The number of bats is falling. Is this an effect of global warming?

• The number of bats in the largest winter bat colony in Poland is declining for the second year in a row
• Counting had to be done in one day. Chiropterologists from 6 countries in the European Union and Great Britain took part

Scientists count bats mainly out of scientific curiosity, but also the obligation of environmental monitoring imposed by the European Union on all member states of the community. Every five years, researchers send reports to Brussels, which serve as the basis for strategies for the protection of individual species and habitats.

- Global warming creates new threats to nature, including bats, which we have been researching since 1999 as part of an international research programme coordinated by the Wrocław University of Environmental and Life Sciences. Bats are protected and useful and must face this problem with or without our help - says Dr. Tomasz Kokurewicz from the Institute of Biology of the UPWr, who coordinates the counting.

Chiropterologists in the Międzyrzecze Fortified Region checked the number and species composition of colonies and the condition of the bats. The collected data will be analysed, including in terms of the impact of climate change on these flying mammals. Bats are a kind of barometer of changes taking place in the climate. They have one, or in exceptional circumstances two cubs a year, so they are very sensitive to any changes in external conditions, and are not able to adapt to them quickly enough. It is therefore possible to predict the state of the environment on the basis of changes in their numbers. And not only temperature changes.

The results of this year's count are not optimistic

- Unfortunately, we are witnessing a second year of decline in the number of bats in this largest winter colony of these animals in our country. Last year, 36,838 bats were counted there. On the other hand, in 2020 a record result was recorded for this winter site, with 38,909 bats spending winter hibernation in the underground site of the Nietoperek reserve - concludes Dr. Kokurewicz.

He emphasises that the two-year downward trend does not raise concerns, but if this trend continues at a similar level next year, this will already be a wake-up call that something is happening to the bats, probably due to global warming and the drying up underground.

- It is mainly a decrease in the water level underground, which can be caused by both climate change and anthropogenic factors, i.e. poorly-planned human activity - the scientist explains.

Among the most numerous wintering in the reserve are thermophilic species, for which the increased temperatures underground are a favourable situation. These include, among others, Daubenton's bat and greater mouse-eared bats. The number of coldwater bats, on the other hand, is falling, as it is increasingly difficult to find sufficiently cool places for hibernation. This group includes, among others, Western barbastelle and brown long-eared bats.

- Apart from the temperature itself, air humidity is also an important factor during bats' hibernation. For most species, it should be at a level of 90-100 percent. The animals can then reduce water loss by evaporation and do not need to wake up frequently to replenish their water supply. They can thus minimise the use of energy, which is greatest when bats are waking from their winter sleep - says Dr. Kokurewicz.

International cooperation and smooth operations

Chiropterologists from six countries in the European Union and Great Britain took part in the counting of bats. From Poland, the most numerous group were chiropterologists from the Wrocław University of Environmental and Life Sciences, including three biology students.

In addition to the substantive and organisational support of the Wrocław University of Environmental and Life Sciences, the Society for the Protection of Nature "NIETOPEREK" - a non-governmental non-profit organisation established by chiropterologists and bat lovers from Wrocław to support the research and protection of bats, also helped to organise the counting.

In order to obtain reliable results and minimise any disturbance for these mammals, it was necessary to perform the counting in one day (from sunrise to sunset), which, with a total length of underground corridors of 32 km, required the participation of 50 people. The underground system of corridors was divided into nine sections, in which the bats were counted simultaneously by nine teams.

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