Bats can adapt to certain changes in habitat that threaten their preferred roosting sites. A new study from Poland shows that bats will widen their criteria for selecting roosting sites if forest management practices limit their preferred sites. However, experts recommend that small patches of old growth forest suitable for bat roosting are maintained as the bats in this study did not adapt to young woodland.
Bat populations in Europe are declining, partly due to loss of natural habitats1. Many species of bat roost in trees, often seeking out holes in old, decaying trees. However, these trees are often removed by forest managers because they are a fire hazard and could spread disease to younger trees nearby. It would therefore be useful for forest managers to know more about how bats choose their roosting trees, in order to understand whether they can adapt their roosting behaviour in areas where dead trees have been felled.
To learn more about bats' roosting habits, the researchers studied a 100 kilometre square area in the Polish section of the Białowieża Forest. The study region included areas of well-preserved, ancient forest and other areas of younger, managed forest. The researchers used radio-transmitters to track two particular species of bats to their roosting sites in the forest. The species were the Common Noctule (Nyctalus noctula) and the Lesser Noctule (Nyctalus leisleri). The Common Noctule is one of Europe's most common bat species, but the Lesser Noctule is relatively rare, although common in Ireland.
The researchers then compared the habitats around the roosting trees selected by the bats with those of all the available habitats in the same area of forest. They looked at a total of 100 roosting trees and tracked 51 bats for an average of one week each.
Their study shows, as might be expected, that these particular species of bat prefer to roost in deciduous woodland, characterised by a wide variety of tree species, including oak, hornbeam, and lime. Although there were some differences between the two species, both were far more likely to select old forest stands (over 100 years) as roosting sites, even though plenty of younger stands were available.
However, where there was a lack of old, deciduous woodland, bats selected roosting sites in old, wet woodland instead. Wet woodland is more uniform than deciduous woodland, composed mostly of alder, ash and spruce.
As the study suggests, bats are able to adapt their behaviour to increase the potential area for roosting. But the bats still avoided younger trees. The researchers therefore recommend that, in all managed forests, logging trees in patches of old growth forest should be reduced or stopped altogether in order to preserve suitable bat roosting habitats. They suggest that, where possible, these patches should be connected by corridors.