Crossing to Sustainability: Bridge of Sighs, or Sizable Bridge?

Building bridges for wildlife to cross major highways is a tragically good idea: tragic because it reflects the ongoing damage that the world's mass road construction projects are having on the planet; but good because if done correctly wildlife crossings may allow some patches of habitat to remain connected and reduce the rates of collisions between wildlife and vehicles that often result in death, or serious injury, to all participants (Dobson 2006). Balmori and Skelly take this concept a step further by designing an overpass that includes a whole suite of environmentally sound design principles: an elegant wooden structure that can include patches of soil at different depths to support different types of plant community; the wood used is rescued from pine forests that have been recovered from forests attacked by the pine bark beetle (Dendroctonus ponderosae); and the structure is wide and deep enough to mask road noise and perhaps convince wildlife that they are passing through an almost natural part of their habitat. Ultimately, this is also a good thing, as it challenges architects and district planners to think more deeply about the aesthetics of infrastructure. This is several steps beyond the traditional utilitarian approach, where aesthetics focus on minimizing the bottom line!

I have 3 scientific concerns about this approach and 1 more philosophical worry. Initially let us consider my scientific concerns. Architects are artists, and their interesting designs get recognition, which increases visitation rate.

1. 1. People will be encouraged to visit the bridge, to inspect its architecture, and to replicate and improve upon its design in other locations. This increased site visitation will create further disturbance for wildlife that will simultaneously reduce the use of the bridge by wildlife—ultimately perhaps, defeating its main purpose.

2. 2. How rot resistant will the structure actually be—and can the olfactory senses of wildlife detect the presence of substances used to treat the wood? Wood is wonderful material to make things from; almost the ultimate natural material for engineering. Yet its use for external structures, particularly those that involve intimate association with soil, considerably enhance the opportunities for fungi and bacteria to attack and digest the wood. Tragically, the bridge is likely to eventually collapse, unless the wood is treated with fungicides, varnishes, and a variety of chemicals that are toxic. Many of these substances "off-gas" to such a level that they provide a powerful deterrent for wildlife, as well as for the invertebrates, particularly the worms that are vital to soil composition. When I was growing up in Scotland, we noticed there were never any molehills within several meters of fence posts we'd treated with creosote to prevent them rotting. This led to a wonderful way to annoy the neighbors and the moles. If you poured a small tea cup of creosote into a fresh mole hill, the next fresh one would like appear many meters away—usually in the neighbor's yard!!

3. 3. How long before we know if the bridge is effective? And what is our control on this experiment? Although I recognize that this is a proposal and not a scientific paper, I do wonder what the control is for this experiment? Certainly leaving the area without a bridge will likely lead to increased wildlife-vehicle collisions on an unfenced road, so now that the damage of adding a road is done, an overpass is needed. But elk (Cervus canadensis) and wolf (Canus lupus) aesthetics may be very different to those of humans. Would these species do better with an underpass, or even a broader, less aesthetically pleasing bridge? Similarly, the bridge may work well, but how important are the approaches to it? Should these be broad funnels coming in from a fence a long way back from the road, or can the fence be run close to the road, or will this over habituate wildlife to cars, trucks, and other forms of human disturbance?

My philosophical worry is perhaps less tangible, but it is one that I think is my biggest concern about this concept; will this structure create the impression we can always find an engineering solution to...