Who’s who on the wildlife bridges?

I hate my e-tag – that ever-present, electronic parasite lodging on the dashboard of my car. Each beep heralding the departure of cash from my bank account as I cruise through the city’s toll roads. It never misses a pass, the insufferable little know-it-all. Sometimes I glare at it.

My (irrational) hatred of a simple monitoring device makes me a hypocrite. As a wildlife researcher, I’ve tagged, collared and marked many, many, many animals in an attempt to learn what they get up to when I’m not watching. Finding out which animals went where and why can border on an obsession, and some researchers go to some amazing lengths to learn more about animal movement (see here, here and here). The ultimate goal is conservation – by understanding a species’ movements, we can make sure we protect the space and paths they need to survive.

In my case, I wanted to know which animals used wildlife bridges to cross the Hume Freeway. You see, back in 2007, we installed rope bridges over the freeway to help gliders, possums and other tree-dwelling critters cross the road safely. You can catch up on some of the background here, but suffice it to say, little animals have a tough time crossing big roads and these rope bridges aim to help.

We know that animals use our bridges. In seven years of spying camera monitoring, we’ve detected 1000s of crossings, including threatened species like the squirrel glider and brush-tailed phascogale, as well as more familiar faces like the ringtail and brushtail possum. I’m happy to say it’s becoming a more and more common finding for wildlife crossing structures of all types across the world – build it and they will cross.

But I want to know more. Which ones cross? How many? How many times each? Are we seeing 20 animals cross each night? Or just one animal cross 20 times? Aside from just satisfying my curiosity, you can see how this information would be important. If we’ve built a bridge that only helps a single animal out of the whole population then we probably haven’t done a very good job. We could also use the movement patterns of individual animals to figure out why the animals might cross. If some animals cross regularly, then it’s probably for something important to their daily lives.  Maybe they have a feeding trees and nesting site on both sides of the road, or a distant relative (or secret lover) to visit?

The first step to answering these questions is to tell individual animals apart. Unless you work with something spotty or stripey, this can be tricky. My gliders and possums are plain old grey. Cute and fluffy, but grey. I can’t tell one fluff-ball from another through the lens of an infrared camera. This brings me back to the e-tag.

Through all our years capturing and studying possums and gliders on the Hume Freeway, we’ve used microchips to tag our animals. It’s just like the one your vet put in your cat or dog (assuming that you’re a responsible pet owner). The techy-term is “Passive integrated transponder” or PIT tag. They’re a handy little device, about 12 mm long and are implanted just below the skin between the shoulder blades. Each tag has a completely unique identifier, a barcode 10 digits long, so that we can always tell one animal from the next. Their own personal e-tag.

So, we got to thinking – what if we could install a system that would scan the tag as each animal went over the bridge? A tollway for wildlife! In 2009 started talking with Microchips Australia and Faunatech to do just that. We tested it in 2010, worked out some bugs, and then rolled the system out over five rope bridges along the Hume between Seymour in Victoria and Holbrook in New South Wales.

It works like this. A flat bed scanner, about the size of a pizza box, sits at each end of the bridge. Whenever an animal with a tag passes over the scanner, the chip is scanned and recorded, along with the time and date. All the while our cameras were still in place, recording photos and videos of any animals (tagged or untagged) that cross.

The information started pouring in! The bridges were definitely used by our tagged animals, males and females, old and young. This is great news, because it shows us that no particular demographic group avoided using the crossing structures.

Our sample size was unfortunately small – of the five bridges, only two were used on a regular basis. The busiest bridge for squirrel gliders was at Longwood, with three different individuals crossing 244 times over 11 months. Brushtail possum traffic was highest at the Violet Town bridge, with four tagged animals lumbering back and forth 277 times in the same period. These animals were often seen crossing multiple times in a night, which is pretty convincing evidence that they had important things to do and places to be on both sides of the freeway. The bridges have given them access to resources that they couldn’t have reached before.

At the two busiest sites, all of the crossings were made by just three or four tagged animals per species. This represents about one-quarter of all the tagged animals that live nearby. Is that enough for us to say a crossing structure is successful? Well, that’s the million-dollar questions and the next on my list.

Microchip scanners have now been used to monitor bandicoots, salamanders and tortoises crossing through wildlife structures, and could work for a whole range of other species. Of course, there are kinks to work out to make the system more reliable (e.g. how to prevent foxes or cockatoos from vandalising the equipment!). Even so, being able to identify individual animals can yield fascinating insights into how and why crossing structures are used by wildlife, and how they might benefit the population. When we start to combine data from microchip scanners with other field and genetic data, we can learn if related animals travel together, if parents teach their offspring to use the structures, and how this changes through the generations.

If only we could charge animals for use, we might be able to fund more wildlife crossing projects…

Learn more

• My paper on the microchip scanners along the Hume
  Soanes K, Vesk PA and van der Ree R (2015) Monitoring the use of wildlife crossing structures by arboreal mammals: insights gained from motion-triggered cameras and PIT-tag readers. Wildlife Research 42, 241–256.
• Some great articles exploring the use of PIT tags in wildlife research   Smyth and Nebel (2013) Passive Integrated Transponder (PIT) Tags in the Study of Animal Movement
• Gibbons and Andrews (2004) PIT Tagging: Simple Technology at Its Best

Originally posted at my old site ksoanesresearch.wordpress.com, January 6, 2016.