Brecon Beacons (Collins New Naturalist Library, Book 126) by Jonathan Mullard

Author:Jonathan Mullard
Language: eng
Format: epub
Publisher: HarperCollins Publishers
Published: 2014-03-12T16:00:00+00:00

FIG 149. Bullhead in Porth yr Ogof. They are often found in the caves and, unlike trout, their colour is not affected by living in the dark. (Andrew Lewington)

BATS

Some of the caves in the east of the Brecon Beacons, such as Ogof Agen Allwedd and Ogof Draenen, contain important bat populations, primarily lesser horseshoe bat, which is the commonest bat found in Welsh caves. Edward Lhwyd noted the habits of hibernating lesser horseshoes in Welsh caves in a letter to John Ray in January 1698/9 (Gunther 1945):

I know not whether I ever mentioned to you (although you have probably frequently observed it yourself) in what manner the Bats are lodged in the caves during winter. The caves of this country (to mention that by the by) are always (I speak of the inland caves), in limestone, and in such places only are all our subterraneous brooks, which in Wales are no great rarity. In these caves the bats choose the driest apartments, where, planting their talons to the roof, they cover their bodies with their wings, and so, hanging perpendicularly in great numbers (but not so as they touch each other) they sleep for some months.

Lesser horseshoes eat half their weight in insects each day, producing large amounts of nitrogen-rich guano, which provides food for beetles and other forms of cave life. An annual survey of the bat population in Agen Allwedd shows that numbers are currently on the increase, and there is no evidence to suggest that cavers cause them undue disturbance.

Ogof Draenen has the largest guano deposits to be found in a British cave. Unfortunately, with the exception of one small chamber, Siambre Ddu, which is located directly above the main Ogof Draenen system, there are now few bats found inside the main cave complex. The entrance has been blocked by a rock collapse and there has been a decline in the local forest cover, as well as a change in its composition. Extensive accumulations of guano from lesser horseshoes though are found in several parts of the system. In places these piles of guano cover many square metres and the heaps can be over 0.5 m high, volumes not found in any other cave system in Britain. Although it is not known when the main Ogof Draenen system was abandoned as a bat roost, six radiocarbon dates from the guano indicate that it was occupied from the Iron Age to the medieval period. As part of the dating process the guano was compared with two modern guano samples, one from Siambre Ddu and one from Agen Allwedd cave, 5 km to the northwest. The latter is currently one of the largest active roosts for lesser horseshoes in Britain and lies close to the present northern limit of the species in Europe. Ogof Draenen appears to have been used both as a summer and a winter roost and, if the largest heap is continuous, it represents the accumulation of around 750 years of guano, at a rate of 0.

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