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Proceedings 10 : "Pozu del Xitu"

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Studies of the Cave Fauna on the 1979-1981 OUCC Expeditions

by Simon Fowler


Top : Introduction : Results : Species : Discussion : Acknowledgements

Collections were made in four caves at varying altitudes; one of the few biospeleological studies to have been done in this area. Four species new to science were discovered, two belonging to a new genus, and some more specimens of a species discovered by Lancaster University cavers were found at a new site. Possible reasons for the low diversity of the cave faunas in the Picos de Europa are discussed.

Introduction and Methods

Top : Summary : Results : Species : Discussion : Acknowledgements

The type of study undertaken by University Expeditions has been aptly called 'rape and pillage' collecting! Basically, in each of the three years, 1979-1981, most of the caves that were being explored by OUCC were collected in. Sometimes, mere searching by eye revealed animals; in others, baits were placed. Several types of bait were used, ranging from illegally poached crayfish from Lago Ercina to the local delicacy of strong blue cheese, scrounged from one of Amador's superb meals in the local refugio. All worked quite well! The methods and apparatus are covered in more detail in OUCC Proc. 9.

The preserved specimens were passed to Phil Chapman, who sent them to various authorities on the separate groups. Most of the specimens have now been identified or classified, and a list is shown below. This is unavoidably full of biological jargon and I apologise! The discussion is written to interest the hypothetical layman, so read on!


Top : Summary : Introduction : Species : Discussion : Acknowledgements

Cave details:

Pozo de Fresno, El Mazuco, Sierra de Cueva. Alt. 335m.
Cave temperature 12° C.

Hoyo la Madre, Rio Casaño gorge, nr Belbin. Alt. 880m.

Cueva del Osu, Los Lagos. Alt. 1230m.
Cave temperature (50m from entrance) 4.5±0.5° C over two weeks in July

Pozu del Xitu, Ario. Alt. 1680m.
Cave temperature (bottom of entrance series, -180m) 5±0.5° C, July 1980 to July 1981! At camp (-790m) 7° C.

More details are given in OUCC Proc. 9. All the caves are in the catchment area of the Rio Cares, so are connected by ridges of limestone.

Species List

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Abbreviations used:
TB - Troglobite - restricted to caves.
TP - Troglophile - lives and breeds in or out of caves.
TX - Trogloxene - spends only part of life cycle in caves (e. g. bats).
PH - Phreatobite - lives primarily in cracks, gravels, etc. below water table.
ED - Edaphobite - primarily a soil living species.

These distinctions are typical arbitrary biological classifications, but do have some limited uses.


Top : Summary : Introduction : Results : Species : Acknowledgements

With the single exception of the empty snail shells of Bythinella saxatilis, animals are rather rare in the caves of the Picos. In Pozo de Fresno, however, even limited sampling revealed a substantial fauna, especially notable for the large number of predatory beetles, indicating that a sizeable herbivore community must be present. The Sierra de Cuera, where Fresno is situated, is close to the Picos. Why then is there such a major difference between the faunas?

I believe there are two main reasons for this. Firstly, the number of available niches is low in the Picos, and secondly, the fauna would have been wiped out in the Pleistocene glaciations (about 10 000 years ago), and may not yet have recovered to its maximum potential level, so not all the available niches will be filled. These ideas are discussed below.

The 'available niches' will be dependent on the number of ways animals can 'make a living' in the cave environment. In caves, the basis of the food chain is organic matter from the surface. This can be washed through from the soil cover, be deposited as faeces or dead bodies of trogloxenes and so on. In the Picos the organic matter entering the caves is likely to have the following features:

a) Small in quantity: much of the area is covered in scree and Lapiaz, what vegetation there is is overgrazed. The soil layer is thin and probably gets rapidly oxidised by the summer sun.

b) Have low variety: if organic matter is swept into the caves in large quantities it is often all beech leaves. Alpine choughs supply faeces etc., but only near entrances. There are very few trogloxenes using the deeper of the caves. The low temperatures and low numbers of external insects are probably the reason for this.

c) Be poorly spread in the cave systems: the limestones are, in general, massively bedded. So most of the organic input to caves is by streams in flood. Most passages lack the inflow of seepage water, probably vital for the maintenance of cave populations.

All these reasons will result in a poor cave fauna in the Picos, even if there has been enough time since the last glaciation for animals to colonize the caves of the region. Data on the amount and nature of organic inputs to the caves are needed, especially with regard to comparisons between caves of the high Picos and those such as Fresno.

The second major reason for the rarity of Picos fauna is the limited invasion of the caves by species since the probable mass local extinctions caused by the severe glaciations in the Pleistocene. An individual species colonization is a random event, but the overall number of species in caves is likely to be influenced by the following factors:

a) the degree of physical isolation of the 'vacant' area. For example, whether deep gorges, non-cavernous rocks etc. form physical barriers to colonization. This depends to a great extent on the type of organism considered. A cavernous millipede will find colonization much more difficult than a winged insect. Interestingly phreatobites may find dispersal, at least within limestone, relatively simple. Quite a number of the Picos species could have been dispersed this way.

b) Something one could call 'ecological isolation', for want of a better term. If the vacant area forms an unusual type of biological habitat, then even if species arrive, they cannot immediately colonize it: they have to evolve. Stenasellus may be able to disperse into the Picos, but is highly cave-adapted to the low altitude, warm, organic rich caves such as Fresno.

c) Size of available pool of colonizers. If the surrounding areas, which will form the arbitrarily defined 'available pool', have only a few species, then the probability of species colonizing the caves is clearly reduced. Contrast the likelihood of a Picos cave being colonized, with a locally depauperate fauna, with the probability of a tropical cave being colonized, with a tremendously diverse local fauna.

These factors will have combined effects, and are not mutually exclusive. To illustrate two possible extremes:

1) The troglobite fauna of the surrounding hills may be very isolated from the Picos, already cave-adapted (no 'ecological isolation' ) and be low in species number (small 'available pool').

2) The above-ground beetle fauna in the surrounding hills may not be physically isolated (able to fly), ecologically quite isolated (need to adapt to caves) and have a reasonably large pool of available species.

In general, it appears that the Picos fauna is derived from the cave-adapted species of the surrounding hills: some species such as Trichoniscoides chapmani and Plusiocampa espanoli are common in the surrounding hills and have evolved only very slightly, if at all, in their Picos populations. Species probably need to change less to exploit relatively low altitude caves such as Madre and Agua, than to exploit high level caves such as Xitu. It would be nice to know the evolutionary ancestry of some of the other troglobites, such as the two species of millipede in the new genus of Asturasoma. Until more is known, the relative importance of factors such as 'ecological isolation' (Stenasellus? ), physical isolation or the saturation of the available riches will remain a subject for speculation.

The abundance of the empty hydrobiid snail shells in Pozu del Xitu (and probably other Picos caves) is rather surprising in the light of what has been discussed above. They are present on rock and mud surfaces at widely varying heights above the stream, from -180m (bottom of the entrance series) to at least -800m. Their distribution is patchy, maximum abundances being about 10 per 100cm² on deposits of mud, which have a 'grazed' appearance. If this is a result of 'grazing' by animals, then the snails seem the likely culprits, as nothing else is present in the necessary levels of abundance. In life, the snails live in cracks in the phreas, but get washed out in periods of flood. I would suggest that the snail populations on mud banks in Xitu are probably a remnant of the last flood, at that level, and that they lived long enough to heavily graze the mud surface, but rapidly overexploited the limited organic matter present in the mud. Since the cave does not flood to any great height (1st stream, less than ½m, 1979 to 1980), the old snail population, at the highest levels, may be remnants of very ancient floods indeed; presumably when that part of the cave was last active. Micro-radio carbon dating has been considered , and could give an accurate age for the last period of activity of the passage, but the process is in its infancy. Mollusc shells are also notoriously subject to dating errors, so whether this project is feasible or not is really not known. However, samples of the snails in Xitu were taken, in anticipation of this, in 1981.


Top : Summary : Introduction : Results : Species : Discussion

To the many members of OUCC whom I dragged along to collect bugs or keep me company.

Particular thanks to Phil Chapman for seeing to all the identifications, and for thought-provoking discussions. Phil is producing a paper for BCRA transactions, which will cover both the OUCC and the extensive LUSS collecting in the Picos, which he kindly let me read before writing this! All details of identifying authorities and references will be given in his paper.

Top : Summary : Introduction : Results : Species : Discussion : Acknowledgements