Research notes No. 214. Surveillance of bats Myotis sp. and their living conditions at Daugbjerg and Mønsted Limestone Mines 2002-2004. Søgaard, B., Baagøe, H.J. & Degn, H.J. 2005. National Environmental Research Institute. 56 pp.
Background and purpose
Mønsted and Daugbjerg Limestone Mines in Jutland are of international importance as hibernacula and resting places for several species of bats Myotis sp. According to the EEC Habitats Directive the mines are designated as special areas of conservation because of the occurrence of the Pond Bat Myotis dasycneme, which together with the other Myotis species need strict protection on their resting and wintering localities.
For several years the mines have been used as storing places for maturation of cheese. In the Mønsted Limestone Mine the storing of cheese has caused the development of a bacterial biofilm on the walls and ceilings of the caves near the place of storage apparently deterring the bats to hang and rest in this section of the mines. No widespread development of bacterial biofilms has yet been observed in the Daugbjerg Limestone Mine, though storing of cheese also takes place here albeit on a much smaller scale.
The aim of the project was therefore to investigate - in both mines – if there is a substantial, negative influence from the development of bacterial biofilms associated with the storage of cheese on the habitat conditions for bats, especially during hibernation. The project is carried out in collaboration with the National Forest and Nature Agency.
Surveillance of bacteria
In the Mønsted Limestone Mine there is an annual cheese production of around 3.500 tons, while the annual cheese production in the Daugbjerg Limestone Mine is around only 100 tons of cheese. The maturing cheeses give off ammonia, which has caused development of bacterial biofilms on the walls and ceilings of the caves. To follow the development of the bacteria a number of test squares (one square metre) were established in both limestone mines and were surveyed yearly in the period 2002-2004.
In the Mønsted Limestone Mine the bacterial biofilms occur over a distance about 750 metre of the caves in and around the storing place for the cheese. In the Mønsted Mine eight tests squares have been established since 2002 at wall sites with and without visible development of bacterial biofilms. At two of these eight test squares a development through two succession stages has been recorded. Furthermore, it has been noticed from 1998 to 2004 that the bacterial biofilms has spread to the ceilings of the galleries around the cheese store. The difference between the development of bacteria in the ceiling and in the test squares is due to the ammonia rising up to the ceilings and remains there, while the concentrations of ammonia at the floor are lower and also influenced by draughts.
In the Daugbjerg Limestone Mine bacteria biofilms occur only in a very limited area (less than one square metre) in the central cheese store, and in the monitoring period bacterial biofilms have not been recorded in the five test squares established in this mine. It is assumes the absence of developing bacterial biofilms is associated with the small-scale production of cheese combined with the existence of a ventilator, which extracts air from the central cheese store.
Surveillance of bats
In the Mønsted Limestone Mine the surveillance of bats in 2003 showed, that there was a hibernating population of Myotis daubentonii estimated to 5.900-8.300 individuals. It is not likely that drastic changes in the population of this species has occurred since 1977. The size of the hibernating population of Myotis dasycneme is estimated to about 2.100 individuals, and for this species there are strong indications that the population has increased since 1977. Furthermore, there exist smaller hibernating populations of Myotis nattereri and Myotis brandtii.
In two smaller sections of the Mønsted Limestone Mine counts of bats was carried out in 1980-1981 and in 2002-2003. In the area "Port", where bacterial biofilms in 2002 have developed on the greater part of the walls and ceilings, only a third of the number of bats counted in 1980-1981 was counted in 2002-2003 and with clear preference for areas without bacterial biofilms. In the area "Skorsten", where bacterial biofilms do not occur, there was conversely almost a doubling in the number of bats. In 1980-1981 there were no bacterial biofilms in neither "Port" nor "Skorsten".
In the Daugbjerg Limestone Mine the surveillance of bats in 2003 shows, that here there is a hibernating population of Myotis daubentonii estimated to 10.000-12.000 individuals, which is thriving. The size of the hibernating population of Myotis dasycneme is estimated to about 1.750 individuals, and there are indications, that the population has increased since 1960. There was in 2003 a small hibernating population of Myotis brandtii, but it is uncertain if there was also a hibernating population of Myotis nattereri, because no individual of these species was caught or observed in the Daugbjerg Limestone Mine in 2003.
For Daugbjerg Limestone Mine no earlier data are available direct to compare with. The present surveillance therefore represents a baseline for subsequent similar registrations.
Final conclusions
In the Mønsted Limestone Mine there have been a reduction of the habitat conditions for the hanging bats in those sections of the mine, where bacterial biofilms have developed through the last 25 years. The total length of the caves is unknown, but is thought to be at least of the order of tens of kilometres. The 750 metres of the mines with occurrence of bacterial biofilms are therefor only a small part of the total systems of mine tunnels. That fact, together with the data showing a sizable population of Myotis daubentonii, pronounced increase in the population of Myotis dasycneme, and the registrations of in- and outflights of bats made in 2004 does not warrant concern for developments in the hibernating populations of bats in the Mønsted Limestone Mine.
Concerning the criteria for favourable conservation status for Myotis dasycneme at hibernation localities, favourable conservation status seems to bee maintained for the population, as the hibernating population appears to be stable and in fact increasing. This is not-withstanding reduction in the extent of suitable hibernation conditions due to spreading of bacterial biofilms on the walls and ceilings of the caves. Thus, it appears that the extent of suitable hibernation conditions is not for the present a crucial factor for the Mønsted Limestone Mine population of Myotis dasycneme.
Possible further spreading of bacterial biofilms in the Mønsted Limestone Mine can however cause concern for the wellbeing of the bat population. If the bacterial biofilms spread to the network of small tunnels in the inner part of the mine, which are very important hibernating areas for the bats, the situation could be critical. Furthermore, the situation in the galleries could become a concern since the bats especially make use of the galleries in the mating season and it is in these parts of the mine that the biofilms are spreading.
With the storing of cheese and the resulting development of bacterial biofilms an "artificial" ecosystem has resulted, where threshold values in relation to certain as yet unknown factors might imply drastic changes in the use by the bats of Mønsted as a hibernaculum. Automatic registration of in- and outflights of the bats from the mines will in this connection be very important as a early warning of changes in the numbers of hibernating bats indicating that the quality of habitat conditions has been seriously reduced.
In the Daugbjerg Limestone Mine there occur no bacterial biofilms in the caves except for a very small area in the central cheese store. The surveillance showed hibernating populations of Myotis dasycneme and Myotis daubentonii, that were thriving indicating favourable conservation status for these species in the Daugbjerg Limestone Mine.
It is concluded, that storing of cheese has not reduced habitat conditions for hibernating bats in the Daugbjerg Limestone Mines. With the present size and character of the cheese stores in Daugbjerg there seems no risk that it should be the case in the future.
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