Over and Under: Reconnecting Australia’s Wildlife.

Today I want to touch on my pet subject – Fauna passages. This was the focus of my PhD research, so as you may have guessed, I have a LOT to talk about. But in this post I will give you a brief introduction to road ecology and to the science of fauna passages.  First off, let me explain – ‘fauna passages’ is the collective term for underpasses, overpasses, culverts and other specialised structures that have been purpose-built (or assumed from another purpose and retrofitted), specifically to allow animals to cross roads. There are a number of reasons for this, but primarily, fauna passages aim to reconnect remnant habitat (natural habitat existing within a modified landscape), or prevent further isolation from urbanisation (F0rman 2003). I will revisit this in a moment, but first it is important to understand exactly what urbanisation does to a landscape, and how we utilise fauna passages to assist in reducing urban effects.

The problem with urbanisation is straightforward; as we ‘urbanise’ a natural area, we tend to remove the pre-existing habitat. The obvious consequence of urbanisation, whether it be urban expansion or the formation of a new urban hub, is this severe loss of habitat. Along with the introduction of native species (who often strongly persist in urban landscapes), we remove vast quantities of previously vegetated habitat, which displaces or kills much of the resident wildlife. During this process, it is common that some areas of habitat are left as ‘green space’, sometimes in the form of reserves or parks. Usually this is a legislated requirement of urban expansion, and is usually required if endangered species reside in the area. What occurs during this process is what is referred to as a habitat (or urban) matrix (which is far less exciting than choosing the red pill). The matrix consists of islands of remnant habitat within a ‘sea’ of urbanisation – often too dense, dangerous or unwelcoming for wildlife to negotiate.


An example of how the urban habitat matrix occurs: previously existing, high quality, habitat (green) (left) is replaced with a network of buildings (grey) and roads (black) (right), creating highly fragmented habitat.

The outcome in this example is four remnant islands of habitat, within which some species may be able to persist. However, the consequences of isolated habitat are significant, and will negatively affect the habitat as well as the wildlife within it (Kautz et al. 2010). Habitat degradation closely follows significant habitat destruction, where the small islands of habitat become vulnerable to weed invasion, introduced predators and disturbance such as vehicles, pollution, noise and light. Larger habitat remnants are likely to persist for longer, while smaller patches degrade and eventually cannot support wildlife. The wildlife themselves have other issues. While some more mobile, or more generalist species (species which have broad ecological requirements and can adapt easily), may be able to negotiate the new urban landscape, many species cannot, or will not (Fahrig 2003, Forman 2003). This severely restricts the movement of individuals to find food, water, nesting resources and mates. The restriction of mating, in particular, can severely impact the availability of gene pool diversity, which often results in species decline and, eventually, extinction (Coffin 2007, Corlatti et al. 2009). On top of this, roads pose a completely new danger: the very real possibility of being hit by cars.

Roads are a critical element in urban ecology, hence why they have their own research subsection (road ecology). Road ecology, or more broadly, linear infrastructure ecology, encompasses roads as well as rail lines, power lines and any other linear infrastructure that bisects the natural landscape. Roads are a major contributor to the urban matrix, as they can extend for hundreds, or in some cases, thousands of kilometres, slicing up the landscape and fragmenting habitat beyond many other aspects of urbanisation (Clevenger and Sawaya 2010). Some roads can be negotiated with little trouble, for example small rural and suburban roads that are not too busy. Other roads, such as highways and freeways, are almost impossible to cross safely, if at all. Road ecology has only been a subject of ‘categorised’ research for about 20 years (see Forman and Alexander 1998) – this makes it the new born baby of ecological research, but we are already learning quickly about exactly how roads impact wildlife and natural areas, as well as how they indirectly impact vast amounts of non-adjacent landscape ( eg. Trombulak and Frissell 2000). There is plenty more I could tell you about road ecology, but I will revisit this another time.

So in the midst of all this doom and gloom, this brings me to the function of fauna passages. Fauna passages have been implemented across the world, and come in a range of designs, performing a myriad of functions. Typically however, they fall into four categories: overpasses, underpasses, arboreal crossings and culverts.


Overpasses are the most recognised of fauna passages, particularly in the road ecology community. They are large, often vegetated bridges which are usually implemented for a particular species – most commonly large mammals (Bond and Jones 2008). In Australia, many of our vegetated overpasses are implemented for kangaroos and wallabies, while in Canada they are built for animals such as moose, and in Europe they are provided for elk, bears, badgers and many other large mammals. The concept of a vegetated overpass is generally to be available for a multitude of species (among different taxa – mammals, reptiles, amphibians etc.), however they are generally built for a single (or a couple) of species.


Two examples of modern fauna overpasses in Europe; M7 in southern Hungary (above) and the A66 motorway in north-west Spain (below). (Photographs from Jones 2010)


In contrast, Australian overpasses are often planted with large trees to resemble the neighbouring forests, and are therefore much more robust in design. Australian overpasses; Compton Road overpass in Brisbane, Queensland, and Bonville overpass over the Pacific Highway at Bonville, NSW. (Photographs M. McGregor and D. Jones).

Underpasses and culverts:

The term underpass is used for a purpose-built passageway underneath a road which allows animals to cross safely. These are not technically the same as culverts, which are not purpose-built for wildlife (they often have other purposes, such as for drainage), but may be retrofitted or otherwise provide a safe passage for animals. Underpass designs are getting more technical and creative with wider implementation, and as we learn more about features that may increase or decrease the success of an underpass. Often they are fitted with ‘furniture’ (logs and shelving) to allow animals a safe crossing, above the concrete floor (Veage and Jones 2007).


The Compton Road fauna underpass, Brisbane, QLD is an example of a typical purpose-built concrete underpass with fauna furniture. (Photo: M. McGregor)


Examples of culverts include box culverts, used for drainage, which can be retrofitted with furniture to allow use by wildlife (left), while natural under-bridge throughways can also be retrofitted to be more appealing to wildlife (right). (Photos: C. Dexter)

Arboreal crossings:

The last typical crossing type (in Australia, at least) is the arboreal crossing. Arboreal, meaning animals that don’t generally live on the ground, for example possums, gliders and phascogales. The two most common designs are pictured below: glider poles, which you can see at Compton Road, but also along the Pacific Highway between Brisbane and Coffs Harbour, as well as on the Highway travelling south into Melbourne; and the rope bridges, which are surprisingly abundant along the entire east coast, particularly over the Highway.The function of the rope bridge is perhaps more obvious than the glider poles (arboreal mammals use it to run across). Gliders will use the poles in lieu of trees, which they climb and launch from the top – aiming to land on the next pole, and so on. I often get asked about rope bridges in particular, by people wanting to know how successful they really are. The answer is yes! they do work – but that is a story for another time.


Glider poles (photo: EHP Queensland) (left) and a rope bridge (photo: FaunaTech) (right), which allow arboreal mammals to cross roads.

So going back to our fabulous matrix diagrams – the promise of fauna passages in the landscape is simple; they aim to overcome habitat fragmentation, giving wildlife at least some connectivity back within the urban landscape. The reality of our imagined habitat is not a positive one, but is, sadly, the reality we see all too often. Instead of wildlife being able to travel for resources and find mates, they are isolated, often in areas where they may not persist, or areas which present dangerous situations with roads or other infrastructure.


Functioning habitat provides opportunity for mating and free access to resources (left), while urbanisation often ends in isolation and population decline (right). Animals being isolated from resources, or being the victim of car collisions are two common scenarios post urbanisation.

Instead, when we implement fauna passages, and pair them with other important wildlife preservation measures such as wildlife corridors (linear, remnant vegetation which provides a safe passage) and patches of revegetation, we can greatly increase the likelihood of persistence for many species.


The implementation of fauna passages, alongside wildlife corridors and revgetation can restore connectivity and greatly improve the persistence of wildlife.

There is a lot more to the concept of habitat fragmentation than what I have discussed here, but I hope this lighthearted and animated insight has helped explain a little about why fauna passages are important. While they may not be a silver bullet on their own, fauna passages are an exciting and successful way of reconnecting natural landscapes around urbanisation. Road ecology and the science of fauna passages is expanding rapidly, and there are many many success stories to be told, as well as lessons we already know. The question of success (and levels of success for different taxa) remains a bit of an untold story for now, but I assure you I will fill you in soon!

Dr Mel.


Bond, A.R. and Jones, D.N. 2008. Temporal trends in use of fauna–friendly underpasses and overpasses. Wildlife Research, 35(2), pp.103–112.

Clevenger, A.P. and Sawaya, M.A. 2010. Piloting a non–invasive genetic sampling method for evaluating population–level benefits of wildlife fauna passages. Ecology and Society, 15(1), pp. 7–29

Coffin, A.W. 2007. From roadkill to road ecology: a review of the ecological effects of roads. Journal of Transport Geography, 15, pp. 396–406.

Corlatti, L., Hackländer, K. and Frey–Roos, F. 2009. Ability of wildlife overpasses to provide connectivity and prevent genetic isolation. Conservation Biology, 23, pp. 548–556.

Fahrig, L. 2003. Effects of habitat fragmentation on biodiversity. Annual review of ecology, evolution, and systematics, pp. 487–515.

Forman, R.T.T. and Alexander, L.E. 1998. Roads and their major ecological effects. Annual Review of Ecology and Systematics, 29, pp. 207–231.

Forman, R.T.T., Sperling, D., Bissonette, J.A., Clevenger, A.P., Cutshall, C.D., Dale, V.H., Fahrig, L., France, R., Goldman, C.R., Heanue, K., Jones, J.A., Swanson, F.J. 2003. Road Ecology: Science and Solutions. Island Press, Washington D.C.

Kautz, R.S., Bittner, S.R. and Logan, T.H. 2010. Wildlife Crossing Handbook. BDA Environmental Consultants, Winter Park, FL.

Trombulak, S.C. and Frissell, C.A. 2000. Review of ecological effects of roads on terrestrial and aquatic communities. Conservation Biology, 14(1), pp. 18–30.

Veage, L–A. and Jones, D. N. 2007. Breaking the Barrier: Assessing the Value of Fauna–friendly Crossing Structures at Compton Road. Brisbane City Council and Centre for Innovative Conservation Strategies, Griffith University, Brisbane, Qld.