Road ecology blog: Tunnel (landscape bridge) Morro Alto (1837 m long), BR 101, near Aguapés, Rio Grande do Sul, Brazil.
There are no strict rules on how to classify a wildlife crossing structure. However, "wildlife overpasses" tend to be specifically build to allow wildlife to pass to the other side of the road. The dimensions and "furniture" (e.g. soil, cover, vegetation, ponds on either side) of a wildlife overpass are typically designed with certain "target species" in mind. The appropriate width of the structure (i.e. the road length covered by the structure) is often a topic of discussion, but most structures are between 30-70 m wide. While most wildlife overpasses do have soil and vegetation, they tend to not be designed to have a wide range of ecosystem processes continue across the road. Ecosystem processes may not only include movements of a wide variety of plant and animal species groups representative for the ecosystem, but also soil and hydrology processes.
Long road tunnels and long bridges (elevated roadways) are typically several hundreds of meters up to several kilometers wide (i.e. the road length covered or spanned by the structure). They have relatively "undisturbed" soil and water on top or below the structure which make them better suited to have ecosystem processes continue under or over the road compared to wildlife overpasses. In this context these large structures can be regarded as a "landscape bridge" rather than just a wildlife crossing structure. However, long tunnels or bridges tend to be primarily build for other reasons (e.g. a mountain or floodplain is in the way of the road). While most long tunnels or elevated roadways are not primarily designed for ecological processes to continue over or under the road, they do have great potential to function as a landscape bridge.
For images of landscape bridges or elevated roadways over long distances click here.
For images of wildlife overpasses click here.
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