Restoring ecological networks
across transport corridors in Bulgaria

 
   



A few simple steps to help preventing habitat fragmentation

Landscape bridge

A landscape bridge is an overpass that aims to restore connectivity between ecosystems/landscapes. Such bridges are at least 200 m wide and allow for the development of natural habitats on top. The objective is not to facilitate passage of a selected group of ‘target’ animal species but to provide possibilities for passage across the road for all fauna (and flora) species of an ecosystem. All the locations where a landscape bridge is recommended in Bulgaria are locations where the transport corridor currently runs through a tunnel. Hence none of these locations require new bridge construction. Recommended measures for these bottleneck locations are:

  1. Habitat improvement on top of the overpass and in the adjacent areas to provide optimal conditions for passing animals;
  2. Removal of all anthropogenic structures (e.g. buildings, fences) that may inhibit or disturb wildlife movements across the overpass;
  3. Construction of wildlife fences to keep the animals off the road and guide them towards the overpass;
  4. Establishing a 1 km “buffer-zone” around the overpass in which public access is prohibited in order to prevent disturbance.

Wildlife overpass

A wildlife overpass is an overpass that aims to restore habitat connectivity for a selected group of ‘target’ animal species such as brown bear, wolf or red deer. Wildlife overpasses may vary in width but a minimum width of 50 m is recommended. The vegetation on top of the overpass should reflect the habitats on either side of the transport corridor to create favorable conditions for the passing animals. A row of tree stumps or stone piles across the overpass and zone with more humid conditions, e.g. a stream or series of small ponds, may further increase the chances that wildlife will use the overpass as a habitat corridor. A wildlife overpass should be accompanied by wildlife fences that keep the animals off the road and guide them towards the overpass and a 500 m “buffer-zone” around the overpass in which public access is prohibited in order to prevent disturbance.

Large wildlife underpass

Large wildlife underpasses allow for the passage of medium-sized and large mammals such as deer, wild boar, wildcat and wolf. Smaller animals may also use these underpasses, especially if special measures are taken to provide them with cover. Large wildlife underpasses are less suitable for connecting habitats, as vegetation growth in the underpass is usually limited. The underpasses may vary in width and height but a minimum width of 20 m and a minimum height of 4 m are recommended. A row of tree stumps or stone piles in the underpass, sufficient vegetation cover around the entrances and a watercourse through the underpass improve the chances of a variety of wildlife using the passage. A large wildlife underpass should be accompanied by wildlife fences that keep the animals off the road and guide them towards the underpass and a 500 m “buffer-zone” around the underpass in which public access is prohibited in order to prevent disturbance.

Small wildlife underpass

Small wildlife underpasses allow for the passage of small mammals, such as mustelids or rodents, amphibians and possibly reptile and invertebrate species. Medium-sized animals, such as badger or wildcat, may also use these underpasses if the underpass size allows. Small wildlife underpasses vary in design and dimensions. Rectangular tunnels with a minimum width of 1.5 m and minimum height of 1 m are recommended as they have been proven to provide a corridor for the greatest variety of species, including amphibians. Guidance of animals towards the underpasses should be provided through appropriate fencing, sufficient vegetation cover and the placement of a row of tree stumps or stones. A small wildlife underpass should further be accompanied by habitat improvement around the tunnel entrances, such as the construction of ponds, and a 50 m “buffer-zone” around the underpass in which public access is prohibited in order to prevent disturbance.

Modified bridge or viaduct

In the current situation rivers, mountain valley or lower level roads are often crossed by bridges or viaducts. In many cases these can easily be adapted to function as wildlife passages. If these bridges and viaducts are of sufficient width and height they can even allow for connecting ecosystems, as the existing light and water conditions may be sufficient for the development of undisrupted plant cover. The bridges and viaducts vary in width and height but a minimum width of 20 m and a minimum height of 5 m are recommended. Where rivers are crossed, the width should allow at least 10 m on either side of the water to allow for the growth of river bank vegetation and provide passage possibilities for aquatic, amphibian as well as terrestrial species. A row of tree stumps or stone piles and sufficient vegetation cover underneath as well as around the bridge/viaduct will improve the chances of a variety of wildlife using the passage. Additional measures include the construction of wildlife fences that keep the animals off the road and guide them towards the underpass, the removal of all obstructing elements (e.g. buildings, fences) that may inhibit or disturb wildlife movements, as well as the establishment of a 500 m “buffer-zone” around the bridge or viaduct, wherever feasible, in which public access is prohibited in order to prevent disturbance.