Ultrasonic detectors reveal the harmful effect of wind turbines on forest sites on bats

As more wind turbines (WT) are installed as part of the energy transition and regulations on distance to human settlements are tightened, suitable locations are becoming increasingly difficult to find. As a result, wind turbines are increasingly installed in forests, to the detriment of forest specialists among bats. In a new study, a team of scientists led by the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) demonstrated that forest specialists among bats, which feed under the treetops and therefore do not have increased risk of collision with turbines, avoid proximity to wind turbines. Forest sites should therefore not be used for wind turbines at all, or only in exceptional cases with mandatory compensatory measures to protect forest bats, the team concludes in a paper now published in the Journal of Applied Ecology.

More and more wind turbines are being installed around the world to meet the objectives of national climate strategies. In Germany, around 30,000 onshore wind turbines are currently in operation. However, the open spaces on which wind turbines are tolerated near towns and villages are limited. This is why wind turbines are increasingly installed in forests. “Forests are sensitive ecosystems and valuable habitats for many rare and protected bat species,” says Dr. Christian Voigt, head of the Department of Evolutionary Ecology at Leibniz-IZW. “Wind turbines in forests can cause problems for bats in several ways. Bats that hunt for insects above the treetops can be killed directly on wind turbines if they collide with the blades of the rotor or do not survive the intense atmospheric pressure differences near the blades. Bats that hunt in vegetation under the treetops lose some of their habitat due to the creation of clearings.” Their habitat could also deteriorate in the wider vicinity of wind turbines and clearings if disturbed by the operation of wind turbines.

Together with colleagues from Phillips-Universität Marburg and the University of Kiel, Voigt and his student mainly examined bats that forage under treetops sheltered from vegetation. To do this, they monitored bat activity using ultrasonic detectors at different distances from the wind turbines at 24 forest sites in Hesse. The scientists categorized the recorded calls into three groups of bats. First, those that forage in open areas (e.g. above treetops), second, species that hunt at edge structures, and third, specialists for foraging in narrow spaces, e.g. forest specialists under the canopy such as bats of the genera mouse-eared bats (Myotis) or long-eared bats (Plecotus). “These forest specialists were much less active near wind turbines, especially near large-rotor wind turbines, and during the summer months,” Voigt says. From a distance of 450 meters towards the wind turbines, the activity of these bats dropped by almost 50% near the wind turbines.

Wind turbines on forest sites therefore not only pose a direct threat to bats that hunt insects above the treetops, but also degrade the habitat of bats that live below the treetops in forests and hunt insects there. “Even though forestry specialists are unlikely to hit wind turbines, they still suffer from wind turbines in the forest due to habitat loss as they avoid operating wind turbines over a distance of several hundred meters,” Voigt concludes.

The authors therefore recommend that wind turbines should not be located inside forests but in the open landscape and in particular that near-natural forests with a varied vegetation structure be avoided. If wind turbines must nevertheless be erected in the forest, compensatory measures are essential. A central element of these mandatory compensatory measures should be to set aside a sufficiently large forest area as a wilderness area for specialized forest bats, so that the loss of habitat caused by the operation of the turbines is compensated.

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Materials provided by Leibniz Institute for Zoo and Wildlife Research (IZW). Note: Content may be edited for style and length.