Hibernating bats in Flemish marlland: an overview of the period 1989-2020

Abstract: Twenty six different marl quarries in Flanders are checked annually for hibernating bats, 18 in the Meuse basin and eight in the Scheldt basin. Most of the quarries were created in the second half of the 16th Century through the extraction of marl as a building material and as a raw material for fertilizing agricultural lands. After the Second World War, mushrooms were grown in both large and small tunnel systems. In 1958, virtually all underground mushroom cultivation ceased. This article provides an overview of the bat counts by the Nature Point Bat Working Group, carried out since 1989, between the last week of December and the end of February. Over the years, a few more quarries have been included bringing the total up to 26 sites, which are counted completely every year. The numbers for 2015 are not included in the data because the count in the Lacroix quarry, the largest of the quarries, which accounts for a substantial proportion of the total bats counted did not take place in that year. In 1989 the two species of whiskered bats, Myotis mystacinus and M. brandtii (no distinction is made between the two in the counts) were the second most counted species with 158 individuals counted. By 2020 this number had risen to 1,622, making the whiskered bat the most commonly observed bat in the Flemish marl quarries. In 1989, Daubenton’s bat (M. daubentonii) was the most commonly observed species with 269 individuals. The numbers continued to rise until 2002 to a peak of 949 animals with a low of 340 animals observed in 2013. Initially, Natterer’s bat (M. nattereri) was still a rarity, but by 2020 there were 1191 hibernating animals. Geoffroy’s bat (M. emarginatus) is the species that has increased the most. Since 1989, their numbers have increased exponentially. The number of pond bats (M. dasycneme) initially increased moderately, to 154 in 2013 and there are now about 100 animals every year. The numbers of brown long-eared bats vary between 30 and 55 animals, which is barely 1% of the total number of bats counted in the Flemish marl quarries. Similarly, Bechstein’s bat (M. bechsteinii) was a very rare sight during the counts; in 2015-2016 the number increased to 16 animals. Greater mouse-eared bat (M. myotis), and common pipistrelle (Pipistrellus pipistrellus) were seldom seen, while a serotine (Eptesicus serotinus) is very exceptionally observed in the marl quarries. The proportion of ‘unidentified bats’ was significant in 1989, but has declined steadily thereafter. The Lacroix, Verbiestberg and De Keel quarries have the most bats. The strong increase of counted hibernating bats is possibly due to an increase in the population as a result of efforts made to close the quarries. Better illumination, increased capacity to identify the bats and better knowledge of the cave systems have also all played a role in more hibernating bats being counted and identified.