Chapter 5: Compensatory Mitigation

Compensating for Adverse Impacts

Updated December 27th, 2022

Compensatory mitigation is an attempt to balance or offset the loss of natural resources – wildlife or their habitat – that cannot be avoided after reasonable efforts have been made to avoid and minimize adverse impacts of a specific activity. In the case of a wind energy project, the approach will depend on several factors, including the type of resource lost (e.g., wildlife fatalities or habitat), the opportunities available to offset the loss, and how impacts and offsets are calculated, as well as timing and spatial considerations.

Compensatory mitigation approaches

The first step in determining a compensatory mitigation approach is to define the resource loss in measurable terms. The two broad categories of resource loss associated with wind energy development are (1) direct loss of individual animals due to collision fatalities, and (2) impacts that result in a loss of habitat either directly or indirectly by displacing or reducing the success of wildlife populations.

Compensation for collision fatalities

Collision fatalities may be relatively rare events, especially for threatened or endangered species. Because compensatory measures may need to be in place before a project is operational, the number of predicted fatalities will need to be calculated. For example, the U.S. Fish and Wildlife Service provides a collision risk model for use in predicting eagle fatalities at a proposed wind project site Eagle Conservation Plan Guidance.

Post-construction fatality monitoring provides a way to verify pre-construction predictions and update risk model inputs, as well as to confirm whether compensatory measures need adjustment to achieve the desired balance. In the case of rare and harder to detect species, carcass counts may underestimate the total number of fatalities, and estimation of actual fatalities based on observed carcasses may require methods that take low detection and low sample size into account.

Collision fatalities that cannot be avoided may be offset by actions that reduce mortality for the same population from a different anthropogenic source. For example, in addition to raising cut-in speeds to minimize bat fatalities, a wind energy project located in Indiana that anticipated impacts to Indiana and Northern long-eared bats compensated for unavoidable fatalities by contributing to a mitigation bank. The bank was intended to fund the installation of a gate to reduce bat mortality at an important winter hibernation site in Kentucky, as well as conservation of the bats’ swarming and summer habitat.

Eagle fatalities at wind projects are offset by retrofits to power poles to prevent electrocutions. Currently this is the only compensatory mitigation method approved by the U.S. Fish and Wildlife Service, but other strategies have been published and may be used in the future. Learn more about compensatory mitigation for eagles.

Compensation for habitat impacts

Depending on the impacts and mitigation opportunities available for a given species and a given project area, compensatory mitigation for habitat impacts may take a variety of forms. These include:

  • Preservation of existing habitat – Acquisition of a conservation easement (a voluntary, legal agreement that permanently limits uses of the land in order to protect its conservation values) may be used to preserve existing habitat within or adjacent to the project area.
  • Enhancement or restoration – Wind developers may be able to offset habitat impacts from project infrastructure by enhancing or restoring equivalent areas of degraded or former habitat.
  • Monetary payment – Wind project proponents may make payment(s) into an established conservation fund or bank, or an in-lieu fee program. The U.S. Fish and Wildlife Service defines a conservation bank as “a site or suite of sites established…to protect, and where feasible, improve habitat for a species. Entities pursuing development that require mitigation can purchase ‘credits’ generated by perpetual conservation easements and conservation projects to offset impacts occurring elsewhere.”

Considerations for compensatory mitigation

Calculation of impacts and offsets

An important consideration for compensatory mitigation measures is that they require an agreed-upon “biologically based currency” that can be used to balance the biological value of mitigation offsets against the impacts created. For example, a 2019 paper describes a method for quantifying the amount of habitat needed to provide equivalent biological value for grassland birds and waterfowl displaced by energy and transportation infrastructure. This method includes examples in which the offset habitat is similar to the affected habitat in terms of habitat type, geographical location, land use, and landscape composition, and in which the biological value of the offset habitat is dissimilar to the affected habitat. 

Timing of impacts and offsets

Timing is another consideration in balancing the value of a compensatory action with that of the impact or resource loss. For example, in the case of a new wind project seeking an incidental take permit (ITP) for eagles, the project proponent must commit to implementing the compensatory action before project operations begin to meet the statutory eagle preservation standard. In other words, the benefits of compensatory mitigation should begin accruing before costs, i.e., fatalities or habitat loss, are incurred.

Spatial considerations

For eagles, take predictions are assessed by the Service both for individual projects and for the cumulative effects of other activities causing take at the scale of the local‐area eagle population. However, in the case of the Indiana bat mitigation described above, impacts occurring in one part of a species’ range were offset by actions to decrease mortality for the same population during the season spent hibernating in a different state.