Fatalities at wind turbines may threaten population viability of a migratory bat (original) (raw)

Abstract

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 most frequently 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.

Introduction

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 highest fatalities 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.

Section snippets

Expert elicitation

We used a structured elicitation method to obtain specific judgments or values from experts. Co-author JAS and colleagues served as eliciting facilitators and identified the conservation problem (“Does mortality from wind turbines pose a threat to population viability of hoary bats in North America?”), selected the experts, and designed the elicitation process. Nine experts (see Supporting Information) were identified based on literature review and discussions with the bat ecology and

Results

Current rates of wind turbine fatalities are sufficiently high to substantially change the probability of population stability and risk of extinction across a range of plausible demographic scenarios for hoary bats (Fig. 1, Fig. 2). Mortality from wind turbines increased the isoline of stable population growth after 50

years indicating that annual population growth rate (λ) would have to be substantially higher, particularly at lower population sizes, to compensate for wind-associated mortality (

Discussion

Reports of large numbers of bats killed at wind energy facilities have attracted conservation attention for the past decade (Kunz et al., 2007). However, the lack of basic demographic information about bats in general and migratory bats specifically, has hindered our ability to empirically address whether bat fatalities from wind energy developments presents a serious threat to the viability of these species (Diffendorfer et al., 2015). Likewise, few studies have directly estimated

Acknowledgements

We thank Todd Katzner and two anonymous reviewers for comments on the manuscript. Robyn Niver, Lori Pruitt, and Dan Nolfi helped with designing and conducting the elicitation workshop and provided comments on earlier drafts. David Nelson and Stephen Keller provided information to experts during the introductory phase of the elicitation workshop. Taal Levi, Ed Arnett, Paul Cryan, and Maarten Vonhof contributed to earlier versions of the modeling effort. The findings and conclusions in this

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