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Maintain habitat connectivity: Large wildlife passage structures

Points on the map represent the most appropriate locations in Massachusetts for large wildlife crossing structures. The brown colors are conductance, a measure of importance for regional-scale connectivity. The darker the color the more valuable the area is for connectivity. Data are from the Critical Linkages Phase II (2013)The final three layers are from The Nature Conservancy's Berkshire Wildlife Linkage project. They include priority road segments for mitigation (use of wildlife crossing structures), priority areas for connectivity, and habitat nodes (core areas).


Points on the map represent the most appropriate locations in Massachusetts for large wildlife crossing structures. The brown colors are conductance, a measure of importance for regional-scale...

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Adaptation Strategies and Actions

Maintain habitat connectivity: Large wildlife passage structures

Adaptation type: 
Roadway infrastructure, crossings, and dams
Species conservation and management


Restore and maintain terrestrial and aquatic connectivity sufficient to maintain healthy ecosystems and wildlife populations

Animal movements (individuals or their offspring) across the landscape are important for maintaining healthy wildlife populations. Climate change is likely to result in changes to habitat conditions (temperature, rainfall, vegetation) that will require many species to adjust the areas they occupy. Restoring and maintaining landscape connectivity sufficient to allow wildlife populations to adjust their distribution over time is a critically important strategy for adapting to climate change.


Construct large wildlife passage structures to facilitate movement across roads for a broad range of species
The construction of large wildlife passage structures that can accommodate a wide range of species may be warranted to maintain landscape connectivity in areas where roads with significant amounts of vehicular traffic intersect high quality habitat. Large species, such as moose, bear and bobcat, are generally hesitant to use culverts or even small bridges. Although river bridges with appropriate dry passage along the banks can meet the needs of these species, some of the largest blocks of continuous forested habitat occur along mountain ridges - not along rivers. To maintain long-term landscape connectivity, it may be necessary to construct passage structures for wildlife in areas that lack rivers crossings.

Given the expense of wildlife underpasses and overpasses, it is particularly important that they be used strategically where roads with substantial vehicular traffic fragment areas of high quality habitat. Results from the Critical Linkages analysis, conducted by UMass Amherst, and the Berkshire Wildlife Linkage project, conducted by The Nature Conservancy, can be used to identify road segments where large wildlife passage structures are likely to be most effective for enhancing landscape connectivity (see map information on this page).

A wildlife overpass (ecoduct) in Europe and large wildlife underpass in Florida for facilitating movement by a variety of wildlife across highways. Photo credit: Scott Jackson.
A wildlife overpass (ecoduct) in Europe and large wildlife underpass in Florida for facilitating movement by a variety of wildlife across highways. Photo credit: Scott Jackson.

Wildlife Underpasses

It is possible to create large underpasses for wildlife, sometimes referred to as wildlife bridges, where roads are built on significant areas of fill. A wildlife underpass is more than simply a tunnel. To be effective they need to be carefully designed and located, and generally equipped with guide fencing to channel wildlife to the structure and prevent passage over the road surface.

A number of characteristics can affect the performance of wildlife underpasses:

  • Size – Large animals, like moose and bear, require large structures. How large is difficult to say. For some species, openness (cross sectional area divided by structure length) may be more important than absolute size.
  • Light – Salamanders and turtles are known to be averse to using wildlife passage structures that lack sufficient light. Other species also might be sensitive to low light levels. Large structures generally allow plenty of ambient light inside. Light may be a concern for very long structures or passage structures with steep approaches in both directions.
  • Moisture – Amphibians are sensitive to dry conditions and can die of dehydration while crossing through dry wildlife passage structures. At the other extreme, improperly designed storm water management systems can create conditions within structures that are too wet.
  • Substrate – In large wildlife passage structures it is generally possible to provide a variety of substrates (rocks, sand, soil) accommodating the needs of a wide range of species. The inclusion of cover objects (large rocks, woody debris) within wildlife passage structures can significantly enhance their effectiveness for small animals.
  • Approaches – Some wildlife are most comfortable in forest habitat; others in more open habitat. It is generally best, to the extent that it is possible, to create conditions at the approaches to the structure that resemble adjacent natural habitat.
  • Fencing – In the period right after wildlife passage structures are constructed they are novel features in the landscape and may be avoided by wildlife. Over time they may come to be accepted and used by wildlife as a safe and convenient route of passage across the road. Wildlife use can be promoted and enhanced by the use of guide fencing to channel animals to the structure and prevent them from crossing over the road surface.

Wildlife Overpasses

It is generally impractical to build underpasses in areas where roads cut through hills or mountains. However, these topographic conditions are well-suited for construction of wildlife overpasses (known in Europe as Ecoducts). Wildlife overpasses are wide bridges that pass over busy roads and are covered with soil and vegetation. The vegetation provides cover for mobile wildlife and habitat for small, less mobile species such as small mammals (moles, voles, chipmunks) and terrestrial invertebrates (ants). Small ponds constructed on wildlife overpasses can provide habitat and rehydration areas for amphibians.

Other Design Considerations

Wildlife crossing structures (underpasses and overpasses) that are heavily used by people (for hiking, hunting, horseback riding, mountain biking) tend to be used less by wildlife than crossing structures where human activity is limited or excluded. A limited amount of human activity may be acceptable given that people are most active during daylight hours and many wildlife species are active at night.

Scope and Constraints

One-time action
Immediate impact
Long-term impact
Higher cost category
Municipal or county jurisdiction required
State jurisdiction required


National Fish Wildlife Plants Climate Adaptation Strategy Goal 1: Conserve habitat, diversity, and connectivity
National Fish Wildlife Plants Climate Adaptation Strategy Goal 2: Manage species, habitats, ecosystem functions
Forestry Goal 5: Re-connect the landscape

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