More night-time traffic equals less bat activity
Written by Kerry Borkin
Department of Conservation, Taupō, New Zealand
February 13th, 2025
Photo credit: C Hillock, Department of Conservation.
Summary
We know some bat species fly and feed along roads, particularly when these are in remote areas and when roadsides in farmland contain trees and shrubs. We also know that the activity rates of many species, including those that use roadsides in agricultural and remote areas, decline close to roads. This raises the question: is overnight traffic volume to blame? We investigate…
Figure 1. New Zealand’s long-tailed bat, Chalinolobus tuberculatus, is an insect-eating Vespertilionid that often flies along forest edges so is sometimes seen flying along roads. Photo credit: Colin O’Donnell Department of Conservation.
Traffic volumes and road length increasing in New Zealand
In the 34 years since record-keeping began in New Zealand, traffic volumes have increased by over 75%, and increases are predicted to continue. Globally, road length and traffic volumes also continue to increase. If traffic volumes influence bat use of an area, this could mean the effective loss of large areas of suitable habitat for bats.
The long-tailed bat is one of two extant species of bats still present on mainland New Zealand, both of which are endemic and now threatened with extinction. Habitat loss is one of the main reasons for the decline of the widely distributed, yet critically threatened long-tailed bat. Long-tailed bats are a small, edge-adapted species that largely feeds on flying invertebrates and are often found flying along the edges of forests feeding. This use of edges means they are often observed flying along roads, so, in the past, many ecologists and road-builders assumed that they were resilient to roads, traffic, and other infrastructure. However, growing evidence from elsewhere in the world raised the concerns of ecologists that traffic and road-building could lead to habitat loss and further threaten their persistence.
Measuring bat activity along highways
To see whether overnight traffic volume influenced long-tailed bat activity rates, we investigated whether their activity correlated with night-time traffic volume on New Zealand’s highways. We recorded bat echolocation calls using ultrasound detectors as a measure of use of, or activity in, an area. Bat activity was measured at 57 traffic monitoring sites throughout New Zealand.
We used pairs of bat detectors: one placed beside the highway; the other ≥ 200 metres from the highway to allow a comparison of rates of detection next to and further away from highways with different overnight traffic volumes. Pairs were more than 250 m away from other pairs to support independence of sites.
Figure 2. We recorded bat calls using detectors at 57 traffic monitoring sites throughout Aotearoa New Zealand and calculated overnight traffic volumes.
Figure 3. We placed bat detectors next to and away from highways. Here they are called “distant” and “forest” respectively.
Bats were less active near highways, and especially busy highways!
We examined relationships between traffic volume, position in relation to the highway, and bat detections.
Bats were detected less often alongside busy highways. This means that they either called less often or more likely spent less time in areas alongside busy roads.
We were 3.87 times less likely to record bats close to highways than at similar sites further away.
As traffic volume at night increased along New Zealand highways, we were even less likely to detect the threatened long-tailed bat using that area. The same pattern didn’t happen at sites further from highways.
Long-tailed bat use of areas along roads can be high when overnight traffic volumes are low or very close to zero, but this declines rapidly as traffic rates reach ≥ 500 vehicles per night. This overnight traffic volume is far lower than recorded near New Zealand cities, where overnight traffic volumes reportedly drop only as low as 500 vehicles per hour – 4000 vehicles over an eight-hour night! With this research, we demonstrated reductions in bat use of an area when there are far lower overnight traffic volumes.
Implications for bat conservation in New Zealand and the world
Traffic volumes in New Zealand and around the world are predicted to increase year on year, so finding long-tailed bats use roads and neighbouring areas less when they have higher traffic volumes is concerning. This reduction in use means that large areas, which bats once used, are now used far less often by this threatened species.
We don’t yet know why bats avoid roads with higher volumes of traffic. However, with higher traffic volume comes increased noise and light, plus the greater risk of bats becoming roadkill. Research has begun to determine how light and noise affect New Zealand bats. Unpacking the causes will aid the development of mitigation methods to reduce impacts on bats when building roads, and to mitigate adverse effects of existing roads.
These results also have implications for bats around the world. Bats use wooded vegetation along roads globally, and if traffic volumes increase, this could result in both loss of spaces used by bats and bat declines worldwide.
Author information
Kerry Borkin
Department of Conservation, Taupō, New Zealand
Co-authors: Des Smith, William Shaw, Jo Mc-Queen (Wildland Consultants Ltd, New Zealand )
Source citation
Borkin, K.M., Smith, D.H.V., Shaw, W.B., McQueen, J.C. 2019. More traffic, less bat activity: the relationship between overnight traffic volumes and Chalinolobus tuberculatus activity along New Zealand highways. Acta Chiropterologica 21(2): 321-329 https://doi.org/10.3161/15081109ACC2019.21.2.007
Editor:
Rodney van der Ree
Cite this summary:
Borkin, K. (2025). More night-time traffic equaly less bat activity. Edited by van der Ree, R. TransportEcology.info, Accessed at: https://transportecology.info/research/batactivity-traffic