In discussing the risks wind energy can pose to wildlife, we define risk in terms of a framework that incorporates three components: hazard, vulnerability, and exposure.
The Risk Framework
A hazard is any activity or thing that causes an adverse impact. Many human-made structures pose a collision threat to birds, including wind turbines – even when they are stationary. Bats are not likely to collide with stationary turbines but can collide with or be struck by rotating turbine blades. Wind energy facilities can also constitute a hazard if they change species’ habitat in ways that adversely affect those species. For example, species may avoid a wind site during the construction phase of a project and may not habituate to its presence once construction is completed and the facility is operational. The presence of turbines and access roads may result in fragmentation of a species’ habitat, or change the balance of prey and predator species, affecting the survival or reproductive success of the species of concern.
Vulnerability pertains to the consequences of being exposed to a hazard. In the case of collisions with wind turbines, the consequence to an individual animal is injury or death. In the case of habitat impacts, vulnerability is a measure of how species’ use of habitat, survival, and reproductive success rates are affected by the presence or proximity of wind energy facilities. An example of how vulnerability could be measured would be to compare the abundance of prairie-chickens on project and control sites before and after construction of a wind facility, and also look at any changes in population fitness measures, such as nesting success and adult survival.
Whether individual fatalities render populations vulnerable to decline depends on the size of the population, its level of exposure to the hazard, and the cumulative impacts of wind energy and other sources of mortality. For example, The U.S. Fish and Wildlife Service has concluded that the population status of golden eagles indicates that the species is vulnerable to any additional mortality. Bald eagles, whose populations have been recovering, are by comparison less vulnerable to additional mortality (within limits) from wind energy impacts.
In the case of collisions, exposure is a measure of the amount of time a bird or bat is within the collision risk zone of a wind turbine. In the case of habitat impacts, exposure is a measure of the likelihood that a wind facility is constructed in habitat used by a species.
How we estimate risk
Collision risk modelling
The U.S. Fish and Wildlife Service’s collision risk model (CRM) for eagles is an example of how collision risk can be modelled for a species. The Service’s Eagle Conservation Plan Guidance eagle take prediction model is a function of eagle exposure – defined as eagle minutes within a certain height range at a potential wind facility site – measured pre-construction, the number of turbines, and collision probability. The Service recently updated the model for golden and bald eagle collision risk to reflect new information gathered at wind energy projects that have pre-construction take predictions and post-construction fatality data.
The Service’s CRM is designed to be used to predict annual eagle fatalities for a proposed site, enabling wind companies to apply for the appropriate permits and plan for compensatory mitigation to offset any predicted eagle fatalities. Once the site is in operation, post‐construction fatality monitoring can be used to update the collision probability input for the model.
Modelling habitat impact-related risk
Because the other risk components associated with habitat changes are not known in most cases, exposure to risk is often assumed to occur for any individual or population that occupies an area within or near a wind facility. To avoid or minimize this assumed exposure, developers are often required or requested to place infrastructure away from critical habitat features, such as leks (mating grounds) used by sage-grouse or prairie-chickens.