These compounds, and the bacteria which exploit them have usually been washed in as part of the cave’s sediment load. But there is at least one energy source which may originate within the fabric of the cave itself. Most limestones contain detectable amounts of organic matter, largely in the form of hydrocarbons. Such material would be of no use to animals directly, but if there are bacteria present which are capable of using it as an energy source, it could be continually liberated into the cave ecosystem at the interface between the cave and the rock, imperceptibly, as the cave is dissolved out by flowing water. Since the organic matter in the limestone will have been derived from organisms present in the seas when the rock was being laid down, its energy content must originally have come from the sun. It is in fact fossil solar energy.
Interesting though they may be as a scientific curiosity, chemo-autotrophic bacteria contribute only slightly to the food base of most cave communities. By far the biggest source of energy is still the sun’s rays, but at second-hand, in the form of introduced detritus from surface communities; for no green plants can survive in the absolute dark of the cave.
The absence of green plants in caves led early biospeleologists to the conclusion that cavernicoles are starved animals. In 1886, Packard insisted that the shortage of food available to cave animals is the reason for their small size. While it is self-evident that large animals with a high metabolic rate can have no place in an entirely heterotrophic, food-poor ecosystem, in reality many cave species are actually far larger than their surface relatives. Most recent studies have shown that cave-evolved animals (troglobites) have unusually low metabolic and growth rates and that they save energy in every way possible, by streamlining their movements and by adopting highly efficient foraging and reproductive strategies. These are obvious specializations to cope with a low food supply, and seem to be a fundamental characteristic of cavernicolous evolution at temperate latitudes like ours. Some cave species are remarkable in their metabolic efficiency and consequent ability to tolerate starvation. Gadeau de Kerville (1926) reported that a specimen of the Slovenian cave salamander, or Olm (Proteus), had been kept in captivity for fourteen and a half years, and for the last eight of these had received no food. He did not report whether it eventually died of starvation or just plain old age.
Some more recent cave biologists have noticed that captive Olms regularly slough and then eat the mucus layer which, like an extra skin, covers and protects their whole body. Mucus is sticky and microscopic examination has shown that in captive amphibians it becomes encrusted with bacteria, algae and protozoa. So de Kerville’s amazing ‘non-feeding Olm’ may all the time have been sneaking clandestine meals of diatoms-in-slime – not the tastiest of fare, but enough to keep it ticking over. Streams sinking into caves must carry with them a fair load of phytoplankton, and it may be that the Olm’s mucus-eating behaviour in captivity has some adaptive significance in the subterranean rivers where it lives.
Fig. 2.8 Olms (Proteus anguinus) – blind, depigmented cave salamanders which retain gills and an aquatic lifestyle as adults.
In a closed system, such as our Olm aquarium, the recycling of dissolved nutrients could go on endlessly, providing the Olm did not lock them up as extra Olm tissue. Once equipped with a source of energy – sunlight from the nearest window will do – the tank becomes a self-perpetuating ‘mini-ecosystem’. Because caves are perpetually dark, production cannot meet demand and they can therefore never be considered as proper self-sustaining ecosystems. But the waters which form, enlarge, infill and eventually destroy caves almost invariably carry organic compounds – complex chemicals gathered from the soil or the breakdown of animal and plant detritus. Organic chemicals (dissolved in water, or clumped together in big lumps of detritus) fill the role of an energy source in the cave. There has been much speculation by cave biologists over the extent to which dissolved organic substances can be absorbed directly by aquatic cavernicoles. So far the evidence is inconclusive, but there is no doubt that they are captured by microfungi, bacteria and protozoa, which are plentiful at least in some allogenic cave waters. So, one way or another, all organic material entering the cave becomes available to the bottom rung of cave animals – the detritivores which fill the equivalent role to the primary consumers of the sunlit world above.
While the photosynthesising parts of plants have no place in caves, there is no reason why their roots should not penetrate into subterranean voids. Indeed, in the lava tubes which run just beneath the skin of the active volcanoes of Hawaii, tree roots form dense subterranean forests which support a range of sap-sucking bugs, root-chewing caterpillars and their predators; pale-skinned, eyeless relatives of species found in the forest above. Roots seldom penetrate into macrocaverns in Britain, but they may constitute a significant food source in a number of other cave habitats, such as the SUC, culverts and slutch caves. No one seems to have investigated root-associated faunas in such locations, but it would surprise me if such a widespread niche is not occupied by at least one insect specialist.
Second-hand plant material