In lieu of an abstract, here is a brief excerpt of the content:

  • Can Precision in Ecological Science Match Elegance of Design?
  • David Drake (bio)

The bridge design has much architectural novelty. However, from my perspective of wildlife science and needs, the authors frequently draw ecological conclusions improperly and overreach. One example is in the "Safe Crossings" section where the authors write, "The survival of entire animal populations is jeopardized by the barriers created by highways. Once fragmentation increases beyond a certain point, it poses a threat to genetic diversity within species. The isolation of too small a population leads to inbreeding". I am not familiar with an "entire animal population" that experienced genetic inbreeding due to the construction of a highway. In very extreme situations, such as isolated populations of endangered species, this may be true, but this is not typical in the conditions being addressed. Very small populations are subject to inbreeding depression, but this is usually true in "island" or very disjunct populations, not where a road-scale disruption is inserted. The authors use dramatic language, but we are not given citations from the technical literature to support this statement.

Other examples of overreaching language include, "Wildlife crossings also help reduce habitat fragmentation" and ". . . wildlife crossings . . . creating new natural areas helps to stretch the boundaries of ecological structures." These suggest a casual foundation to the specific ecological requirements of the site. Wildlife crossings don't necessarily reduce habitat fragmentation; the habitat remains fragmented regardless of the presence or absence of the crossing structure. Instead, crossing structures can connect fragmented habitats. Moreover, the amount of habitat created and offered by wildlife crossings is relatively nominal, and most/all wildlife using these structures use them for passage in a short amount of time to get from one side of the roadway to the other rather than dwelling in the crossing habitat for extended periods of time. The incremental improvement of using such crossings in the overall landscape structure has not been quantified.

A final example exists in the "Land-shape" section where the authors write "The mold has the added advantage . . . of eliminating traffic jams in the construction phase, which are harmful to the environment." This also seems to be a bit of an overreaching statement; I understand about idling vehicles and emissions, but how long will the traffic be held up during construction? The amount of traffic in the area of crossing construction is relatively minor compared to traffic in western urban and suburban areas, for example, which is continuous and severe.

My biggest criticism of this manuscript is the lack of species focus relative to design focus. I realize the authors are probably not wildlife ecologists but are trained as architects or in a related profession. However, despite a couple of nods to the "ecological composition being the decisive factor" ("Bridge, Ground Level, and Natural Layer" section) and "When planning a wildlife crossing, it is important to look carefully to see what kind of animals are likely to use it since each species requires different conditions" (Consultation Culture: "Poldering" section), wildlife use of the crossing structures seems almost an afterthought. Precision and detail by structural engineers and materials consultants are normal parts of bridge design, from the beginnings of a contraction project. One could argue that wildlife professionals sitting at the table during design development can serve as suppliers of the same level of detail.

I would like to hear how the authors decided which species would be the focal species for using the crossing structures. Background framework was given to the competing teams, but more discovery of conditions and community structure at this specific site is needed. For example, perhaps in North America the best example of wildlife crossings is in Banff National Park (Clevenger et al. 2009). Prior to any construction, the people involved with the Banff project very carefully conducted long-term monitoring to determine what species were in the area, which species were most likely to use crossing structures, and how best to design crossing structures. In addition to building crossing structures, they also constructed underpasses after discovering some animals (i.e. mule deer) were reluctant to cross over the highway. The authors seem to be trying to offer all types of habitat structure to a...

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