Understanding the population structure and dynamics of bats is essential for assessing and mitigating risk and ensuring population viability. For many cave-roosting species, it is relatively easy to quantify and track populations because they congregate in large numbers during winter and can be monitored over time. However, for tree-roosting species, collecting and interpreting population data is challenging. These species do not aggregate in conspicuous locations but instead roost individually and are dispersed across the landscape, making it impractical for traditional surveys to provide estimates of census population sizes.
Several species of bats are exposed to both natural and human-induced environmental stressors resulting in population declines. For cave-roosting species, White-nose Syndrome has decimated populations. Conversely, wind turbines represent a potential population-level threat for migratory bats, such as hoary bats (Lasiurus cinereus), eastern red bats (L. borealis), and silver-haired bats (Lasionycteris noctivagans), given their relatively low reproductive rates and the level of documented mortality across wind energy facilities in North America.
Recent studies have assessed the potential population-level impact of wind energy facilities on hoary bats and given a set of assumptions for population growth rate and mortality rate from wind turbines, suggested a 38% reduction in turbine-related mortality is necessary to manage extinction risk for a starting population of 2.25 million bats. Reducing uncertainty in the model may not be immediately achievable as census data cannot be derived from existing methods. However, using a variety of techniques, including systematic acoustic sampling and genomic analysis, it is possible to build a weight of evidence on bat population trends and assess whether 1) mortality associated with wind turbines is sustainable, 2) minimal or substantial mitigation is required, and 3) mitigation measures are effective in ensuring population stability.
Consistent, long-term data collection remains the most reasonable option for reducing uncertainty and offering clarity into population trends for migratory tree-roosting bats. Using statistically robust methods for collecting acoustic data through the North American Bat Monitoring Program (NABat) or estimating effective population size at repeated intervals may provide data points over time to show species trends. Although there are no options for short-term actions that will result in significantly reducing uncertainty, there is opportunity for relatively short-term investments that can support long-term success of data collection efforts.
For the wind energy industry, one option is to reallocate resources used for standard preconstruction acoustic monitoring to follow the NABat field methods and data submission protocols or provide funding support for NABat regional hubs in lieu of monitoring. Contributing to NABat will enable broad-scale inferences about bat populations. This approach has several additional benefits in that the data are not associated with a given wind energy project and the level of effort/cost, even for multiple years of data collection, may be equivalent to standard preconstruction acoustic surveys conducted in one year at a single wind energy project. There already is broad support for NABat, but the speed at which data are collected and available for analysis can be increased with support by the wind energy and wildlife community.
Another option is to invest resources to collect genetic samples from bats collected at wind energy facilities during postconstruction mortality monitoring. This collection should be targeted for migratory tree-roosting species at two different scales: 1) sample intensively in a few select geographic areas that have high expected levels of bat mortality; and 2) sample broadly across the United States to capture the rangewide level of impacts, which will ensure that we have sampled the entire population for each species. The collection, processing, and storage of tissue samples, and carcasses if available, would require cooperation and coordination that is above and beyond standard postconstruction mortality monitoring efforts. The anticipated cost of tissue sample collection and storage are not expected to be a barrier to participation, but additional funding sources will be required to conduct the genetic analyses.
Through the systematic collection of acoustic and genetic data over time, a weight of evidence may be built that will support an improved understanding around population stability for migratory tree-roosting bat species. This information can be leveraged to infer the influence stressors have on population stability, and subsequently determine the level of response necessary to sustain healthy populations. Here, we synthesize the existing information related to bat population status and trends, uncertainty regarding existing estimates, current and novel methods for assessing population-level data, and recommendations for near-term and long-term studies to reduce uncertainty surrounding population data. Although this report focuses on wind energy development and bats, the methods used are broadly applicable and are being implemented by other industries, academia, government agencies, and nongovernmental organizations. Through efforts outside of the wind energy industry, it is likely we will gain a better understanding of bat populations, but the pace of data collection and analysis will be expedited by further participation by the wind energy industry. In return, the wind energy industry will benefit by contributing to these efforts either by 1) discovering that the population status of the species of interest is stable or growing, which may prevent the need for listing one or more species; or 2) recognizing the population status of certain species is in decline and implementing relatively moderate measures to reduce wind turbine-related mortality in the near term to stave off more stringent measures, which may be necessary if action is delayed.