As concerns over environmental impacts of fossil fuels grow, wind energy is increasingly popular. However, large numbers of bats are killed at some wind energy facilities and this raises concerns about cumulative impacts on bat populations. An estimated 0.84 and 1.7 million bats have been killed in the U.S. and Canada from 2000-2011, 78% of which are of migratory tree-roosting bats - 38% hoary bats (Lasiurus cinereus), 21% eastern red bat (L. borealis), and 18% silver-haired bat (Lasionycteris noctivagans). As wind energy continues to expand, fatality rates have increased, with over 500,000 fatalities annually. Given these estimates, and the push towards increasing capacity of renewable energy, there are worries that this additional source of mortality may threaten the population viability of bats. However, population estimates are lacking for bats, especially for migratory tree-roosting species, so we used parameters derived from expert elicitation to model population trends of hoary bats, the species most commonly killed at turbines. We also examined how acoustic detections, capture rates, rabies submission rates, and wind-energy related fatality rates of migratory tree-roosting bats change through time to look for evidence of population declines. If all these metrics consistently show declines, then this may indicate declines in population sizes.
Population modelling derived from expert elicitation suggests that for hoary bat populations to persist, the mean annual population growth rate must be substantially higher than what appears “most likely” and that current fatality levels could cause a 91% decrease in the continental population size of hoary bats within 50 years. We also found multiple signs of population declines in all three species of migratory tree-roosting bats, including significant declines in fatality rates at wind energy sites. Our analyses highlight the need for effective policy and mitigation strategies that embrace adaptive and flexible management and address cumulative impacts.