Not all species with observed collision fatalities, changes in behavior, or habitat loss experience population-level impacts. In examining what we know about the effects of collisions and habitat impacts on wildlife populations, there are several important considerations:
- Population dynamics hinge on a wide range of factors, several of which are better understood for some species than for others.
- The impacts of wind energy development may operate in combination with other sources of mortality that together could lead to population-level effects.
- The net cumulative impacts of a buildout scenario that maximizes wind energy’s potential to replace fossil fuel-generated electricity may have implications for some species beyond what we might consider when thinking about wind energy on a project-by-project basis, while the potential positive benefits to species’ populations from replacing fossil fuels with renewable energy are difficult to predict.
How do we assess impacts to populations?
Evaluating impacts to populations from wind energy projects requires understanding collision and habitat-related impacts from wind energy as well as population dynamics – either at the local level or for an entire species – and other factors that contribute to cumulative impacts on the population.
Species population parameters
In thinking about population impacts, if the number of expected fatalities is very small relative to the total population size, collision impacts are unlikely to lead to population declines. It is also important to distinguish between local populations and species global populations. When determining the impact of wind energy-related fatalities or habitat loss on a population, we want to know: how big is the global population? Is it subdivided into smaller local populations, and if so, how big is each local population?
Stable isotopes and genetic studies can help identify the origins of individuals killed at a wind facility, which can inform our understanding of potential consequences to local populations. For example, if a large proportion of a local population of red-tailed hawks are killed in a wind resource area, the viability of that local population may be threatened; but if the same number of hawks killed at that wind facility are from multiple geographically distinct populations, then we need to consider whether those fatalities pose a threat to any of the contributing hawk populations.
As indicated in the U.S. Fish & Wildlife Service’s Land-based Wind Energy Guidelines (p. 23), species like sage-grouse, which require large territories and are sensitive to habitat fragmentation, may experience increased risk to their demographic and genetic viability if wind energy development results in a single population being fragmented into isolated sub-populations. Research to better understand the habitat-based impacts for grouse populations is ongoing.
In addition to population size and distribution, population dynamics help determine whether adverse wind energy impacts are likely to have population-level effects. In general, long-lived species that have higher adult survival and fewer offspring each year may be more susceptible to population-level effects from collisions with wind turbines. Understanding whether fatalities caused by wind turbines are compensatory, i.e., the turbine-caused deaths simply replace some other form of mortality that would have occurred – or additive, i.e., the turbine-caused deaths represent individuals that would have otherwise continued to survive and reproduce – is key when considering population-level impacts from wind energy or from any anthropogenic activity, although this is very difficult to assess.
Cumulative impacts represent the aggregate of past, present, and reasonably foreseeable future impacts on a given species, resource, or ecosystem. Many species’ populations face stresses from various types of human activity. The expansion of agriculture, urban and suburban development, and transportation networks have decreased, degraded, or fragmented habitat for many species. Climate change and the deliberate or inadvertent introduction of invasive species are other examples of human activity exacerbating threats to wildlife populations.
Understanding how wind energy development may compound these other stresses is vital for the effective long-term conservation of biodiversity. Wind energy impacts on wildlife are typically assessed on a project-by-project basis; however, it is important to consider the potential for cumulative impacts from future buildout at the scale needed to address climate change.
The reduced carbon emissions resulting from wind energy buildout is assumed to have beneficial consequences for species and ecosystems that are typically not considered in cumulative impact assessments of wind energy impacts. A full accounting of the cumulative impacts of wind energy may need to evaluate the net impact of the projected buildout on wildlife, although this may require development of new analytical tools to be successful.
What do we know about population-level impacts?
Nationally, average fatality rates (adjusted for known sources of detection error and sampling intensity) range from 3-6 birds per megawatt (MW) of installed wind energy capacity per year (learn more about collision risk to birds.) At a global level, fatality rates do not appear likely to lead to population declines in most bird species, but we know less about whether some local populations are affected by turbine collisions.
For small passerines (songbirds) in the U.S. for which data are available, cumulative collision mortality at wind energy facilities was estimated to represent less than 0.01% of population size, although this study is out of date. Even with the substantial increase in installed capacity, collision fatalities likely remain orders of magnitude less than leading anthropogenic sources of mortality. Long-lived species, including most raptors, may be more susceptible than short-lived species to population-level effects from collisions with wind turbines, and possibly to nest disturbance as well. Shorebirds and waterfowl are not considered to be at risk of population declines from collision mortality from current wind energy buildout.
Population-level consequences due to habitat impacts have not been documented. However, some species, including some grassland birds, ducks, and prairie grouse, avoid using areas near turbines, and concern exists that the cumulative effect of this avoidance may prove significant.
Nationally, average fatality rates (adjusted for known sources of detection error and sampling intensity) range from 4-7 bats per MW of installed wind energy capacity per year (learn more about collision risk to bats.) The population status and distribution of some bat species is poorly understood, and the population impact of collision mortality, alone or in combination with other causes of mortality, is not known. Because bats are long-lived with low reproductive rates, they are assumed to be more susceptible to population impacts from collision mortality compared to species that are short-lived and have higher reproductive rates.
The majority of documented bat collision fatalities at wind energy facilities are from three species of migratory tree-roosting bats: hoary, silver-haired, and eastern red bats. Demographic modeling indicates the potential for population-level impacts in hoary bat. wind energy-related fatalities are low for most cave-hibernating bat species, but other threats – notably white-nose syndrome, a fungal pathogen that has devastated bat populations in the U.S. and Canada – have heightened concerns about the consequences of additive sources of fatalities.