Following the devastating nuclear blast of Fukushima, many countries considered investing into renewable energy. Therefore, wind power was largely promoted in the aftermath of Fukushima, particularly in Germany. Lately, it was acknowledged by conservationists and politicians alike that wind turbines cause large numbers of bat fatalities. It is currently estimated that about 10-12 bats die at German wind turbines1. Considering that about 23,000 on-shore wind turbines are active in Germany2, it is anticipated that more than 200,000 bats might be at risk of dying in Germany by getting in lethal contact with wind turbines.From our point of view, this is a major CONSERVATION CRISIS for European bats that should not be ignored!
1 Korner-Nievergelt, F., Korner-Nievergelt, P., Behr, O., Niermann, I., Brinkmann, R., Hellriegel, B. (2011) A new method to determine bird and bat fatlitiy at wind energy turbines from carcass searches. Wildlife Biology 17: 350-363.
2Berkhout, V., Faulstich, S., Görg, P. Kühn, P., Linke, K., Lyding, P., Pfaffel, S., Rafik, K., Rohrig, K., Rothkegel, R., Stark, E. & Witte, J. (2013), Wind energy report Germany 2012. Fraunhofer-Institut für Windenergie und Energiesystemtechnik – IWES – Kassel.
It is currently poorly understood why bats are so susceptible of dying at wind turbines. On the one hand, mostly migratory bats die at wind farms3, and it is therefore assumed that wind turbines work at a height above ground where bat migrants travel. On the other hand, killed bats seem also to be attracted to the prominent landscape features of wind turbines. Thus, it seems likely that bats approach wind turbines for the reason of social interactions with conspecifics (swarming during the autumn mating season) or for hunting for insects4. The specific reason for bats flying too closely to wind turbines may vary among species, and therefore it is difficult to draw a general picture. This multifaceted scenario makes it very difficult to establish features at wind turbines that act repellent.
3Rydell, J., Bach L., Dubourg-Savage, M.-J., Green, M., Rodriguez, L. & Hedenström, A. (2010). Bat mortality at wind turbines in northwestern Europe. Acta Chiropterol, 12,261-274.
4Cryan, P.M., Jameson, J.W., Baerwald, E.F., Willis, C.K.R., Barclay, R.M.R., Snider, E.A., Crichton, E.G. (2012) Evidence of late-summer mating readiness and early sexual maturation in migratory tree-roosting bats found dead at wind turbines. PlosONE 7: e47586.
In general, bats die because of a so-called blunt-force trauma when colliding directly with the tower or the moving rotor blades, because of a barotrauma when getting in close contact with the vortices in the tail-wind of wind turbines or because of a combination of these two factors5,6. Barotrauma comes along with various symptoms such as ruptured inner organs (e.g. stomach, kidney, liver, lung), blood-filled thorax or abdomen (hemothorax or hemo-abdomen) 5,6. Usually, barotraumatized bats show no external signs of injuries, but usually intact arm and finger bones. In contrast, bats dying of blunt-force trauma usually show broken skulls, fingers and/or arms. Bats colliding directly with wind turbines are usually found below wind turbines because fractures prevent them fro flying long distances afterwards. Barotraumatized bats, however, may fly some distances when showing only mild symptoms. These animals may eventually die later because of their internal injuries. This postponed death of barotraumatized bats has been ignored largely by studies focusing on the negative impact of wind turbines on bat populations. Most likely, bat fatality rates at wind turbines are higher than previously anticipated.
5Baerwald, EF., D’Amours, G.H., Klug, B.J. & Barclay, R.M.R. (2008). Barotrauma is a significant cause of bat fatalities at wind turbines. Current Biol., 18, R695-R696.
6Grodsky, S.M., Behr, M.J., Gendler, A., Drake, D., Dieterle, B.D., Rudd, R.J. & Walrath, N.L. (2011). Investigating the causes of death for wind turbine-associated bat fatalities. J. Mammal., 92, 917-925.
Besides the moral responsibility of mankind to consider any negative affect of its action on nature, there are several other important reasons to look into ways to reduce bat fatality rates at wind turbines. From a legal point of view, all European bat species are protected by EU law following the Habitats Directive 92/32/CEE (Annexes II and IV). In addition, all bat species are protected in Germany by the so-called Bundesnaturschutzgesetz (§44). Further, migratory bats are as well protected by the ‘Convention on Migratory Species of Wild Animals’ (Bonn, 1979) through the Eurobats agreement (London, 1991). In addition to these legal aspects, we also need to aim at mitigating the negative effects because bats provide important ecosystem services by consuming large amounts of insects, including several species that are relevant for agriculture and silviculture7.
7Ghanem, S.J., Voigt, C.C. (2012) Increasing awareness of ecosystem services provided by bats. Advances in the Study of Behavior44: 279-302.
Yes, it is possible to reduce the number of bat fatalities at wind turbines. In general, bats become less active with increasing wind speed, i.e. when wind turbines start to produce net energy. Yet, there is a range of low wind speeds where bat activity overlaps with net energy production of wind turbines (see graph). If wind turbines are stopped to operate when bats are active (i.e. at night usually after sunset, at temperatures above 20°C and when not hibernating in winter), bat fatality rates are significantly reduced. This intelligent, yet easy-to-implement method is a practical way of preventing many bat deaths at wind turbines.
Bat activity and net energy production of wind turbines in relation to wind speed. Grey areas indicate the number of bat passes at the height of rotating blades and the solid black line the sigmoidal increase of net energy production of wind turbines with increasing wind speed. The conflict zone (i.e. the range of wind speeds at which bats are active and wind turbines produce energy) is indicated as a red box. Stopping the operation of wind turbines at low and intermediate speeds causes drastic reductions in bat fatalities at minimal revenue loss for companies, since wind power plants work efficiently only at higher wind speeds.
A recent review revealed that 70% of bats killed at wind turbines are migratory species8. For conservation purposes, it is important to know which populations are negatively affected by wind turbine related fatalities. We investigated this question at German wind turbines where bat carcasses were found. Our study revealed that bats may originate both from local but also from distant populations, as distant as Baltic countries, Belarus and Russia. This highlights that environmental policies at the national scale may have large-scale negative effects on the national and international scale.
8Rydell, J., Bach L., Dubourg-Savage, M.-J., Green, M., Rodriguez, L. & Hedenström, A. (2010). Bat mortality at wind turbines in northwestern Europe. Acta Chiropterol, 12,261-274.
9Voigt, C.C., Popa-Lisseanu, A., Niermann, I. & Kramer-Schadt, S. (2012). The catchment area of wind farms for European bats: A plea for international regulations. Biol. Conserv., 153, 80-86.