Poulson and Kane’s finding may go some way to explaining the unattractiveness of my cheese and liver baits in OFD. It could be that unusual concentrations of protein-rich food entering an oligotrophic (food-poor) cave, may be shunned by the cave community (except guanobia) as they represent a level of energy availability well above that to which the cavernicoles are adapted. But there may be a simpler explanation. Kathleen Lavoie of the University of Illinois noted that certain cave fungi grew better on large droppings (such as those of pack rats) than on those of smaller species. Rapid colonization by fungal hyphae formed a meshwork barrier which seemed to exclude cave arthropods from this otherwise tasty food resource. So, far from cave arthropods shunning large items of food, it may be that other, more opportunistic forms of life actively keep them at bay.
So far, we have considered only detritus as a food-source. While in sheer numbers, detritus-eaters must of course form the bulk of the cave community, it is predators and omnivores which have the greatest scope for dietary specialization, providing they are able to survive at low population densities. The predators in British caves are unspectacular creatures: mites, spiders, beetles and their larvae, amphipods, caddis larvae and flatworms. Once again, we can only speculate as to the degree of ecological specialization which they enjoy – they have not yet received any detailed study in Britain. The Americans, however, have studied their own equivalent species in considerable detail and, as there is no reason to expect cave predators of temperate American caves to behave very differently from our own, their research findings are worth repeating here.
Tom Barr studied six cave-evolved ground beetles which co-exist in Mammoth Cave. One of them, Neaphaenops tellkampfi, feeds almost exclusively on the eggs of cave crickets. It finds them by smell, digs them out of the sand in which they were laid, then punctures them with specially-elongated mandibles and sucks out the contents. The other five beetles are all closely related in the genus Pseudanophthalmus, and all feed on different prey or in different microhabitats. Even the two near-identical-looking species P. menetriesii and P. striatus feed quite differently: one hunts for small arthropods in rotting wood, the other digs for tubificid worms in cave silt. Our own cavernicolous carabid, Trechus micros, feeds in a similar way. I have watched it work over a tidal mud bank in Otter Hole in the forest of Dean, pocking the surface with hundreds of tiny pits, as it thrust its head repeatedly into the soft sediment in search of prey.
The tidal mud of Otter Hole is an unusual cave sediment which supports an exceptional fauna. Estuarine muds of the Bristol Channel have been estimated to have a productivity four times that of good arable soil. They are rich in organic material and correspondingly smelly. The mud is kept sloshing up and down the upper reaches of the estuary by fierce tides which have an amplitude here of 13 m, one of the greatest tidal ranges anywhere on our side of the Atlantic. During the highest Spring tides, when the cave stream is itself swollen by rains, it is usual for the entrance passages of the cave to flood right up to the roof and to stay flooded for several weeks. Cavers know this, and avoid the cave during this danger period. Eventually the trapped waters drain slowly away, but in the meantime their sediment load has settled out as a rich brown deposit which coats the walls and ceiling of the cave right up to the high-tide mark. The new mud is soon invaded by a wealth of invertebrates, including a millipede, Brachychaetuma melanops, and a rove beetle, Aloconota subgrandis, found in no other British caves. The richness of the cave fauna is a direct result of the floods.
The importance of seasonal flooding to cave invertebrates has been noted by many biospeleologists. In the food-poor alpine caves of the Pyrenees and Spanish Cantabrians, almost the whole fauna lives in the ‘intertidal zone’ of flood-prone passages. In the enclosed space of a cave, flooding can take one of two forms: ‘flash floods’ and ‘ponding’. In the former, a sudden rush of water temporarily approaches the carrying capacity of a streamway, sweeping all before it. This is the type of flood dreaded by cavers, and passages which are prone to such events are rigorously avoided whenever there is a risk of heavy rain in the catchment area. ‘Ponding’ floods happen more gently. They usually occur where collapsed blocks or sediment impede the flow of escaping water, so that any slight increase of input causes a temporary pond or lake to form behind the obstruction. The water may have been flowing quite quickly up to the barrier, but now it slows, depositing its sediment load. As the waters recede, the local invertebrates rush out of hiding to ‘beachcomb’ for the juciest morsels.
In temperate caves, flooding is a strongly seasonal, and therefore predictable, phenomenon – and many cave species time their cycles of activity and reproduction around it. The advantage of synchronized breeding is obvious in cave species which occur at low densities and in which only a small proportion of females are capable of reproduction in a given year. Tom Poulson, of Yale University, has made a special study of aquatic cave communities in the USA. He explains the complex relationship between cavernicoles and floods as follows:
Fig. 2.9 The ground beetle Trechus micros digging an enchytraeid worm from the silt of Otter Hole in the Wye Valley.
“Annual growth and breeding cycles in caves are cued by spring floods, specifically by changes in temperature, food supply, amounts of solute, turbidity and current … Scale growth rings of amblyopsid cave fish, and probably other cave fish, form during floods while plankton and organic matter are being replenished, but the fish are secretive, inactive and not feeding. The amblyopsid cave fish Chologaster agassizi, Typhlichthys, Amblyopsis spelaea and A. rosae breed in spring towards the end of the yearly floods or high water, and the young appear during the period of low water 3 to 8 months later when residual food is still present and chances of injury from floods are low. … Some snails, isopods, amphipods and decapods also breed in spring and early summer … [for example] breeding in the shrimp Paleomonetes ganteri and the crayfish Orconectes pellucidus precedes high water, with maximum organic inwash, by 4 to 6 months.”
James Keith has found a clear seasonal reproductive cycle in the American cave beetle Pseudanophthalmus tenuis, which inhabits flood-prone mud banks in Murray Spring Cave, Indiana. Across the Pacific, Chris Pugsley has studied the New Zealand glow-worm Arachnocampa luminosa whose starry displays form the centre-piece of a tourist development at the famous Glow-worm Grotto at Waitomo. Although most stages are present throughout the year, numbers of larvae (the feeding stage) show a clear peak in late spring / early summer, when their food (winged imagines of aquatic insect larvae) are abundant, and the evaporation rate in the cave is at its lowest level.
Seasonal variations in food supply in caves are not due solely to flooding. Bats have a predictable seasonal occurence in temperate caves, and this might be expected to strongly affect members of the guano communities which depend on their presence. Harris (1970) has described the cycle associated with occupation of a cave by a maternity colony of Bent-winged Bats in Australia. As the bats arrive, there are fast changes in food, temperature, moisture relations and pH. Food