Across the United States and Canada, hoary bats (Lasiurus cinereus) are the most common species among reported bat fatalities at wind farms. Although large gaps in our knowledge about hoary bat population size and dynamics makes the impact of wind turbine fatality difficult to assess, expert opinion places the population parameters in a range in which the risk of decline or extinction may be high. A key question for the wind energy industry, regulators, and conservation groups is whether fatality minimization strategies such as operational curtailment or the use of bat deterrent devices could manage population-level risks, particularly considering continued growth in installed capacity.
The risk of hoary bat population decline or extinction in the presence of wind turbine mortality was projected from 2012 through 2050 using a simple population model. Future levels of turbine mortality were a function of expected capacity buildout in the United States and Canada. High- and low-buildout scenarios were explored. Non-probabilistic scenarios of population growth rate and abundance addressed the large uncertainties around hoary bat population status.
Risks to hoary bats and the level of fatality reduction necessary to minimize them were highly dependent on population size. In the absence of fatality reduction, declines of at least 50% occurred at all initial population sizes tested and in some scenarios were likely to occur prior to 2020. However, extinction risk without fatality reduction was present only in populations that started with fewer than 5 million hoary bats, even in cases in which the population was assumed to be declining independently of wind mortality.
Adopting fatality reductions in 2020 with efficacies within the range observed from curtailment and deterrent studies was sufficient to manage extinction risk in populations of at least about 2 million. However, this reduction could not be implemented in time to prevent significant declines in populations of fewer than about 3 million hoary bats.
Applications, Value, and Use
Understanding risks to hoary bats posed by wind turbine mortality can help in the development of strategies to ensure that listing is unnecessary. Though existing uncertainty about population status is large enough to produce a variety of forecasts, the model itself provides a tool to integrate new information from sources such as post-construction monitoring, genetic analyses, or acoustic surveys.