Punchlines aside, in this blog Kylie Soanes shares insights from her recent article, Evaluating the success of wildlife crossing structures using genetic approaches and an experimental design: Lessons from a gliding mammal.
Wildlife crossing structures are a common answer to the age-old question: ‘How did the animal cross the road?’ Tunnels and bridges for wildlife are being built and used by animals all over the world, from pygmy possums to salamanders, grizzly bears to elephants (though, ironically, not the humble chicken).
In this study, we were interested in what happens after the animals get to the other side. Can these crossings reconnect animal populations that have been isolated by roads? Do they promote dispersal and gene flow?
We investigated this for the squirrel glider, a pocket-sized gliding possum, in south-east Australia. Squirrel gliders are an ideal study species to test questions about barriers and movement. These tree-dwellers don’t like to move along the ground, and instead choose to glide from tree to tree (using a skin membrane like a wing). But they can only glide so far, and large treeless gaps, like those over major roads, can restrict movement.
Thanks to radio-tracking and camera trap studies, we know that squirrel gliders will use artificial structures, like rope bridges and glider poles, to cross roads. But what does all this movement mean for gene flow?
To find out, we partnered with a local road agency to set up an experiment. We added crossing structures to an existing freeway and used population genetic analyses in a before-after-control-impact design to evaluate the effects on squirrel glider movement and gene flow.
It went like this. Earlier radio-tracking research, suggested that squirrel gliders would have trouble crossing the freeway at six sites where the treeless gap across the road was wider than 50 m. At four of these sites, we built crossing structures – either a rope bridge or a glider pole. The other two sites were left alone, unmitigated, for the duration of the study. We also looked at sites where tall trees provided natural stepping-stones for squirrel gliders (known as ‘vegetated medians’), as well as non-freeway sites. At all of these sties, we collected DNA from squirrel gliders living on both sides of the freeway to analyze:
- genetic structure – whether animals on opposite sides of the road formed separate genetic groups;
- migration – whether animals born on one side of the road had moved to the other; and
- parentage – whether animals from opposite sides of the road could mate and produce ‘cross-freeway’ offspring.
Comparing among the different site types, before and after mitigation, helped us to understand how well, or poorly, the crossing structures were performing. It meant that we were able to ask ‘What happens after the crossing structures are added?’, ‘How does this compare to doing nothing?’, ‘How does this compare to the natural connectivity maintained by tall trees? ‘, and ‘How does this compare to not building the freeway at all?’.
We found that the freeway was not a strong genetic barrier for squirrel gliders. Even at those unmitigated sites, where the width of the treeless gap was greater than the average distance a squirrel glider could glide, the freeway had no apparent impact on genetic structure or gene flow (only one site was an exception).
Considering what we know about the way squirrel gliders move, this was a bit of a surprise. It should be harder for gliders cross the freeway at sites that don’t have crossing structures or natural stepping-stones. In fact, all our other work showed us that this is the case. So why wasn’t this reflected in their gene flow?
It may be that by focusing on site-level impacts, we took too narrow a view of what influences genetic connectivity for squirrel gliders. While there weren’t any obvious crossing points at the ‘unmitigated’ sites, our study landscape is littered with scattered trees and narrow, linear corridors along fence-lines and roads – even along the freeway itself in some areas. These might allow squirrel gliders to take detours through the landscape, and eventually reach sites where it was easier to cross the freeway.
A closer look at the road-crossing animals in our migrant analyses supported this idea. At sites with crossing structures or vegetated medians, the evidence suggested that squirrel gliders crossed the road “directly” – that is, an animal migrated from the habitat patch right on the opposite side of the freeway. But at unmitigated sites, direct crossings were rare. Instead, animals seemed to take “indirect” paths through the landscape, coming from neighboring patches rather than from straight across the 50 m gap at the freeway. After crossing structures were added to these sites, the overall amount of migration didn’t change, but the apparent path did. Squirrel gliders could now cross directly, instead of taking longer detours.
Does this mean that the crossing structures weren’t helpful? Well, no, because gene flow wasn’t the only goal of these structures. They still provide other benefits like access to food and nesting resources on both sides of the freeway. Our related radio-tracking and camera trap studies show that without crossing structures or tall trees, these kinds of daily movements don’t occur. But it does mean that the crossing structures here weren’t necessary to allow gene flow – except for one, that is.
At the one location where the freeway had created a local genetic barrier, we found that installing a rope bridge restored connectivity within just five years. Before mitigation, the site was a particularly bare stretch of freeway, with no tall trees for more than a kilometer in either direction. After the rope bridge was installed we started to see cross-freeway movements, and genetic structure all but disappeared. We could even match gene flow to individual animals that were detected crossing the rope bridge during a concurrent camera monitoring study. The same squirrel gliders that used the crossing structure found loving company on the other side, and produced cross-freeway offspring.
So, what were the punch lines? How did the squirrel glider’s genes cross the road?
- They can use roadside trees. Even linear strips and scattered trees are likely to be really important to help arboreal animals to move through heavily-modified landscapes, and can prevent potential barrier effects of a freeway.
- They can use crossing structures. Canopy bridges and glider poles can restore gene flow where large gaps in tree cover inhibit road crossing.
Combining genetic approaches and an experimental framework really improved our ability to evaluate the success of wildlife crossing structures. Future studies that use genetic approaches to investigate landscape-scale patterns and identify localized barriers will be useful not just to evaluate the success of wildlife crossing structures, but also to guide their placement in the landscape. These kinds of studies are critical to understanding just how valuable crossing structures are as a conservation tool.
Read Associate Editor, Yolanda Wiersma’s comments on this article in this post from September.