Large numbers of migratory bats are killed every year at wind energy facilities. However, population-level impacts are unknown as we lack basic demographic information about these species. We investigated whether fatalities at wind turbines could impact population viability of migratory bats, focusing on the hoary bat (Lasiurus
cinereus),the species mostfrequently killed by turbines in North America. Using expert elicitation and population projection models, we show that mortality from wind turbines may drastically reduce population size and increase the risk of extinction. For example, the hoary bat population could decline by as much as 90% in the
next 50 years if the initial population size is near 2.5 million bats and annual population growth rate is similar to rates estimated for other bat species (λ = 1.01). Our results suggest that wind energy development may pose a substantial threat to migratory bats in North America. If viable populations are to be sustained, conservation measures to reduce mortality from turbine collisions likely need to be initiated soon. Our findings inform policy decisions regarding preventing or mitigating impacts of energy infrastructure development on wildlife.
Wind energy development is growing rapidly across the globe as a renewable energy source. However, wind energy facilities are not without environmental costs (Saidur et al., 2011). For example, large numbers of bats are killed at wind energy facilities (Arnett et al., 2016; O'Shea et al., 2016). Over 300,000 bats are estimated to be killed annually at wind energy facilities in Germany (Lehnert et al., 2014; Voigt et al., 2012) and over 500,000 are estimated to be killed annually across Canada and the United States (Arnett and Baerwald, 2013; Hayes, 2013; Smallwood, 2013). Over the past decade, substantial numbers of bat fatalities and increased growth in wind energy have raised concern about the impacts of wind energy development on bat populations (Kunz et al., 2007). A critical question for conservation planning is whether these fatalities could drive populations to dangerously low levels or even extinction. Addressing this question is challenging because bats that migrate
latitudinally over long distances have the highestfatalities at wind energy facilities and are among the least studied (Kunz et al., 2007). Basic demographic parameters and even rough empirical estimates of population size do not exist (Lentini et al., 2015). In general, reproductive rates for bats are low, which can impact their ability to respond to mortality threats (Barclay and Harder, 2003). Lack of empirical demographic and population data for migratory bats, especially for non-colonial species, limits the ability to quantitatively assess the potential impact of wind energy on these species (Diffendorfer et al., 2015). The challenges associated with empirical estimation will likely remain insurmountable into the foreseeable future given the ecology of these organisms.
Determining the threat of wind energy development on migratory bats highlights the common problem of how to assess threats to species when critical data are lacking. Data from similar species or structured elicitation of expert opinion can be used for conservation decision-making when empirical data for a focal species are unavailable (Burgman et al., 2011; Drescher et al., 2013; Martin et al., 2012). In recent decades, expert elicitation has been used for a variety of conservation problems (Donlan et al., 2010; Martin et al., 2005; Oberhauser et al., 2016; Runge et al., 2011; Smith et al., 2007), and evaluations of the elicitation method provide structured approaches to help guard against subjective biases when eliciting expert opinion (Martin et al., 2012). Deciding whether conservation measures are necessary to prevent or mitigate impacts from wind energy development on populations of migratory bats requires use of expert judgments and/or use of data from similar taxa to quantify reasonable scenarios of population growth and trajectories. We use population projection models to explore whether fatalities from wind turbines threaten the population viability of hoary bats (Lasiurus cinereus), a wide-spread migratory species comprising the highest proportion of bat fatalities (38%) at wind energy facilities in North America (Arnett and Baerwald, 2013). Given the lack of empirical data on key population parameters for hoary bats, we used data from similar species as well as expert elicitation (Martin et al., 2012) to identify available data sources, provide estimates of unknown parameters, and quantify uncertainty. Our objective was to assess the likelihood that mortality from wind energy turbines poses a species-level threat to hoary bats in North America to inform conservation decision-makers about the potential impacts of energy infrastructure development on migratory bats. We hypothesized that mortality from wind energy turbines at installed capacity by 2014 was sufficiently high to substantially reduce the probability of population stability and increase the probability of extinction over the next 50 to 100 years.