Tying a Wildlife Bridge into the Ecological Landscape

The flexible, modular design of this wildlife crossing appears to be exceptionally innovative and practical, but I don't have the background to evaluate it in terms of structural integrity and cost effectiveness. Instead I will focus on whether the design is likely to be effective in terms of the overall goal of providing a corridor that will reduce wildlife mortality and increase dispersal between populations of relatively large mammals on the two sides of an interstate highway in Colorado.

Before I read the proposal I wrote down the main criteria I would use for evaluating the ecological effectiveness of the design: (1) Is it based on previous studies of the general behavior of the target species, particularly their movement patterns?, (2) Is it based on specific information on movement of animals at West Vail Pass where the wildlife crossing will be constructed?, and (3) Will the effectiveness of the crossing be systematically evaluated during the years after its construction? The final criterion is particularly important because this wildlife bridge will be a major investment, and it can potentially serve as a model for future wildlife crossing projects.

The first criterion, that the design should be based on information about movement patterns of the target species, is met in a general way by drawing upon the "the personal experience of the project ecologists." As the authors point out, there are few similar wildlife crossings in North America to draw upon for more precise information about dispersal of particular species across bridges covered with natural vegetation. The effectiveness of wildlife bridges and tunnels in Banff National Park in Canada was assessed for a period of several years (Clevenger and Waltho 2005), however, and the results may provide useful background information on species that are the same or similar to the target species in Colorado. For example, the Banff study showed that deer (Odocoileus sp.) and elk (Cervus elaphus) are more likely to cross open areas that are wider. The strip of trees down the center of the proposed bridge substantially reduces the width of the grassy corridor, which may reduce the effectiveness of the bridge for these species. Many wildlife crossings have also been constructed in Europe (particularly in the Netherlands), and studies of the effectiveness of these crossings may provide insights even though the particular species of mammals differ.

The second criterion, that the design considers movement patterns of animals at the site, has been fulfilled. The proposed bridge site is located in an area where a large number of mammals were observed crossing the highway. Also, animal trails and other evidence of movement patterns of mammals have been documented adjacent to the highway.

The third criterion has also been fulfilled. The proposal includes a provision for a long-term monitoring program to assess the use of the bridge by target species. The use of webcams and cameras triggered by infrared crossing beams should provide a standardized source of information on the frequency of use by different species of animals. I agree with the authors that the use of these surveillance devices is superior to installing a wildlife-viewing platform that might deter animals from crossing the bridge. Human activity at wildlife crossings can reduce the frequency of their use by large mammals (Clevenger and Waltho 2005).

The designers astutely took into account the likelihood that the habitats on both sides of the corridor are likely to change because of events such as wildfires and pine beetle infestations. Not only is this taken this into account in the initial design, but the structure of the bridge also permits a flexible response to long-term changes in wildlife movement patterns by expanding or reducing the width of the bridge or even moving the bridge to another location.

An integral component of most bridges and tunnels that are designed as wildlife crossings is a system of fences that funnel moving animals into the corridor. This feature is not described in the narrative but is depicted in the design diagrams. An important question is whether these fences will only direct large and medium-sized mammals onto the bridge. If smaller organisms are...