Wind Turbines and Bats

WIndmill_MtStormWV_HarrisonShull_Aurora_Getty.jpg
A wind farm on Mt. Storm, West Virginia. These types of ridge top projects tend to cause mortality for migrating birds and bats. Harrison Shull/Aurora/Getty

With over 40 species, bats represent a large component of mammal biodiversity in the United States. They also contribute substantial ecosystem services: bats that consume flower nectar pollinate plants, fruit-eating bats disperse fruit seeds, and insectivorous bats feed on insects, including many pests. Estimates of the value bats provide in pest management services vary, but it is considerable. Bat colonies located near agricultural fields can save the farmers thousands of dollars in pesticide expenses every year.

The rapid growth in the wind energy business has been increasingly exposing birds and bats to spinning blades all over North America. The effects of wind turbines on birds appear to be substantial in absolute numbers, but still much behind other threats like domestic cats and communication towers. What’s the story with bats?

Estimating Bat Mortality Due to Wind Turbines

A recent study estimated the number of bats killed by wind turbines in the United States at 600,000 in 2012 alone. However, researchers are finding that it is extremely difficult to estimate bat mortality from turbines. Documenting carcasses under turbines yields only a fraction of the victims: they are very small and easy to overlook, the fast-spinning wind blades can propel the bodies a considerable distance, and scavengers may remove the evidence. In addition, studies conducted at single wind farms may not be representative of projects elsewhere.

Wind farms located on top of mountain ridges seem to be responsible for the death of larger numbers of migrating bats.

To compound the problem, it appears that a collision with a turbine is not just the result of unlucky bats bumbling their way into the blades’ path randomly. There is emerging evidence that bats are actually attracted to tall towers, including wind turbines, compounding mortality problems.

It appears that bats view turbines as large trees, and they may be investigating them for potential cavities where they could roost.

Any Negative Effects on Populations?

To understand how severe a threat is on an animal population, we need to have an estimate of its size. Unfortunately, the nocturnal and secretive lives of bats make population sizes very difficult to estimate, so it is not possible to truly evaluate the severity of wind turbine mortality on bat species. There are indications that we should view the threat seriously, though.

First, the life history characteristics of bats make them vulnerable to adult mortality. They reproduce slowly, with generally just one young per year, and young bats suffer high mortality rates the first year of their life. Species with low reproduction rates need to be able to live a long time for the chance to have at least one of their young reach adult age. Premature mortality affects this delicate reproduction/survival balance and can lead to declining populations.

Second, wind turbine mortality may be an important stressor on bat populations, which are already greatly affected by climate change, habitat loss, and white nose syndrome. Regarding the latter threat, there’s a silver lining: the cave bats, which are currently in serious danger from white nose syndrome, are much less frequently killed by turbines.

The species known collectively as tree bats are the most common victims, and they have been mostly spared by the white nose syndrome epidemic.

Can Anything Be Done?

Unlike climate change and white nose syndrome, there is a potential solution to reduce bat mortality from wind turbines. If periods of high risk can be identified (for example, certain weather combinations during heavy bat migration nights), turbines can be stopped for a few hours, and reactivated when the bats have moved on. More research is needed to better identify high risk periods, but it could have important conservation applications.

Sources

Hayes. 2013. Bats Killed in Large Numbers at United States Wind Energy Facilities. BioScience.

Jameson & Willis. 2014. Activity of Tree Bats at Anthropogenic Tall Structures: Implications for Mortality of Bats at Wind Turbines.

Animal Behaviour.

Kunz et al. 2011. Ecosystem Services Provided by Bats. Annals of the New York Academy of Sciences.