Date Published: 14 June 2012

Bat bridges over roads don't work according to Leeds University researchers

Wire bridges built to guide bats safely across roads do not work, according to recent research from Leeds University (England, UK).

Roads act as barriers to bats. They cut colonies of bats off from their established feeding sites, reducing the bats' ability to feed themselves and their young. Most species of bat fly close to the ground or close to trees and hedges for protection against the weather and potential predators. Bats that cross roads tend to do so at traffic height, with a consequent high risk of collision. In a previous study published last year, the Leeds team showed that the number and diversity of bats can be significantly depleted near major roads. Wire bridges such as that shown are a common conservation measure aimed at encouraging bats to cross above the traffic.

A team from Leeds University's Faculty of Biological Sciences has recently monitored four wire bridges spanning major roads in the north of England. All of these "bat bridges" have been built during the last nine years to replace hedgerows ? the bats' established commuting routes ? when these routes were severed by new roads.

Using bat detectors that detect bats' high frequency echolocation calls, together with night video equipment, the team measured the height at which bats crossed the road and their proximity to the wire bridges and compared the results with bats crossing at nearby severed hedgerows without wire bridges.

The research team found that most of the bats they studied preferred their former commuting routes to the wire bridges and continued to cross the road at a low height (approx. "traffic height"). There was no evidence to suggest that the bats changed their behaviour in response to the bridges, even over time. A well-established wire bridge built nine years ago and only 15 metres from the severed commuting route it replaced was still spurned by the bats.

Study leader, Professor John Altringham said:

" The results of this research are relevant to small insectivorous bats worldwide and highlight the impact of roads on wildlife in general."

Under UK law the government has an obligation to ensure that development does not have a detrimental effect on populations of protected species. Wire bridges are assumed to act as artificial aerial hedges, guiding bats safely over traffic and are an increasingly favoured mitigation tool across Europe. However, referring to the costs associated with the installation of bat bridges, Professor Altringham said:

" Conservation measures are sometimes widely implemented without evidence to support their effectiveness, and large sums of public money are being spent without any proven benefit to nature."

According to Earl Attlee on 4 Oct 2011, bat bridges constructed in the UK since 2008 have cost up to £300,000 but typically some £10,000s, (see http://bit.ly/LSr1v9).

Anna Berthinussen, said:

" Many bat species forage up to several kilometres from their roost, so our road system is an ever expanding network of life-threatening hurdles the bats must overcome. Our findings raise concerns about how we can build or improve roads without impacting on these protected species.
_ We need to find solutions that really work and suggest that alternative designs are investigated and, most importantly, tested more effectively than they have been in the past."

She has also been investigating the extent to which bats make use of underpasses built to carry roads and footpaths under roads. Of three underpasses studied, only one was used extensively by bats. There may be potential to improve their attractiveness to bats.

Ref. to Paper:
Anna Berthinussen, John Altringham, Do bat gantries and underpasses help bats cross roads safely? is published on PLoS ONE on 13 June 2012 (http://dx.plos.org/10.1371/journal.pone.0038775)

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