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Maintain habitat connectivity: Modify stream crossings to allow wildlife passage
If data are not visible, use the plus sign to zoom in. Priority areas for: a) terrestrial connectivity for wetland? and aquatic ecosystems are represented as green dots on roads. You can layer in results as top 5%, top 10%, top 15% and top 20%; darker colors represent higher priorities; b) terrestrial connectivity via large wildlife crossing structures are represented as pink polygons on roads; and c) road-stream crossings that serve or could be modified to serve as important passage structures for terrestrial connectivity are represented in the pink-purple dots. You can layer in results as top 5%, top 10%, top 15%, top 20% and top 25%; darker colors represent higher priorities. Data are from the Critical Linkages Phase I (2011) and Critical Linkages Phase II (2013) analyses. The final three layers are from The Nature Conservancy's Berkshire Wildlife Linkage project. They include priority road segments for mitigation (i.e., use of wildlife crossing structures), priority areas for connectivity, and habitat nodes (core areas).Hide
If data are not visible, use the plus sign to zoom in. Priority areas for: a) terrestrial connectivity for...Read More
Adaptation Strategies and Actions
Maintain habitat connectivity: Modify stream crossings to allow wildlife passage
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.
Modify river and stream crossings to facilitate wildlife passage across roads
Using river and stream crossings for wildlife passage across roads
Many people are aware of the dangers that wildlife face when crossing roads. Roadkill serves as a constant reminder of how poorly equipped many wildlife species are for dealing with the speed and size of vehicles traveling on roads. In addition to road mortality, wildlife populations can be adversely affected by physical barriers that prevent movement (e.g., Jersey barriers and right-of-way fencing), as well as the psychological barriers caused by the noise and commotion of road traffic and the lack of natural cover on the road surface and along the shoulders.
Wildlife crossing structures (tunnels, bridges) can be used to reduce road mortality and facilitate wildlife movement. These conservation tools are expensive and require the cooperation of highway agencies, and therefore, they tend to be reserved for a limited number of strategic locations. Places where rivers and streams cross roads offer another opportunity for wildlife to cross roads without the danger of passing over the road surface. In more developed areas, river and stream corridors may be the last remaining connecting habitat available to facilitate the movement of wildlife. Where bridges and culverts at river/stream crossings are large enough and have other appropriate characteristics, wildlife are able to travel through the landscape even in areas that are highly fragmented by agriculture or development.
Many bridges and some culverts already provide conditions that are suitable for wildlife passage. At other locations, stream crossings may be too small and constricted, have physical barriers such as outlet drops, or be too wet (lacking dry bank areas) to be used by terrestrial wildlife. These crossings offer opportunities to enhance landscape connectivity through modification or replacement of the crossing structures to create conditions more suitable for wildlife passage.
Characteristics of river and stream crossings that can affect use by wildlife:
- Size – Large wildlife (bear, moose) require large structures.
- Openness – some species are insecure about passing through confined spaces. Relatively large structures may feel confining to wildlife if they are very long. An openness factor calculated by dividing the cross sectional area of the structure opening by the structure length is often used to assess suitability for animals, such as deer or bear.
- Substrate – Highly artificial surfaces, such as corrugated metal, are not as suitable for wildlife as a smooth (e.g. concrete) bottom, or better yet, natural substrate. Small animals, such as chipmunks, weasels, rodents and rabbits, are likely to prefer structures with natural cover objects (e.g. large rocks, woody debris or vegetation that can grow under bridges).
- Dry passage – Some wildlife (raccoons, otters) are comfortable wading or swimming through water but other, more terrestrial, species rarely swim. These more terrestrial species benefit from banks or other areas of continuous dry substrate through the structure and connecting to banks upstream and downstream.
- Absence of physical barriers – Some characteristics of road-stream crossings, such as outlet drops or beaver fencing, can make it difficult or impossible for some species to use these crossings.
Massachusetts River and Stream Crossing Standards
The Massachusetts River and Stream Crossing Standards contain two sets of standards to provide landscape connectivity: 1) Best known are the General Standards that have been incorporated into state and federal wetlands regulations to ensure that road-stream crossings don’t block the movement of aquatic and semi-aquatic organisms (e.g., fish, turtles), 2) Optimum Standards are recommended for road-stream crossings that are designed for use by terrestrial and large semi-aquatic wildlife (e.g., river otter). The Optimum Standards are recommended for “…areas of particular statewide or regional significance for their contribution to landscape level connectedness…”
Replacing culverts or otherwise upgrading stream crossings to meet the Optimum Standards is one way to improve terrestrial connectivity and provide more resilient habitats and wildlife populations. Results from the Critical Linkages analysis conducted by UMass Amherst can be used to identify road-stream crossings that are likely to be significant for landscape level connectedness (see map on this page). If it is not possible to upgrade a crossing to meet these standards, it may still be possible to modify bridges (perhaps even some culverts) to make them more suitable for terrestrial wildlife passage.
Modifications that make a crossing suitable for terrestrial wildlife:
- Wildlife Benches – Areas of dry passage can be constructed within a crossing using wood, concrete or gabion baskets.
- Vegetated Rock Fill – Dry passage can be constructed within the structure on one or both sides of the stream using rock fill with soil and gravel to create a vegetated “bank” suitable for use by wildlife.
- Stream Channel and Natural Bank Restoration – The stream banks and side slopes under bridges are covered in large angular riprap that is not well suited for wildlife passage. In some cases, it may be possible to remove some or all of the riprap and create (restore) more natural bank and channel characteristics.
- Fencing – In some cases a road-stream crossing may not be ideal but still be good enough for use by some wildlife species. In these cases it is not unusual for some individuals to pass through the structure while others continue to cross over the road surface. To reduce mortality at these crossing points, it may be possible to use fencing to block access to the road and force animals to use the structure. However, fencing should be carefully considered before use. Fencing may facilitate movement of some wildlife across the road by forcing them to use the existing structure. However, if the structure is unsuitable for use by other species, fencing may foreclose their best opportunity for crossing the road (e.g. over the road surface late at night) thereby reducing terrestrial connectivity.
It is important to note that all work affecting streams must be permitted under applicable state and federal wetland? regulations.
Many opportunities exist to modify or replace crossing structures to facilitate wildlife passage across roads and increase landscape connectivity. Available information should be used to identify the best opportunities so that we make the most of limited resources available for climate change adaptation.
Target Species, Species Groups, Habitats and Stressors
Scope and Constraints
1. Scott D. Jackson and C. R. Griffin. "A Strategy for Mitigating Highway Impacts on Wildlife" Wildlife and Highways: Seeking Solutions to an Ecological and Socio-economic Dilemma. Ed. T.A. Messmer and B. West. The Wildlife Society, 2000. 143-159.
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