Do you know the most common question I get about wildlife crossing structures? Surprisingly, it’s not ‘do they work?’. It’s ‘how do you teach them how to use it?’. I understand why people are skeptical. Sure, a big, safe bridge for wildlife sounds like a great idea, but animals aren’t smart enough to know the difference between a safe spot and a dangerous spot. Are they? After all, they can’t even read! Thankfully, we don’t have to rely on wildlife commonsense. We road ecologists have a few tricks up our sleeves to make sure that animals do use these structures and don’t just cross wherever they feel like.
Location location location
Ideally, we’ve had a chance to study the animals living nearby before the road and crossing structures are put in. We spy on them with cameras, track them with GPS collars and generally stalk them about the country-side. If we can’t track them, we often know a bit about where they like to move through – do they travel along gullies or follow forest patches? From this we can build a map that shows their main movement corridors and pathways. Then we overlay where the new road will be built. Wherever the animal path crosses the road path is a good place for a structure. That way the animals don’t have to change their behaviour too much. They were going that way anyway.
A little bit of guidance
Still, animals have minds of their own and ultimately, can cross wherever they see fit. Maybe one day a new family moves in and decide they’d like to make a new path, a few 100 m away. How do we stop all our careful planning going to waste?
A fence. I know, it’s obvious when you think about it. We can put up fences where we don’t want animals to cross. Fences serve two important functions. First, they stop animals getting onto the road, creating traffic havoc and potentially getting killed. Second, fences guide animals to the structures, so that the structure is the only place the can cross. Unfortunately, fences can’t go on forever (that’s a lot of fencing!) but targeting them around key movement paths and crossing structures can be really effective.
It doesn’t happen overnight
Even with great planning and fencing, there can be a learning curve. Imagine you’re a deer. You’ve woken up in the morning and are heading off to your favourite waterhole for a quick drink before a day of frolicking (I assume this is how deer behave). Trotting merrily along, all of a sudden you come across this…thing, in your path. A strange, hard tunnel. You can see the waterhole on the other side, but the tunnel smells odd, and looks funny and generally gives you the heeby-jeebies. Better not risk it. Next time you come across the tunnel you’re feeling braver. The smell is fading and grass has started to grow around it. You might take a few steps inside before you chicken out. Then you take a few steps more, and more, until one day, you’ve made it all the way through. You think ‘Nothing bad happened. Maybe I’ll try that again… ‘ Anthropomorphism aside, animals usually take time to get used to the structures. How much time depends on the species. Black lion tamarins used a canopy bridge “as soon as it was assembled” (Valladares-Padua 1995), while larger, long-lived animals could take decades to adapt. My own work on squirrel gliders showed took up to 2 years before they started regularly using canopy bridges and glider poles (Soanes et al 2013).
Not just a cute idea
So, that’s how we do it. Scientists, engineers, road planners and environmental managers put a lot of thought into making each one as good as possible. After a structure is built there’s more work to do, figuring out how well it works and perfecting designs to make future structures even better. I’m not saying we’ve got it all figured out yet – there’s still a long way to go – but we’re learning all the time how to make safer roads for people and animals. Although this guy seems to have the situation under control…
References and further reading
Valladares Padua C, Cullen Jr L and Padua S (1995) A pole bridge to avoid primate road kills. Neotropical Primates 3, 13-15.
Soanes K, Lobo MC, Vesk PA, Moore J, McCarthy MA and van der Ree R (2013) Movement re-established but not restored: Inferring the effectiveness of road-crossing mitigation for a gliding mammal by monitoring use. Biological Conservation 159, 434-441.
van der Ree R, Gulle N, Holland K, van der grift E, Mata C, Suarez F (2007) Overcoming the barrier effect of roads – how effective are mitigation strategies?, In International Conference on Ecology and Transportation. eds C.L. Irwin, D. Nelson, K.P. McDermott, pp. 423-431, Centre of Transportation and The Environment, North Carolina State University, Raleigh, North Carolina and Little Rock, Akansas, USA.