How well do wildlife crossing structures help animals across roads?
Written by Kylie Soanes
School of Agriculture, Food and Ecosystem Sciences, University of Melbourne, Australia
January 22nd, 2025
Summary
Wildlife crossing structures are becoming a common part of road construction projects around the world. From pipe tunnels to vegetated land bridges, most people have seen viral photos of animals using their bespoke pathways, safely separated from traffic.
But how well do they actually work? Are they helping animals move across roads safely?
It's a surprisingly difficult question to answer and one we tackled in our latest paper published in the Journal of Applied Ecology. Our approach was to conduct a research synthesis – bringing together studies from around the world to look at the big picture. We had already taken a similar approach looking at mitigation for reducing roadkill – now it was the barrier effect’s turn.
We found 313 studies, including published papers, technical reports, and theses published across 34 countries. This is what they said…
Figure 1. Animals using wildlife crossing structures. Clockwise from top left: roe deer and calves, badger, squirrel glider, European hare. Images: Edgar van der Grift and Kylie Soanes
The good news
Crossing structures definitely allow animals to cross roads (and railways, canals, and pipelines too, though these weren’t part of this study). Less than 2% of the studies found that no animals used the crossing structure. Every type of animal was represented, including insects, amphibians, reptiles, birds, and mammals of all shapes and sizes.
The bad news
Most studies are not set up to measure whether wildlife crossing structures actually change the number of animals crossing the road. Only 14% of them asked ‘Did crossing structures improve movement?’, ‘Did they restore movement?’, or ‘Did they prevent movement from declining?’
The key element missing was a benchmark comparison – a point of reference that allows us to say whether the movement across a wildlife crossing structure is better or worse than before (e.g. comparing with pre-construction data) or better than if we took no action at all (e.g. comparing with an unmitigated road).
What this means is that we still do not have a good stockpile of evidence to build an overall picture of how well wildlife crossing structures work. This is a major problem, because if we aren’t setting our research studies up to ask those questions we will simply see an animal on a bridge or in an underpass and assume we have done a good job – when all the while the population could still be declining.
Figure 2. For each of the 313 studies, we counted the number of times a specific outcome (e.g. no net loss, some loss, or no movement) was reported for each of the ten species groups pictured (insects to bats). Dashes indicate no data were available.
The concerning news
Of the studies that did measure changes in movement, some of the findings were concerning.
So far, wildlife crossing structures haven’t been successful at preventing the negative impacts of road construction on animal movement. In fact, cross-road movement declined post-construction in 16 out of the 25 times it was measured, even though wildlife crossing structures were present.
Only two studies measured how well wildlife crossing structures could restore movement across an existing road. Both were on arboreal mammals (a possum and a glider) and in both cases movement was lower at the crossing structures than at non-road conditions.
The encouraging news
There is consistent evidence that adding a wildlife crossing structure is better than doing nothing. Most of the 37 datasets that looked at this question found that movement was higher at the crossing structures than at unmitigated sections of road. Even adding vegetation cover or ledges to existing structures (such as basic drainage culverts) helped, particularly for small mammals.
Figure 3. For each of the 313 studies, we counted the number of times a specific outcome (e.g. no net loss, some loss, or no movement) was reported for each of the ten species groups pictured (insects to bats). Dashes indicate no data were available.
In a nutshell
Does all this mean that wildlife crossing structures can’t work? Absolutely not! We have tens of thousands of examples of wildlife happily using them, and clear evidence that they can increase wildlife movement across roads and even prevent barrier effects from disrupting movement in some cases.
What is equally clear, however, is that we are not doing enough to fully mitigate the barrier effect. Improving outcomes may require more structures, better designs, more careful placement, taking into account species needs, or including appropriate fencing. But knowing how to improve depends on more robust, more thorough evaluation ¬– not just observing wildlife using structures, but comparing this to appropriate benchmarks.
Author information
Kylie Soanes
Faculty of Sciences, University of Melbourne, Australia
Co-authors: Trina Rytwinski, Lenore Fahrig, Marcel P. Huijser, Jochen A.G. Jaeger, Fernanda Z. Teixeira, Rodney van der Ree and Edgar A. van der Grift.
Source citation
Soanes K., Rytwinski T, Fahrig L, Huijser MP, Jaeger JAG, Teixeira FZ, van der Ree R, van der Grift EA (2024) Do wildlife crossing structures mitigate the barrier effect of roads on animal movement? A global assessment. Journal of Applied Ecology. 61, 417–430. https://doi.org/10.1111/1365-2664.14582
Editor:
Julia Kintsch
Cite this summary:
Soanes, K.. (2025). How well do wildlife crossing structures help animals across roads? Edited by Kintsch, J. TransportEcology.info, Accessed at: https://transportecology.info/research/effectivness-wildlifecrossingstructures