Wind Turbines: An Underestimated Impact on Bats

Wind turbines are symbols of the green energy transition, but their impact on biodiversity is a cause for concern. These giant turbines are reportedly responsible for significant bat mortality—higher than previous estimates had suggested. The ultrasonic detectors used to assess the number of animals in wind farms have thus only quantified a small fraction of the affected bats. In its current form, monitoring underestimates the impact of wind turbines on bat populations, which are crucial to ecosystems.   With the rise of renewable energy, wind turbines are becoming an increasingly common sight in our rural landscapes as part of efforts to reduce CO2 emissions and mitigate global climate change. However, the energy production from a wind turbine is not environmentally neutral. In fact, these turbines require vast areas to operate, which can encroach on the natural habitat of local wildlife. A large number of birds and bats are killed each year by wind turbines. This conflict between climate-related environmental goals and the protection of biodiversity remains unresolved and requires the collection of more data, which is currently lacking. The vertebrates involved in the highest number of fatal incidents related to wind turbines are bats. In fact, these turbines are the most frequently observed human-caused cause of mortality among these species worldwide. Environmental impact assessments of wind turbines are mandatory in many countries during the first year or years following their construction, in order to measure their impact on animal populations, including bats. To do this, researchers often use automated ultrasonic detectors, placed at the bottom of the nacelle, to estimate the risk of mortality among bats. These devices automatically record the animals’ echolocation calls, which are used to identify species and their activity while taking environmental conditions into account. This information enables the implementation of strategies to reduce the risk of collision. For example, since bats appear to be less active as wind speed increases, there are plans to program wind turbines to start up only when the wind is blowing at a higher speed than currently set, in order to minimize interference with these animals. However, this strategy is only valid if the data provided by automated ultrasonic detectors accurately reflects what is actually happening on the ground. However, comparing acoustic data with other methods, such as visual surveys, shows that only 30 to 50 percent of bat flights are recorded by ultrasonic detectors. Furthermore, the acoustic activity of bats at the height of the nacelle does not necessarily correspond to actual mortality. Researchers from Germany and the United Kingdom sought to better understand this discrepancy and identify the factors causing ultrasonic detectors to malfunction in risk areas around wind turbines where bats are at greater risk of injury. The scientists show that geometric and atmospheric attenuation of echolocation calls, combined with the limited sensitivity of ultrasonic microphones, significantly restrict the detection range for bats. Given the acoustic shadow cast by the nacelle, and assuming a blade length of 60 m, ultrasonic detectors cover only about 23% of the risk area around wind turbines for bats that vocalize at 20 kHz. This coverage drops to 4% for vocalizations at 40 kHz. And the percentage continues to decline as blade length increases in more modern wind turbines. Furthermore, the directionality of echolocation and the aerodynamic behavior of bats limit the detectability of the animals. This is without taking into account the large intraspecific variations and small interspecific variations in echolocation, which can make species identification difficult and thus limit the predictive power of existing models. The researchers therefore conclude that, in its current form, acoustic monitoring aimed at estimating the impact of wind turbines on bat populations is severely limited by technical, physical, and biological factors. It may be necessary to install more ultrasonic detectors at various locations on the wind turbines, or to combine them with other detection methods such as radar or thermal cameras, to gain a more accurate picture of the actual impact of these giant turbines on bats. For now, the data collected is too fragmented.